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Carbon Cycle

1. : Temporal variation in carbon and nitrogen sequestration rates in boreal soils across a variety of ecosystems Wed, 27 Jan 2016 23:20:26 +0100 Biogeosciences+
   
   Temporal variation in carbon and nitrogen sequestration rates in boreal soils across a variety of ecosystems
   K. L. Manies, J. W. Harden, C. C. Fuller, and M. R. Turetsky
   Biogeosciences Discuss., doi:10.5194/bg-2016-24,2016
   Manuscript under review for BG (discussion: open, 0 comments)
   Boreal soils are important in the global C cycle. So we need to understand what controls how C accumulates & is lost from this soil. To help we examined accumulation rates for five boreal ecosystems. Most ecosystems were similar. But the rich fen had higher long-term C & N accumulation rates, suggesting that C & N cycling here is different, likely due to different nutrient cycling & because it burns less. Therefore, if there are shifts among most ecosystems regional C & N dynamics won’t change.
2. : Sources, cycling and export of nitrogen on the Greenland Ice Sheet Mon, 25 Jan 2016 23:20:26 +0100 Biogeosciences+
   
   Sources, cycling and export of nitrogen on the Greenland Ice Sheet
   J. L. Wadham, J. Hawkings, J. Telling, D. Chandler, J. Alcock, E. Lawson, P. Kaur, E. A. Bagshaw, M. Tranter, A. Tedstone, and P. Nienow
   Biogeosciences Discuss., doi:10.5194/bg-2015-484,2016
   Manuscript under review for BG (discussion: open, 0 comments)
   Fjord and continental shelf environments in the Polar Regions are host to some of the planet’s most productive ecosystems, and support economically important fisheries. Their productivity, however, is often critically dependent upon nutrient supply from up-stream terrestrial environments delivered via river systems. One of the most extensive glacially-fed coastal ecosystems is that bordering the Greenland Ice Sheet. The future primary productivity of this marine ecosystem, however, is uncertain. A potential increase in primary productivity driven by reduced sea ice extent and associated increased light levels may be curtailed by insufficient nutrient supply, and specifically nitrogen. Research on small valley glaciers indicates that glaciers are important sources of nitrogen to downstream environments. However, no data exists from ice sheet systems such as Greenland. Time series of nitrogen concentrations in runoff are documented from a large Greenland glacier, demonstrating seasonally elevated fluxes to the ocean. Fluxes are highest in mid-summer, when nitrogen limitation is commonly reported in coastal waters. It is estimated that approximately half of the glacially-exported nitrogen is sourced from microbial activity within glacial sediments at the surface and bed of the ice sheet, doubling nitrogen fluxes in runoff. Summer dissolved inorganic nitrogen fluxes from the Greenland Ice Sheet (30–40 Gg) are a similar order of magnitude to those from a large Arctic river (40 Gg, Holmes et al., 2012). Nitrogen yields from the ice sheet (100–160 kg TDN km⁻² a⁻¹), however, are approximately double those from Arctic riverine catchments. We assert that this ice sheet nitrogen subsidy to Arctic coastal ecosystems may be important for understanding coastal biodiversity, productivity and fisheries, and should be considered in future biogeochemical modelling studies of coastal marine productivity in the Arctic regions.
3. : Phytoplankton dynamics driven by vertical nutrient fluxes during the spring inter-monsoon period in the northeastern South China Sea Fri, 22 Jan 2016 23:20:26 +0100 Biogeosciences+
   
   Phytoplankton dynamics driven by vertical nutrient fluxes during the spring inter-monsoon period in the northeastern South China Sea
   Q. P. Li, Y. Dong, and Y. Wang
   Biogeosciences, 13, 455-466, doi:10.5194/bg-13-455-2016, 2016
   Phytoplankton patchiness in the northeastern SCS during May 2014 could be largely controlled by vertical nutrient fluxes including turbulent diffusion and curl-driven upwelling. There was an increasing turbulent diffusion but decreasing curl-driven upwelling from the coastal upwelling zones to the offshore pelagic zones. Elevated fluxes near Dongsha led to net growth of a diatom-rich community, whereas low fluxes near southwest Taiwan resulted in a decline of a picoplankton bloom.
4. Chris S.M. Turney, Jonathan Palmer, Alan Hogg, Christopher J. Fogwill, Richard Jones, Christopher Bronk Ramsey, Pavla Fenwick, Pauline Grierson, Janet Wilmshurst, Alison O'Donnell, Zoë Thomas, Mathew Lipson: Multi-decadal variations in Southern Hemisphere atmospheric 14C: Evidence against a Southern Ocean sink at the end of the Little Ice Age CO2 anomaly 2016-01-18T02:31:56.304353-05:00 Global Biogeochemical Cycles+
   
   Northern Hemisphere-wide cooling during the Little Ice Age (LIA; CE 1650-1775) is associated with a ~5 ppmv decrease in atmospheric carbon dioxide. Changes in terrestrial and ocean carbon reservoirs have been postulated as possible drivers of this relatively large shift in atmospheric CO2, potentially providing insights into the mechanisms and sensitivity of the global carbon cycle. Here we report decadally-resolved radiocarbon (14C) levels in a network of tree rings series spanning CE 1700-1950 located along the northern boundary of, and within, the Southern Ocean. We observe regional dilutions in atmospheric radiocarbon (relative to the Northern Hemisphere) associated with upwelling of 14CO2–depleted abyssal waters. We find the inter-hemispheric 14C offset approaches zero during increasing global atmospheric CO2 at the end of the LIA, with reduced ventilation in the Southern Ocean and a Northern Hemisphere source of old carbon (most probably originating from deep Arctic peat layers). The coincidence of the atmospheric CO2 increase and reduction in the inter-hemispheric 14C offset imply a common climate control. Possible mechanisms of synchronous change in the high latitudes of both hemispheres are discussed.
5. D. Talmy, A.C. Martiny, C. Hill, A.E. Hickman, M.J. Follows: Microzooplankton regulation of surface ocean POC:PON ratios 2016-01-13T16:11:50.826251-05:00 Global Biogeochemical Cycles+
   
   The elemental composition of particulate organic matter in the surface ocean significantly affects the efficiency of the oceanâĂŹs store of carbon. Though the elemental composition of primary producers is an important factor, recent observations from the western North Atlantic Ocean revealed that carbon-to-nitrogen ratios (C:N) of phytoplankton were significantly higher than the relatively homeostatic ratio of the total particulate pool (Particulate Organic Carbon: Particulate Organic Nitrogen; POC:PON). Here we use an idealized ecosystem model to show how interactions between primary and secondary producers maintain the mean composition of surface particulates, and the difference between primary producers and bulk material. Idealized physiological models of phytoplankton and microzooplankton, constrained by laboratory data, reveal contrasting autotrophic and heterotrophic responses to nitrogen limitation: under nitrogen limitation, phytoplankton accumulate carbon in carbohydrates and lipids while microzooplankton deplete internal C reserves to fuel respiration. Global ecosystem simulations yield hypothetical global distributions of phytoplankton and microzooplankton C:N ratio predicting elevated phytoplankton C:N ratios in the high light, low nutrient regions of the ocean despite a lower, homeostatic POC:PON ratio due to respiration of excess carbon in systems subject to top-down control. The model qualitatively captures, and provides a simple interpretation for, a global compilation of surface ocean POC:PON data.
6. Arne Bratkič, Mitja Vahčič, Jože Kotnik, E. Malcolm S. Woodward, Milena Horvat: Mercury presence and speciation in the South Atlantic Ocean along the 40°S transect 2016-01-07T22:22:42.790198-05:00 Global Biogeochemical Cycles+
   
   Mercury (Hg) natural biogeochemical cycle is complex and a significant portion of biological and chemical transformation occurs in the marine environment. To better understand the presence and abundance of Hg species in the remote ocean regions, waters of South Atlantic Ocean along 40°S parallel were investigated during UK-GEOTRACES cruise GA10. Total mercury (THg), methylated mercury (MeHg) and dissolved gaseous mercury (DGM) concentrations were determined. The concentrations were very low in the range of pg/L (femtomolar). All Hg species had higher concentration in western than in eastern basin. THg did not appear to be a useful geotracer. Elevated methylated Hg species were commonly associated with low-oxygen water masses and occasionally with peaks of Chlorophyll a, both involved with carbon (re)cycling. The overall highest MeHg concentrations were observed in the mixed layer (500 m) and in the vicinity of the Gough Island.. Conversely, DGM concentrations showed distinct layering and differed between the water masses in a nutrient-like manner. DGM was lowest at surface, indicating degassing to the atmosphere; and was highest in the Upper Circumpolar Deep Water, where the oxygen concentration was lowest. DGM increased also in Antarctic Bottom Water. At one station, dimethylmercury was determined and showed increase in region with lowest oxygen saturation. Altogether, our data indicate that the South Atlantic Ocean could be a source of Hg to the atmosphere and that its biogeochemical transformations depend primarily upon carbon cycling and are thereby additionally prone to global ocean change.
7. Toby K. Westberry, Patrick Schultz, John P. Dunne, Michael R. Hiscock, Stephane Maritorena, Jorge L. Sarmiento, David A. Siegel, Michael J. Behrenfeld: Annual cycles of phytoplankton biomass in the Subarctic Atlantic and Pacific Ocean 2016-01-07T21:56:41.867831-05:00 Global Biogeochemical Cycles+
   
   High latitude phytoplankton blooms support productive fisheries and play an important role in oceanic uptake of atmospheric carbon dioxide. In the subarctic North Atlantic Ocean, blooms are a recurrent feature each year, while in the eastern subarctic Pacific only small changes in chlorophyll (Chl) are seen over the annual cycle. Here, we show that when evaluated using phytoplankton carbon biomass (Cphyto) rather than Chl, an annual bloom in the North Pacific is evident and can even rival blooms observed in the North Atlantic. The annual increase in subarctic Pacific phytoplankton biomass is not readily observed in the Chl record because it is paralleled by light- and nutrient-driven decreases in cellular pigment levels (Cphyto:Chl). Specifically, photoacclimation and iron stress effects on Cphyto:Chl oppose the biomass increase, leading to only modest changes in bulk Chl. The magnitude of the photoacclimation effect is quantified using descriptors of the near-surface light environment and a photophysiological model. Iron-stress effects are diagnosed from satellite chlorophyll fluorescence data. Last, we show that biomass accumulation in the Pacific is slower than the Atlantic, but is closely tied to similar levels of seasonal nutrient uptake in both basins. Annual cycles of satellite-derived Chl and Cphyto are reproduced by in situ autonomous profiling floats. These results contradict the long-standing paradigm that environmental conditions prevent phytoplankton accumulation in the subarctic Northeast Pacific and suggest a greater seasonal decoupling between phytoplankton growth and losses than traditionally implied. Further, our results highlight the role of physiological processes in shaping bulk properties, such as Chl, and their interpretation in studies of ocean ecosystem dynamics and climate change.
8. Marie-Elodie Perga, Stephen C. Maberly, Jean-Philippe Jenny, Benjamin Alric, Cécile Pignol, Emmanuel Naffrechoux: A century of human-driven changes in the carbon dioxide concentration of lakes 2016-01-06T09:19:43.690445-05:00 Global Biogeochemical Cycles+
   
   Now that evasion of carbon dioxide (CO2) from inland waters is accounted for in global carbon models, it is crucial to quantify how these fluxes have changed in the past and forecast how they may alter in the future in response to local and global change. Here, we developed a sediment proxy for the concentration of summer surface dissolved CO2 concentration and used it to reconstruct changes over the past 150 years for three large lakes that have been affected by climate warming, changes in nutrient load and detrital terrigenous supplies. Initially CO2-neutral to the atmosphere, all three lakes subsequently fluctuated between near-equilibrium and supersaturation. Although catchment inputs have supplied CO2 to the lakes, internal processes and re-allocation have ultimately regulated decadal changes in lake surface CO2 concentration. Nutrient concentration has been the dominant driver of CO2 variability for a century although the reproducible, non-monotonic relationship of CO2 to nutrient concentration suggests an interplay between metabolic and chemical processes. Yet, for two of these lakes, climatic control of CO2 concentrations has been important over the last 30 years, promoting higher surface CO2 concentrations, likely by decreasing hypolimnetic carbon storage. This new approach offers the unique opportunity to scale, a posteriori, the long-term impact of human activities on lake CO2.
9. J.A.C. Davies, E. Tipping, E.C. Rowe, J.F. Boyle, E. Graf Pannatier, V. Martinsen: Long-term P weathering and recent N deposition control contemporary plant-soil C, N and P 2016-01-06T08:19:51.461438-05:00 Global Biogeochemical Cycles+
   
   Models are needed to understand how plant-soil nutrient stores and fluxes have responded to the last two centuries of widespread anthropogenic nutrient pollution and predict future change. These models need to integrate across carbon, nitrogen and phosphorus (C, N, & P) cycles and simulate changes over suitable timescales using available driving data. It is also vital that they are constrainable against observed data to provide confidence in their outputs. To date, no models address all of these requirements. To meet this need, a new model, N14CP, is introduced, which is initially applied to Northern hemisphere temperate and boreal ecosystems over the Holocene. N14CP is parameterized and tested using 88 northern Europe plot-scale studies, providing the most robust test of such a model to date. The model simulates long-term P weathering, based on the assumption of a starting pool of weatherable P (Pweath0, g m−2), which is gradually transformed into organic and sorbed pools. Nitrogen fixation (and consequently primary production) is made dependent on available P. In the absence of knowledge about the spatial variability of Pweath0, N14CP produces good average soil and plant variables, but cannot simulate variations among sites. Allowing Pweath0 to vary between sites improves soil C, N and P results greatly, suggesting contemporary soil C, N and P are sensitive to long-term P weathering. Most sites were found to be N limited. Anthropogenic N deposition since 1800 was calculated to have increased plant biomass substantially, in agreement with observations, and consequently increased soil carbon pools.
10. Alessandro Tagliabue, Olivier Aumont, Ros DeAth, John P. Dunne, Stephanie Dutkiewicz, Eric Galbraith, Kazuhiro Misumi, J. Keith Moore, Andy Ridgwell, Elliot Sherman, Charles Stock, Marcello Vichi, Christoph Völker, Andrew Yool: How well do global ocean biogeochemistry models simulate dissolved iron distributions? 2015-12-28T15:21:07.38187-05:00 Global Biogeochemical Cycles+
    
    Numerical models of ocean biogeochemistry are relied upon to make projections about the impact of climate change on marine resources and test hypotheses regarding the drivers of past changes in climate and ecosystems. In large areas of the ocean, iron availability regulates the functioning of marine ecosystems and hence the ocean carbon cycle. Accordingly, our ability to quantify the drivers and impacts of fluctuations in ocean ecosystems and carbon cycling in space and time relies on first achieving an appropriate representation of the modern marine iron cycle in models. When the iron distributions from thirteen global ocean biogeochemistry models are compared against the latest oceanic sections from the GEOTRACES programme we find that all models struggle to reproduce many aspects of the observed spatial patterns. Models that reflect the emerging evidence for multiple iron sources or subtleties of its internal cycling perform much better in capturing observed features than their simpler contemporaries, particularly in the ocean interior. We show that the substantial uncertainty in the input fluxes of iron results in a very wide range of residence times across models, which has implications for the response of ecosystems and global carbon cycling to perturbations. Given this large uncertainty, iron-fertilisation experiments based on any single current generation model should be interpreted with caution. Improvements to how such models represent iron scavenging and also biological cycling are needed to raise confidence in their projections of global biogeochemical change in the ocean.
11. Filippa Fransner, Jonas Nycander, Carl-Magnus Mörth, Christoph Humborg, H. E. Markus Meier, Robinson Hordoir, Erik Gustafsson, Barbara Deutsch: Tracing terrestrial DOC in the Baltic Sea - a 3-D model study 2015-10-28T21:30:23.634165-05:00 Global Biogeochemical Cycles+
    
    The fate of terrestrial organic matter brought to the coastal seas by rivers, and its role in the global carbon cycle, are still not very well known. Here the degradation rate of terrestrial dissolved organic carbon (DOCter) is studied in the Baltic Sea, a subarctic semi-enclosed sea, by releasing it as a tracer in a 3-D circulation model and applying linear decay constants. A good agreement with available observational data is obtained by parameterizing the degradation in two rather different ways; one by applying a decay time on the order of 10 years to the whole pool of DOCter, and one by dividing the DOCter into one refractory pool and one pool subject to a decay time on the order of 1 year. The choice of parameterization has a significant effect on where in the Baltic Sea the removal takes place, which can be of importance when modeling the full carbon cycle and the CO2 exchange with the atmosphere. In both cases the biogeochemical decay operates on time scales less than the water residence time. Therefore only a minor fraction of the DOCter reaches the North Sea, whereas approximately 80% is removed by internal sinks within the Baltic Sea. This further implies that DOCter mineralization is an important link in land-sea-atmosphere cycling of carbon in coastal- and shelf seas that are heavily influenced by riverine DOC.
12. Julia Boutorh, Brivaëla Moriceau, Morgane Gallinari, Olivier Ragueneau, Eva Bucciarelli: Effect of trace metal-limited growth on the postmortem dissolution of the marine diatom Pseudo-nitzschia delicatissima 2016-01-23T16:01:48.931938-05:00 Global Biogeochemical Cycles+
    
    We investigated the effects of iron (Fe) and copper (Cu) limitations on biogenic silica (bSiO2) dissolution kinetics of the marine diatom Pseudo-nitzschia delicatissima during a 3 week batch dissolution experiment. The dissolution of this species was faster during the first week than thereafter. Modeling results from four dissolution models and scanning electron microcopy images suggested the successive dissolution of two phases of bSiO2, with two different dissolution constants. Micronutrient limitation during growth affected the respective proportion of the two phases and their dissolution constants. After 3 weeks of dissolution, frustules from micronutrient-limited diatoms were better preserved than those of replete cells. Our results also confirm that micronutrient-limited cells may export more Si relative to N than replete cells and may increase the silicate pump: This may not only be due to a higher degree of silicification of the live cells but also to a decoupling between the recycling of Si and N during dissolution. We suggest that a mechanistic understanding of the evolution of the dissolution constant during dissolution is needed. This would improve the parameterization of dissolution in ecosystem models, and ultimately their predictions on the amount of bSiO2 that dissolves in the photic zone, and the amount of bSiO2 that is exported to the seafloor.
13. Andrea D. Almeida Castanho, David Galbraith, Ke Zhang, Michael T. Coe, Marcos H. Costa, Paul Moorcroft: Changing Amazon biomass and the role of atmospheric CO2 concentration, climate, and land use 2016-01-19T18:44:00.211132-05:00 Global Biogeochemical Cycles+
    
    The Amazon tropical evergreen forest is an important component of the global carbon budget. Its forest floristic composition, structure, and function are sensitive to changes in climate, atmospheric composition, and land use. In this study biomass and productivity simulated by three dynamic global vegetation models (Integrated Biosphere Simulator, Ecosystem Demography Biosphere Model, and Joint UK Land Environment Simulator) for the period 1970–2008 are compared with observations from forest plots (Rede Amazónica de Inventarios Forestales). The spatial variability in biomass and productivity simulated by the DGVMs is low in comparison to the field observations in part because of poor representation of the heterogeneity of vegetation traits within the models. We find that over the last four decades the CO2 fertilization effect dominates a long-term increase in simulated biomass in undisturbed Amazonian forests, while land use change in the south and southeastern Amazonia dominates a reduction in Amazon aboveground biomass, of similar magnitude to the CO2 biomass gain. Climate extremes exert a strong effect on the observed biomass on short time scales, but the models are incapable of reproducing the observed impacts of extreme drought on forest biomass. We find that future improvements in the accuracy of DGVM predictions will require improved representation of four key elements: (1) spatially variable plant traits, (2) soil and nutrients mediated processes, (3) extreme event mortality, and (4) sensitivity to climatic variability. Finally, continued long-term observations and ecosystem-scale experiments (e.g. Free-Air CO2 Enrichment experiments) are essential for a better understanding of the changing dynamics of tropical forests.
14. N. S. Lord, A. Ridgwell, M. C. Thorne, D. J. Lunt: An impulse response function for the “long tail” of excess atmospheric CO2 in an Earth system model 2016-01-09T03:26:12.305219-05:00 Global Biogeochemical Cycles+
    
    The ultimate fate of (fossil fuel) CO2 emitted to the atmosphere is governed by a range of sedimentological and geological processes operating on timescales of up to the ca. hundred thousand year response of the silicate weathering feedback. However, how the various geological CO2 sinks might saturate and feedbacks weaken in response to increasing total emissions is poorly known. Here we explore the relative importance and timescales of these processes using a 3-D ocean-based Earth system model. We first generate an ensemble of 1 Myr duration CO2 decay curves spanning cumulative emissions of up to 20,000 Pg C. To aid characterization and understanding of the model response to increasing emission size, we then generate an impulse response function description for the long-term fate of CO2 in the model. In terms of the process of carbonate weathering and burial, our analysis is consistent with a progressively increasing fraction of total emissions that are removed from the atmosphere as emissions increase, due to the ocean carbon sink becoming saturated, together with a lengthening of the timescale of removal from the atmosphere. However, we find that in our model the ultimate CO2 sink—silicate weathering feedback—is approximately invariant with respect to cumulative emissions, both in terms of its importance (it removes the remaining excess ~7% of total emissions from the atmosphere) and timescale (~270 kyr). Because a simple pulse-response description leads to initially large predictive errors for a realistic time-varying carbon release, we also develop a convolution-based description of atmospheric CO2 decay which can be used as a simple and efficient means of making long-term carbon cycle perturbation projections.
15. Seth M. Bushinsky, Steven Emerson: Marine biological production from in situ oxygen measurements on a profiling float in the subarctic Pacific Ocean 2015-12-10T05:55:28.95091-05:00 Global Biogeochemical Cycles+
    
    Evaluating the organic carbon flux from the surface ocean to the interior (the marine biological pump) is essential for predictions of ocean carbon cycle feedback to climate change. One approach for determining these fluxes is to measure the concentration of oxygen in the upper ocean over a seasonal cycle, calculate the net O2 flux using an upper ocean model, and then use a stoichiometric relationship between oxygen evolved and organic carbon produced. Applying this tracer in a variety of ocean areas over seasonal cycles requires accurate O2 measurements on autonomous vehicles. Here we demonstrate this approach using an O2 sensor on a profiling float that is periodically calibrated against atmospheric pO2. Using accurate data and a model that includes all physical and biological processes influencing oxygen, we determine an annual net community production of 0.7 ± 0.5 mol C m−2 yr−1 in the northeast Pacific Ocean (50°N, 145°W) from June 2012 to June 2013. There is a strong seasonal cycle in net biological oxygen production with wintertime fluxes caused by bubble processes critical to determining the annual flux. Approximately 50% of net autotrophic production during summer months is consumed by net respiration during the winter. The result is a biological pump in the subarctic Pacific Ocean that is less than that determined by similar methods in the subtropics to the south. This estimate is significantly lower than that predicted by satellite remote sensing and global circulation models.
16. KS Chin, J Lento, JM Culp, D Lacelle, SV Kokelj: Permafrost thaw and intense thermokarst activity decreases abundance of stream benthic macroinvertebrates 2016-01-14T11:28:35.247479-05:00 Global Change Biology+
    
    Intensification of permafrost thaw has increased the frequency and magnitude of large permafrost slope disturbances (mega slumps) in glaciogenic terrain of northwestern Canada. Individual thermokarst disturbances up to 40 ha in area have made large volumes of previously frozen, highly weatherable fine-grained sediments available for leaching and transport to adjacent streams, significantly increasing sediment and solute loads in these systems. To test the effects of this climate-sensitive disturbance regime on the ecology of Arctic streams, we explored the relationship between physical and chemical variables and benthic macroinvertebrate communities in disturbed and undisturbed stream reaches in the Peel Plateau in the Northwest Territories, Canada. Highly disturbed and undisturbed stream reaches differed with respect to taxonomic composition and invertebrate abundance. Minimally disturbed reaches were not differentiated by these variables but rather were distributed along a disturbance gradient between highly disturbed and undisturbed sites. In particular, there was evidence of a strong negative relationship between macroinvertebrate abundance and total suspended solids, and a positive relationship between abundance and the distance from the disturbance. Increases in both sediments and nutrients appear to be the proximate cause of community differences in highly disturbed streams. Declines in macroinvertebrate abundance in response to slump activity have implications for the food webs of these systems, potentially leading to negative impacts on higher trophic levels, such as fish. Further, the disturbance impacts on stream health can be expected to intensify as climate change increases the frequency and magnitude of thermokarst. This article is protected by copyright. All rights reserved.
17. Chongyang Xu, Hongyan Liu, A. Park Williams, Yi Yin, Xiuchen Wu: Trends toward an earlier peak of the growing season in Northern Hemisphere mid-latitudes 2016-01-11T06:42:04.474098-05:00 Global Change Biology+
    
    Changes in peak photosynthesis timing (PPT) could substantially change the seasonality of the terrestrial carbon cycle. Spring PPT in dry regions has been documented for some individual plant species on a stand scale, but both the spatio-temporal pattern of shifting PPT on a continental scale and its determinants remain unclear. Here we use satellite measurements of vegetation greenness to find that the majority of Northern Hemisphere, mid-latitude vegetated area experienced a trend toward earlier PPT during 1982-2012, with significant trends of approximately 0.61 day·year−1 across 19.4% of areas. These shifts correspond to increased annual accumulation of growing degree days (GDD) due to warming and are most highly concentrated in the eastern United States and Europe. Earlier mean PPT is generally a trait common among areas with summer temperatures higher than 27.62.9°C, summer precipitation lower than 84.241.5 mm, and fraction of pre-growing season precipitation greater than 89.21.5%. The trends toward earlier PPT discovered here have co-occurred with overall increases in vegetation greenness throughout the growing season, suggesting that summer drought is not a dominant driver of these trends. These results imply that continued warming facilitate continued shifts toward earlier PPT and cause these trends to become more pervasive, with important implications for terrestrial carbon, water, nutrient, and energy budgets. This article is protected by copyright. All rights reserved.
18. Stefanos Mystakidis, Edouard L. Davin, Nicolas Gruber, Sonia I. Seneviratne: Constraining future terrestrial carbon cycle projections using observation-based water and carbon flux estimates 2016-01-06T01:47:17.097707-05:00 Global Change Biology+
    
    The terrestrial biosphere is currently acting as a sink for about a third of the total anthropogenic CO2 emissions. However, the future fate of this sink in the coming decades is very uncertain, as current Earth System Models (ESMs) simulate diverging responses of the terrestrial carbon cycle to upcoming climate change. Here, we use observation-based constraints of water and carbon fluxes to reduce uncertainties in the projected terrestrial carbon cycle response derived from simulations of ESMs conducted as part of the 5th phase of the Coupled Model Intercomparison Project (CMIP5). We find in the ESMs a clear linear relationship between present-day Evapotranspiration (ET) and Gross Primary Productivity (GPP), as well as between these present-day fluxes and projected changes in GPP, thus providing an emergent constraint on projected GPP. Constraining the ESMs based on their ability to simulate present-day ET and GPP leads to a substantial decrease of the projected GPP and to a ca. 50% reduction of the associated model spread in GPP by the end of the century. Given the strong correlation between projected changes in GPP and in NBP in the ESMs, applying the constraints on Net Biome Productivity (NBP) reduces the model spread in the projected land sink by more than 30% by 2100. Moreover, the projected decline in the land sink is at least doubled in the constrained ensembles and the probability that the terrestrial biosphere is turned into a net carbon source by the end of the century is strongly increased. This indicates that the decline in the future land carbon uptake might be stronger than previously thought, which would have important implications for the rate of increase of the atmospheric CO2 concentration and for future climate change. This article is protected by copyright. All rights reserved.
19. Tom N. Walker, Mark H. Garnett, Susan E. Ward, Simon Oakley, Richard D. Bardgett, Nicholas J. Ostle: Vascular plants promote ancient peatland carbon loss with climate warming 2016-01-04T02:22:38.523425-05:00 Global Change Biology+
    
    Northern peatlands have accumulated one third of the Earth's soil carbon stock since the last Ice Age. Rapid warming across northern biomes threatens to accelerate rates of peatland ecosystem respiration. Despite compensatory increases in net primary production, greater ecosystem respiration could signal the release of ancient, century- to millennia-old carbon from the peatland organic matter stock. Warming has already been shown to promote ancient peatland carbon release, but, despite the key role of vegetation in carbon dynamics, little is known about how plants influence the source of peatland ecosystem respiration. Here, we address this issue using in situ 14C measurements of ecosystem respiration on an established peatland warming and vegetation manipulation experiment. Results show that warming of approximately 1 °C promotes respiration of ancient peatland carbon (up to 2100 years old) when dwarf-shrubs or graminoids are present, an effect not observed when only bryophytes are present. We demonstrate that warming likely promotes ancient peatland carbon release via its control over organic inputs from vascular plants. Our findings suggest that dwarf-shrubs and graminoids prime microbial decomposition of previously ‘locked-up’ organic matter from potentially deep in the peat profile, facilitating liberation of ancient carbon as CO2. Furthermore, such plant-induced peat respiration could contribute up to 40% of ecosystem CO2 emissions. If consistent across other sub-arctic and arctic ecosystems, this represents a considerable fraction of ecosystem respiration that is currently not acknowledged by global carbon cycle models. Ultimately, greater contribution of ancient carbon to ecosystem respiration may signal the loss of a previously stable peatland carbon pool, creating potential feedbacks to future climate change. This article is protected by copyright. All rights reserved.
20. Tanner J. Williamson, Wyatt F. Cross, Jonathan P. Benstead, Gísli M. Gíslason, James M. Hood, Alexander D. Huryn, Philip W. Johnson, Jill R. Welter: Warming alters coupled carbon and nutrient cycles in experimental streams 2015-12-31T02:32:55.744079-05:00 Global Change Biology+
    
    Although much effort has been devoted to quantifying how warming alters carbon cycling across diverse ecosystems, less is known about how these changes are linked to the cycling of bioavailable nitrogen and phosphorus. In freshwater ecosystems, benthic biofilms (i.e. thin films of algae, bacteria, fungi and detrital matter) act as biogeochemical hotspots by controlling important fluxes of energy and material. Understanding how biofilms respond to warming is thus critical for predicting responses of coupled elemental cycles in freshwater systems. We developed biofilm communities in experimental streamside channels along a gradient of mean water temperatures (7.5-23.6°C), while closely maintaining natural diel and seasonal temperature variation and a common water and propagule source. Both structural (i.e. biomass, stoichiometry, assemblage structure) and functional (i.e. metabolism, N2-fixation, nutrient uptake) attributes of biofilms were measured on multiple dates to link changes in carbon flow explicitly to the dynamics of nitrogen and phosphorus. Temperature had strong positive effects on biofilm biomass (2.8- to 24-fold variation) and net ecosystem productivity (44- to 317-fold variation), despite extremely low concentrations of limiting dissolved nitrogen. Temperature had surprisingly minimal effects on biofilm stoichiometry: carbon:nitrogen (C:N) ratios were temperature-invariant, while carbon:phosphorus (C:P) ratios declined slightly with increasing temperature. Biofilm communities were dominated by cyanobacteria at all temperatures (>91% of total biovolume) and N2-fixation rates increased up to 120-fold between the coldest and warmest treatments. Although ammonium-N uptake increased with temperature (2.8- to 6.8-fold variation), the much higher N2-fixation rates supplied the majority of N to the ecosystem at higher temperatures. Our results demonstrate that temperature can alter how carbon is cycled and coupled to nitrogen and phosphorus. The uncoupling of C fixation from dissolved inorganic nitrogen supply produced large unexpected changes in biofilm development, elemental cycling, and likely downstream exports of nutrients and organic matter. This article is protected by copyright. All rights reserved.
21. Vanessa Haverd, Benjamin Smith, Cathy Trudinger: Dryland vegetation response to wet episode, not inherent shift in sensitivity to rainfall, behind Australia's role in 2011 global carbon sink anomaly 2015-12-23T09:42:16.834743-05:00 Global Change Biology+
    
    There is compelling new evidence that semi-arid ecosystems are playing a pivotal role in the inter-annual variability and greening trend of the global carbon cycle (Ahlström et al., 2015). The situation is exemplified by the vast inland region of Australia, the driest inhabited continent. Using a global model, Poulter et al. (2014) inferred that Australian ecosystems contributed 57% of a record global carbon uptake anomaly in 2011, and have entered a regime of enhanced sensitivity to rainfall since the mid-1990s. This article is protected by copyright. All rights reserved.
22. Sophia Walther, Maximilian Voigt, Tea Thum, Alemu Gonsamo, Yongguang Zhang, Philipp Koehler, Martin Jung, Andrej Varlagin, Luis Guanter: Satellite chlorophyll fluorescence measurements reveal large-scale decoupling of photosynthesis and greenness dynamics in boreal evergreen forests 2015-12-18T10:39:24.317543-05:00 Global Change Biology+
    
    Mid-to-high latitude forests play an important role in the terrestrial carbon cycle, but the representation of photosynthesis in boreal forests by current modelling and observational methods is still challenging. In particular the applicability of existing satellite-based proxies of greenness to indicate photosynthetic activity is hindered by small annual changes in green biomass of the often evergreen tree population and by the confounding effects of background materials like snow. As an alternative, satellite measurements of sun-induced chlorophyll fluorescence (SIF) can be used as a direct proxy of photosynthetic activity. In this study, the start and end of the photosynthetically active season of the main boreal forests are analysed using space-borne SIF measurements retrieved from the GOME-2 instrument and compared to that of green biomass, proxied by vegetation indices including the Enhanced Vegetation Index (EVI) derived from MODIS data. We find that photosynthesis and greenness show a similar seasonality in deciduous forests. In high-latitude evergreen needleleaf forests, however, the length of the photosynthetically-active period indicated by SIF is up to six weeks longer than the green biomass changing period proxied by EVI, with SIF showing a start-of-season of approximately one month earlier than EVI. On average, the photosynthetic spring recovery as signalled by SIF occurs as soon as air temperatures exceed the freezing point (2-3 °C) and when the snow on the ground has not yet completely melted. These findings are supported by model data of gross primary production and a number of other studies which evaluated in-situ observations of CO2 fluxes, meteorology and the physiological state of the needles. Our results demonstrate the sensitivity of space-based SIF measurements to light-use efficiency of boreal forests and their potential for an unbiased detection of photosynthetic activity even under the challenging conditions interposed by evergreen boreal ecosystems. This article is protected by copyright. All rights reserved.
23. J-P. Jenny, P. Francus, A. Normandeau, F. Lapointe, M-E. Perga, A.E.K. Ojala, A. Schimmelmann, B. Zolitschka: Global spread of hypoxia in freshwater ecosystems during the last three centuries is caused by rising local human pressure 2015-12-15T00:17:48.706851-05:00 Global Change Biology+
    
    The spread of hypoxia is a threat to aquatic ecosystem functions and services as well as to biodiversity. However, sparse long-term monitoring of lake ecosystems has prevented reconstruction of global hypoxia dynamics while inhibiting investigations into its causes and assessing the resilience capacity of these systems. This study compiles the onset and duration of hypoxia recorded in sediments of 365 lakes worldwide since AD 1700, showing that lacustrine hypoxia started spreading before AD 1900, 70 years prior to hypoxia in coastal zones. This study also shows that the increase of human activities and nutrient release is leading to hypoxia onset. No correlations were found with changes in precipitation or temperature. There is no evidence for a post-1980s return to well-oxygenated lacustrine conditions in industrialized countries despite the implementation of restoration programs. The apparent establishment of stable hypoxic conditions prior to AD 1900 highlights the challenges of a growing nutrient demand, accompanied by increasing global nutrient emissions of our industrialized societies, and climate change. This article is protected by copyright. All rights reserved.
24. Mark A. Stevenson, Suzanne McGowan, N John Anderson, Robert H. Foy, Peter R. Leavitt, Yvonne R. McElarney, Daniel R. Engstrom, Sergi Pla-Rabés: Impacts of forestry planting on primary production in upland lakes from north-west Ireland 2015-12-14T23:38:56.63947-05:00 Global Change Biology+
    
    Planted forests are increasing in many upland regions world-wide, but knowledge about their potential effects on algal communities of catchment lakes is relatively unknown. Here the effects of afforestation were investigated using palaeolimnology at six upland lake sites in the north-west of Ireland subject to different extents of forest plantation cover (4-64% of catchment area). 210Pb dated sediment cores were analysed for carotenoid pigments from algae, stable isotopes of bulk carbon (δ13C) and nitrogen (δ15N), and C/N ratios. In lakes with >50% of their catchment area covered by plantations, there were two- to six-fold increases in pigments from cryptophytes (alloxanthin) and significant but lower increases (39-116%) in those from colonial cyanobacteria (canthaxanthin), but no response from biomarkers of total algal abundance (β-carotene). In contrast, lakes in catchments with <20% afforestation exhibited no consistent response to forestry practices, although all lakes exhibited fluctuations in pigments and geochemical variables due to peat cutting and upland grazing prior to forest plantation. Taken together, patterns suggest that increases in cyanobacteria and cryptophyte abundance reflect a combination of mineral and nutrient enrichment associated with forest fertilisation and organic matter influx which may have facilitated growth of mixotrophic taxa. This study demonstrates that planted forests can alter the abundance and community structure of algae in upland humic lakes of Ireland and Northern Ireland, despite long histories of prior catchment disturbance. This article is protected by copyright. All rights reserved.
25. David L. Hoover, Brendan M. Rogers: Not all droughts are created equal: the impacts of interannual drought pattern and magnitude on grassland carbon cycling 2015-11-14T10:20:52.313403-05:00 Global Change Biology+
    
    Climate extremes, such as drought, may have immediate and potentially prolonged effects on carbon cycling. Grasslands store approximately one third of all terrestrial carbon and may become carbon sources during droughts. However, the magnitude and duration of drought-induced disruptions to the carbon cycle, as well as the mechanisms responsible, remain poorly understood. Over the next century, global climate models predict an increase in two types of drought: chronic but subtle “press-droughts”, and shorter-term but extreme “pulse-droughts”. Much of our current understanding of the ecological impacts of drought comes from experimental rainfall manipulations. These studies have been highly valuable, but are often short-term and rarely quantify carbon feedbacks. To address this knowledge gap, we used the Community Land Model 4.0 to examine the individual and interactive effects of pulse- and press-droughts on carbon cycling in a mesic grassland of the US Great Plains. A series of modeling experiments were imposed by varying drought magnitude (precipitation amount) and interannual pattern (press- vs. pulse-droughts) to examine the effects on carbon storage and cycling at annual to century timescales. We present three main findings. First, a single year pulse-drought had immediate and prolonged effects on carbon storage due to differential sensitivities of ecosystem respiration and gross primary production. Second, short-term pulse-droughts caused greater carbon loss than chronic press-droughts when total precipitation reductions over a twenty-year period were equivalent. Third, combining pulse- and press-droughts had intermediate effects on carbon loss compared to the independent drought types, except at high drought levels. Overall, these results suggest that interannual drought pattern may be as important for carbon dynamics as drought magnitude, and that extreme droughts may have long-lasting carbon feedbacks in grassland ecosystems. This article is protected by copyright. All rights reserved.
26. Mingjie Shi, Joshua B. Fisher, Edward R. Brzostek, Richard P. Phillips: Carbon cost of plant nitrogen acquisition: global carbon cycle impact from an improved plant nitrogen cycle in the Community Land Model 2016-01-06T06:58:46.125704-05:00 Global Change Biology+
    
    Plants typically expend a significant portion of their available carbon (C) on nutrient acquisition – C that could otherwise support growth. However, given that most global terrestrial biosphere models (TBMs) do not include the C cost of nutrient acquisition, these models fail to represent current and future constraints to the land C sink. Here, we integrated a plant productivity-optimized nutrient acquisition model – the Fixation and Uptake of Nitrogen Model – into one of the most widely used TBMs, the Community Land Model. Global plant nitrogen (N) uptake is dynamically simulated in the coupled model based on the C costs of N acquisition from mycorrhizal roots, nonmycorrhizal roots, N-fixing microbes, and retranslocation (from senescing leaves). We find that at the global scale, plants spend 2.4 Pg C yr−1 to acquire 1.0 Pg N yr−1, and that the C cost of N acquisition leads to a downregulation of global net primary production (NPP) by 13%. Mycorrhizal uptake represented the dominant pathway by which N is acquired, accounting for ~66% of the N uptake by plants. Notably, roots associating with arbuscular mycorrhizal (AM) fungi – generally considered for their role in phosphorus (P) acquisition – are estimated to be the primary source of global plant N uptake owing to the dominance of AM-associated plants in mid- and low-latitude biomes. Overall, our coupled model improves the representations of NPP downregulation globally and generates spatially explicit patterns of belowground C allocation, soil N uptake, and N retranslocation at the global scale. Such model improvements are critical for predicting how plant responses to altered N availability (owing to N deposition, rising atmospheric CO2, and warming temperatures) may impact the land C sink.
27. Diego Abalos, Shannon E. Brown, Andrew C. Vanderzaag, Robert J. Gordon, Kari E. Dunfield, Claudia Wagner-Riddle: Micrometeorological measurements over 3 years reveal differences in N2O emissions between annual and perennial crops 2016-01-06T06:57:05.878381-05:00 Global Change Biology+
    
    Perennial crops can deliver a wide range of ecosystem services compared to annual crops. Some of these benefits are achieved by lengthening the growing season, which increases the period of crop water and nutrient uptake, pointing to a potential role for perennial systems to mitigate soil nitrous oxide (N2O) emissions. Employing a micrometeorological method, we tested this hypothesis in a 3-year field experiment with a perennial grass-legume mixture and an annual corn monoculture. Given that N2O emissions are strongly dependent on the method of fertilizer application, two manure application options commonly used by farmers for each crop were studied: injection vs. broadcast application for the perennial; fall vs. spring application for the annual. Across the 3 years, lower N2O emissions (P < 0.001) were measured for the perennial compared to the annual crop, even though annual N2O emissions increased tenfold for the perennial after ploughing. The percentage of N2O lost per unit of fertilizer applied was 3.7, 3.1 and 1.3 times higher for the annual for each consecutive year. Differences in soil organic matter due to the contrasting root systems of these crops are probably a major factor behind the N2O reduction. We found that a specific manure management practice can lead to increases or reductions in annual N2O emissions depending on environmental variables. The number of freeze-thaw cycles during winter and the amount of rainfall after fertilization in spring were key factors. Therefore, general manure management recommendations should be avoided because interannual weather variability has the potential to determine if a specific practice is beneficial or detrimental. The lower N2O emissions of perennial crops deserve further research attention and must be considered in future land-use decisions. Increasing the proportion of perennial crops in agricultural landscapes may provide an overlooked opportunity to regulate N2O emissions.
28. Pete Smith: Soil carbon sequestration and biochar as negative emission technologies 2016-01-06T01:12:40.397463-05:00 Global Change Biology+
    
    Despite 20 years of effort to curb emissions, greenhouse gas (GHG) emissions grew faster during the 2000s than in the 1990s, which presents a major challenge for meeting the international goal of limiting warming to <2 °C relative to the preindustrial era. Most recent scenarios from integrated assessment models require large-scale deployment of negative emissions technologies (NETs) to reach the 2 °C target. A recent analysis of NETs, including direct air capture, enhanced weathering, bioenergy with carbon capture and storage and afforestation/deforestation, showed that all NETs have significant limits to implementation, including economic cost, energy requirements, land use, and water use. In this paper, I assess the potential for negative emissions from soil carbon sequestration and biochar addition to land, and also the potential global impacts on land use, water, nutrients, albedo, energy and cost. Results indicate that soil carbon sequestration and biochar have useful negative emission potential (each 0.7 GtCeq. yr−1) and that they potentially have lower impact on land, water use, nutrients, albedo, energy requirement and cost, so have fewer disadvantages than many NETs. Limitations of soil carbon sequestration as a NET centre around issues of sink saturation and reversibility. Biochar could be implemented in combination with bioenergy with carbon capture and storage. Current integrated assessment models do not represent soil carbon sequestration or biochar. Given the negative emission potential of SCS and biochar and their potential advantages compared to other NETs, efforts should be made to include these options within IAMs, so that their potential can be explored further in comparison with other NETs for climate stabilization.
29. Monica B. Emelko, Micheal Stone, Uldis Silins, Don Allin, Adrian L. Collins, Chris H. S. Williams, Amanda M. Martens, Kevin D. Bladon: Sediment-phosphorus dynamics can shift aquatic ecology and cause downstream legacy effects after wildfire in large river systems 2015-12-26T08:54:42.793053-05:00 Global Change Biology+
    
    Global increases in the occurrence of large, severe wildfires in forested watersheds threaten drinking water supplies and aquatic ecology. Wildfire effects on water quality, particularly nutrient levels and forms, can be significant. The longevity and downstream propagation of these effects as well as the geochemical mechanisms regulating them remain largely undocumented at larger river basin scales. Here, phosphorus (P) speciation and sorption behavior of suspended sediment were examined in two river basins impacted by a severe wildfire in southern Alberta, Canada. Fine-grained suspended sediments (<125 μm) were sampled continuously during ice-free conditions over a two-year period (2009–2010), 6 and 7 years after the wildfire. Suspended sediment samples were collected from upstream reference (unburned) river reaches, multiple tributaries within the burned areas, and from reaches downstream of the burned areas, in the Crowsnest and Castle River basins. Total particulate phosphorus (TPP) and particulate phosphorus forms (nonapatite inorganic P, apatite P, organic P), and the equilibrium phosphorus concentration (EPC0) of suspended sediment were assessed. Concentrations of TPP and the EPC0 were significantly higher downstream of wildfire-impacted areas compared to reference (unburned) upstream river reaches. Sediments from the burned tributary inputs contained higher levels of bioavailable particulate P (NAIP) – these effects were also observed downstream at larger river basin scales. The release of bioavailable P from postfire, P-enriched fine sediment is a key mechanism causing these effects in gravel-bed rivers at larger basin scales. Wildfire-associated increases in NAIP and the EPC0 persisted 6 and 7 years after wildfire. Accordingly, this work demonstrated that fine sediment in gravel-bed rivers is a significant, long-term source of in-stream bioavailable P that contributes to a legacy of wildfire impacts on downstream water quality, aquatic ecology, and drinking water treatability.
30. David K. A. Barnes, Louise Ireland, Oliver T. Hogg, Simon Morley, Peter Enderlein, Chester J. Sands: Why is the South Orkney Island shelf (the world's first high seas marine protected area) a carbon immobilization hotspot? 2015-12-18T05:46:35.670403-05:00 Global Change Biology+
    
    The Southern Ocean archipelago, the South Orkney Islands (SOI), became the world's first entirely high seas marine protected area (MPA) in 2010. The SOI continental shelf (~44 000 km2), was less than half covered by grounded ice sheet during glaciations, is biologically rich and a key area of both sea surface warming and sea-ice losses. Little was known of the carbon cycle there, but recent work showed it was a very important site of carbon immobilization (net annual carbon accumulation) by benthos, one of the few demonstrable negative feedbacks to climate change. Carbon immobilization by SOI bryozoans was higher, per species, unit area and ice-free day, than anywhere-else polar. Here, we investigate why carbon immobilization has been so high at SOI, and whether this is due to high density, longevity or high annual production in six study species of bryozoans (benthic suspension feeders). We compared benthic carbon immobilization across major regions around West Antarctica with sea-ice and primary production, from remotely sensed and directly sampled sources. Lowest carbon immobilization was at the northernmost study regions (South Georgia) and southernmost Amundsen Sea. However, data standardized for age and density showed that only SOI was anomalous (high). High immobilization at SOI was due to very high annual production of bryozoans (rather than high densities or longevity), which were 2x, 3x and 5x higher than on the Bellingshausen, South Georgia and Amundsen shelves, respectively. We found that carbon immobilization correlated to the duration (but not peak or integrated biomass) of phytoplankton blooms, both in directly sampled, local scale data and across regions using remote-sensed data. The long bloom at SOI seems to drive considerable carbon immobilization, but sea-ice losses across West Antarctica mean that significant carbon sinks and negative feedbacks to climate change could also develop in the Bellingshausen and Amundsen seas.
31. William H. Schlesinger, Michael C. Dietze, Robert B. Jackson, Richard P. Phillips, Charles C. Rhoades, Lindsey E. Rustad, James M. Vose: Forest biogeochemistry in response to drought 2015-11-18T06:22:11.226599-05:00 Global Change Biology+
    
    Trees alter their use and allocation of nutrients in response to drought, and changes in soil nutrient cycling and trace gas flux (N2O and CH4) are observed when experimental drought is imposed on forests. In extreme droughts, trees are increasingly susceptible to attack by pests and pathogens, which can lead to major changes in nutrient flux to the soil. Extreme droughts often lead to more common and more intense forest fires, causing dramatic changes in the nutrient storage and loss from forest ecosystems. Changes in the future manifestation of drought will affect carbon uptake and storage in forests, leading to feedbacks to the Earth's climate system. We must improve the recognition of drought in nature, our ability to manage our forests in the face of drought, and the parameterization of drought in earth system models for improved predictions of carbon uptake and storage in the world's forests.
32. Clayton J. Williams, Paul C. Frost, Ana M. Morales-Williams, James H. Larson, William B. Richardson, Aisha S. Chiandet, Marguerite A. Xenopoulos: Human activities cause distinct dissolved organic matter composition across freshwater ecosystems 2015-12-11T05:32:06.211935-05:00 Global Change Biology+
    
    Dissolved organic matter (DOM) composition in freshwater ecosystems is influenced by the interactions among physical, chemical, and biological processes that are controlled, at one level, by watershed landscape, hydrology, and their connections. Against this environmental template, humans may strongly influence DOM composition. Yet, we lack a comprehensive understanding of DOM composition variation across freshwater ecosystems differentially affected by human activity. Using optical properties, we described DOM variation across five ecosystem groups of the Laurentian Great Lakes region: large lakes, Kawartha Lakes, Experimental Lakes Area, urban stormwater ponds, and rivers (n = 184 sites). We determined how between ecosystem variation in DOM composition related to watershed size, land use and cover, water quality measures (conductivity, dissolved organic carbon (DOC), nutrient concentration, chlorophyll a), and human population density. The five freshwater ecosystem groups had distinctive DOM composition from each other. These significant differences were not explained completely through differences in watershed size nor spatial autocorrelation. Instead, multivariate partial least squares regression showed that DOM composition was related to differences in human impact across freshwater ecosystems. In particular, urban/developed watersheds with higher human population densities had a unique DOM composition with a clear anthropogenic influence that was distinct from DOM composition in natural land cover and/or agricultural watersheds. This nonagricultural, human developed impact on aquatic DOM was most evident through increased levels of a microbial, humic-like parallel factor analysis component (C6). Lotic and lentic ecosystems with low human population densities had DOM compositions more typical of clear water to humic-rich freshwater ecosystems but C6 was only present at trace to background levels. Consequently, humans are strongly altering the quality of DOM in waters nearby or flowing through highly populated areas, which may alter carbon cycles in anthropogenically disturbed ecosystems at broad scales.
33. Ranae Dietzel, Matt Liebman, Robert Ewing, Matt Helmers, Robert Horton, Meghann Jarchow, Sotirios Archontoulis: How efficiently do corn- and soybean-based cropping systems use water? A systems modeling analysis 2015-11-20T00:44:41.061209-05:00 Global Change Biology+
    
    Agricultural systems are being challenged to decrease water use and increase production while climate becomes more variable and the world's population grows. Low water use efficiency is traditionally characterized by high water use relative to low grain production and usually occurs under dry conditions. However, when a cropping system fails to take advantage of available water during wet conditions, this is also an inefficiency and is often detrimental to the environment. Here, we provide a systems-level definition of water use efficiency (sWUE) that addresses both production and environmental quality goals through incorporating all major system water losses (evapotranspiration, drainage, and runoff). We extensively calibrated and tested the Agricultural Production Systems sIMulator (APSIM) using 6 years of continuous crop and soil measurements in corn- and soybean-based cropping systems in central Iowa, USA. We then used the model to determine water use, loss, and grain production in each system and calculated sWUE in years that experienced drought, flood, or historically average precipitation. Systems water use efficiency was found to be greatest during years with average precipitation. Simulation analysis using 28 years of historical precipitation data, plus the same dataset with ± 15% variation in daily precipitation, showed that in this region, 430 mm of seasonal (planting to harvesting) rainfall resulted in the optimum sWUE for corn, and 317 mm for soybean. Above these precipitation levels, the corn and soybean yields did not increase further, but the water loss from the system via runoff and drainage increased substantially, leading to a high likelihood of soil, nutrient, and pesticide movement from the field to waterways. As the Midwestern United States is predicted to experience more frequent drought and flood, inefficiency of cropping systems water use will also increase. This work provides a framework to concurrently evaluate production and environmental performance of cropping systems.
34. Steven L. Voelker, J. Renée Brooks, Frederick C. Meinzer, Rebecca Anderson, Martin K.-F. Bader, Giovanna Battipaglia, Katie M. Becklin, David Beerling, Didier Bert, Julio L. Betancourt, Todd E. Dawson, Jean-Christophe Domec, Richard P. Guyette, Christian Körner, Steven W. Leavitt, Sune Linder, John D. Marshall, Manuel Mildner, Jérôme Ogée, Irina Panyushkina, Heather J. Plumpton, Kurt S. Pregitzer, Matthias Saurer, Andrew R. Smith, Rolf T. W. Siegwolf, Michael C. Stambaugh, Alan F. Talhelm, Jacques C. Tardif, Peter K. Van de Water, Joy K. Ward, Lisa Wingate: A dynamic leaf gas-exchange strategy is conserved in woody plants under changing ambient CO2: evidence from carbon isotope discrimination in paleo and CO2 enrichment studies 2016-01-04T05:43:43.372364-05:00 Global Change Biology+
    
    Rising atmospheric [CO2], ca, is expected to affect stomatal regulation of leaf gas-exchange of woody plants, thus influencing energy fluxes as well as carbon (C), water, and nutrient cycling of forests. Researchers have proposed various strategies for stomatal regulation of leaf gas-exchange that include maintaining a constant leaf internal [CO2], ci, a constant drawdown in CO2 (ca − ci), and a constant ci/ca. These strategies can result in drastically different consequences for leaf gas-exchange. The accuracy of Earth systems models depends in part on assumptions about generalizable patterns in leaf gas-exchange responses to varying ca. The concept of optimal stomatal behavior, exemplified by woody plants shifting along a continuum of these strategies, provides a unifying framework for understanding leaf gas-exchange responses to ca. To assess leaf gas-exchange regulation strategies, we analyzed patterns in ci inferred from studies reporting C stable isotope ratios (δ13C) or photosynthetic discrimination (∆) in woody angiosperms and gymnosperms that grew across a range of ca spanning at least 100 ppm. Our results suggest that much of the ca-induced changes in ci/ca occurred across ca spanning 200 to 400 ppm. These patterns imply that ca − ci will eventually approach a constant level at high ca because assimilation rates will reach a maximum and stomatal conductance of each species should be constrained to some minimum level. These analyses are not consistent with canalization toward any single strategy, particularly maintaining a constant ci. Rather, the results are consistent with the existence of a broadly conserved pattern of stomatal optimization in woody angiosperms and gymnosperms. This results in trees being profligate water users at low ca, when additional water loss is small for each unit of C gain, and increasingly water-conservative at high ca, when photosystems are saturated and water loss is large for each unit C gain.
35. Björn Gücker, Ricky C. S. Silva, Daniel Graeber, José A. F. Monteiro, E. N. Jack Brookshire, Ronaldo C. Chaves, Iola G. Boëchat: Dissolved nutrient exports from natural and human-impacted Neotropical catchments 2016-01-14T22:20:18.743306-05:00 Global Ecology and Biogeography+
    
    Aim Neotropical biomes are highly threatened by land-use changes, but the catchment-wide biogeochemical effects are poorly understood. Here, we aim to compare exports of dissolved nitrogen (N) and phosphorus (P) from natural and human-impacted catchments in the Neotropics. Location Neotropics. Methods We measured streamwater nutrient concentrations and exports in 20 south-eastern Brazilian catchments with different land uses (natural Cerrado/semi-deciduous forest, pasture, intensive agriculture and urban areas) and conducted a meta-analysis on nutrient exports from Neotropical catchments, both natural and human-impacted. Results Organic forms dominated dissolved nutrient exports in central/south-east Brazil in both natural and human-dominated catchments. Our meta-analysis suggests that there is wide geographic variability in the natural dominance of organic versus inorganic nutrient exports across the Neotropics, and for N a tendency for inorganic and organic forms to vary inversely across sites. We found strong land-use effects, especially in urban areas. In areas naturally dominated by organic N exports, land use did not overturn that dominance, but rather increased the concentration of both inorganic and organic N. In catchments dominated by inorganic N exports or showing equivalent organic and inorganic exports, land use also caused organic exports to be dominant. Land-use effects on P were unclear, probably owing to a paucity of available data, but our results from south-east Brazil suggested that land use also increases dissolved P exports. Main conclusions We show that dominance of inorganic versus organic nutrient exports is highly variable across natural Neotropical catchments, a pattern contrasting with the general tendency for dominance of organic N in natural temperate catchments. Further, we found a general dominance of organic N in most human-impacted catchments, which is in contrast to strong dominance of inorganic N in most human-impacted temperate catchments. Finally, we find that urbanization exerts the strongest impacts on nutrient exports, thus underscoring wastewater treatment as a critical management priority under future land-use change.
36. Yan Sun, Shilong Piao, Mengtian Huang, Philippe Ciais, Zhenzhong Zeng, Lei Cheng, Xiran Li, Xinping Zhang, Jiafu Mao, Shushi Peng, Benjamin Poulter, Xiaoying Shi, Xuhui Wang, Ying-Ping Wang, Hui Zeng: Global patterns and climate drivers of water-use efficiency in terrestrial ecosystems deduced from satellite-based datasets and carbon cycle models 2015-12-23T01:40:12.848158-05:00 Global Ecology and Biogeography+
    
    Aim To investigate how ecosystem water-use efficiency (WUE) varies spatially under different climate conditions, and how spatial variations in WUE differ from those of transpiration-based water-use efficiency (WUEt) and transpiration-based inherent water-use efficiency (IWUEt). Location Global terrestrial ecosystems. Methods We investigated spatial patterns of WUE using two datasets of gross primary productivity (GPP) and evapotranspiration (ET) and four biosphere model estimates of GPP and ET. Spatial relationships between WUE and climate variables were further explored through regression analyses. Results Global WUE estimated by two satellite-based datasets is 1.9 ± 0.1 and 1.8 ± 0.6 g C m−2 mm−1 lower than the simulations from four process-based models (2.0 ± 0.3 g C m−2 mm−1) but comparable within the uncertainty of both approaches. In both satellite-based datasets and process models, precipitation is more strongly associated with spatial gradients of WUE for temperate and tropical regions, but temperature dominates north of 50° N. WUE also increases with increasing solar radiation at high latitudes. The values of WUE from datasets and process-based models are systematically higher in wet regions (with higher GPP) than in dry regions. WUEt shows a lower precipitation sensitivity than WUE, which is contrary to leaf- and plant-level observations. IWUEt, the product of WUEt and water vapour deficit, is found to be rather conservative with spatially increasing precipitation, in agreement with leaf- and plant-level measurements. Main conclusions WUE, WUEt and IWUEt produce different spatial relationships with climate variables. In dry ecosystems, water losses from evaporation from bare soil, uncorrelated with productivity, tend to make WUE lower than in wetter regions. Yet canopy conductance is intrinsically efficient in those ecosystems and maintains a higher IWUEt. This suggests that the responses of each component flux of evapotranspiration should be analysed separately when investigating regional gradients in WUE, its temporal variability and its trends.
37. Alison R. Marklein, Joy B. Winbourne, Sara K. Enders, David J. X. Gonzalez, Tiff L. Huysen, Jorge E. Izquierdo, Derrick R. Light, Daniel Liptzin, Kimberley E. Miller, Scott L. Morford, Robert A. Norton, Benjamin Z. Houlton: Mineralization ratios of nitrogen and phosphorus from decomposing litter in temperate versus tropical forests 2015-12-23T01:25:37.085976-05:00 Global Ecology and Biogeography+
    
    Aim Terrestrial ecosystems sequester about 25% of anthropogenic CO2 emissions annually; however, nitrogen (N) and phosphorus (P) limitation of plant productivity and microbial functioning could curtail this key ecosystem service in the future. Our aim is to address variations in nutrient resupply during decomposition – especially whether the N:P ratio of nutrient recycling via mineralization varies within and across diverse forest biomes. Location Global forest ecosystems. Methods We compiled data on in situ litter decomposition experiments (leaf, wood and root) from the primary literature to examine the relationships between net N and P mineralization across temperate versus tropical forests world-wide. We define net nutrient mineralization ratios as the average N:P released from decomposing substrates at a given ecosystem site. Results We show that net N and P mineralization are strongly correlated within biomes, suggesting strong coupling between N and P recycling in forest ecosystems. The net N:P of leaf-litter mineralization is higher in tropical forests than in temperate forests, consistent with latitudinal patterns in foliar and leaf-litter N:P. At the global scale, the N:P of net mineralization tracks, but tends to be lower than that of litter N:P, pointing to preferential P (versus N) mineralization in forest ecosystems. Main conclusions Our results do not support the view that there is a single, globally consistent mineralization N:P ratio. Instead, our results show that the N:P of net mineralization can be predicted by the N:P of litter, offering a method for incorporating P into global-scale models of carbon–nutrient–climate interactions. In addition, these results imply that P is scarce relative to microbial decomposer demands in tropical forests, whereas N and P may be more co-limiting when compared with microbial biomass in the temperate zone.
38. Xiao Liu, Naomi Marcil Levine: Submesoscale frontal dynamics enhances phytoplankton chlorophyll in the North Pacific Subtropical Gyre 2016-01-25T00:42:50.343289-05:00 Geophysical Research Letters+
    
    Subtropical gyres contribute significantly to global ocean productivity. As the climate warms, the strength of these gyres as a biological carbon pump is predicted to diminish due to increased stratification and depleted surface nutrients. We present results suggesting that the impact of submesoscale physics on phytoplankton in the oligotrophic ocean is substantial and may either compensate or exacerbate future changes in carbon cycling. A new statistical tool was developed to quantify surface patchiness from sea surface temperatures. Chlorophyll concentrations in the North Pacific Subtropical Gyre were shown to be enhanced by submesoscale frontal dynamics with an average increase of 38% (max. 83%) during late winter. The magnitude of this enhancement is comparable to the observed decline in chlorophyll due to a warming of ~1.1°C. These results highlight the need for an improved understanding of fine-scale physical variability in order to predict the response of marine ecosystems to projected climate changes.
39. Chengfeng Le, John C. Lehrter, Chuanmin Hu, Daniel R. Obenour: Satellite-based empirical models linking river plume dynamics with hypoxic area and volume 2016-01-20T17:05:24.676583-05:00 Geophysical Research Letters+
    
    Satellite-based empirical models explaining hypoxic area and volume variation were developed for the seasonally hypoxic (O2 < 2 mg L−1) northern Gulf of Mexico adjacent to the Mississippi River. Annual variations in mid-summer hypoxic area and volume were related to MODIS-derived monthly estimates of river plume area (km2) and average, inner shelf chlorophyll a concentration (Chla, mg m−3). River plume area in June was negatively related with mid-summer hypoxic area (km2) and volume (km3), while July inner shelf Chla was positively related to hypoxic area and volume. Multiple regression models using river plume area and Chla as independent variables accounted for most of the variability in hypoxic area (R2 = 0.92) or volume (R2 = 0.89). These models explain more variation in hypoxic area than models using Mississippi River nutrient loads as independent variables. The results here also support a hypothesis that confinement of the river plume to the inner shelf is an important mechanism controlling hypoxia area and volume in this region.
40. Megumi O. Chikamoto, Axel Timmermann, Masakazu Yoshimori, Flavio Lehner, Audine Laurian, Ayako Abe-Ouchi, Anne Mouchet, Fortunat Joos, Christoph C. Raible, Kim M. Cobb: Volcanic eruptions boost tropical Pacific biological productivity 2016-01-15T16:33:21.578357-05:00 Geophysical Research Letters+
    
    Major volcanic eruptions generate widespread ocean cooling, which reduces upper-ocean stratification. This effect has the potential to increase nutrient delivery into the euphotic zone and boost biological productivity. Using externally forced Last Millennium simulations of three climate/Earth System models (MIROC, CESM and LOVECLIM), we test the hypothesis that large volcanic eruptions intensify nutrient-driven export production. It is found that strong volcanic radiative forcing enhances the likelihood of eastern Pacific El Niño-like warming in CESM and LOVECLIM. This leads to an initial reduction of nutrients and export production in the eastern equatorial Pacific. However, this initial response reverses after about three years in association with La Niña cooling. The resulting delayed enhancement of biological production resembles the multiyear response in MIROC. The model simulations show that volcanic impacts on tropical Pacific dynamics and biogeochemistry persist for several years, thus providing a new source for potential multiyear ecosystem predictability.
41. Frédéric Cyr, Hans van Haren, Furu Mienis, Gerard Duineveld, Daniel Bourgault: On the influence of cold-water coral mound size on flow hydrodynamics, and vice-versa 2016-01-08T17:32:39.786414-05:00 Geophysical Research Letters+
    
    Using a combination of in situ observations and idealistic 2D non-hydrostatic numerical simulations, the relation between Cold-Water Coral (CWC) mound size and hydrodynamics is explored for the Rockall Bank area in the North-Atlantic Ocean. It is shown that currents generated by topographically-trapped tidal waves in this area cause large isopycnal depressions resulting from an internal hydraulic control above CWC mounds. The oxygen concentration distribution is used as a tracer to visualize the flow behavior and the turbulent mixing above the mounds. By comparing two CWC mounds of different size and located close to each other, it is shown that the resulting mixing is highly dependent on the size of the mound. The effects of the hydraulic control for mixing, nutrient availability and ecosystem functioning are also discussed.
42. Mathew Koll Roxy, Aditi Modi, Raghu Murtugudde, Vinu Valsala, Swapna Panickal, S. Prasanna Kumar, M. Ravichandran, Marcello Vichi, Marina Lévy: A reduction in marine primary productivity driven by rapid warming over the tropical Indian Ocean 2016-01-19T17:33:32.465356-05:00 Geophysical Research Letters+
    
    Among the tropical oceans, the western Indian Ocean hosts one of the largest concentrations of marine phytoplankton blooms in summer. Interestingly, this is also the region with the largest warming trend in sea surface temperatures in the tropics during the past century—although the contribution of such a large warming to productivity changes has remained ambiguous. Earlier studies had described the western Indian Ocean as a region with the largest increase in phytoplankton during the recent decades. On the contrary, the current study points out an alarming decrease of up to 20% in phytoplankton in this region over the past six decades. We find that these trends in chlorophyll are driven by enhanced ocean stratification due to rapid warming in the Indian Ocean, which suppresses nutrient mixing from subsurface layers. Future climate projections suggest that the Indian Ocean will continue to warm, driving this productive region into an ecological desert.
43. : SWAT Modeling of Water Quantity and Quality in the Tennessee River Basin: Spatiotemporal Calibration and Validation Mon, 25 Jan 2016 01:59:26 +0100 Hydrology and Earth System Sciences+
    
    SWAT Modeling of Water Quantity and Quality in the Tennessee River Basin: Spatiotemporal Calibration and Validation
    Gangsheng Wang, Henriette I. Jager, Latha M. Baskaran, Tyler F. Baker, and Craig C. Brandt
    Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-34,2016
    Manuscript under review for HESS (discussion: open, 0 comments)
    Model-data comparisons are always challenging, especially when working at a large spatial scale and evaluating multiple response variables. We implemented the Soil and Water Assessment Tool (SWAT) to simulate water quantity and quality for the Tennessee River Basin. We developed three innovations to overcome hurdles associated with limited data for model evaluation: 1) we implemented an auto-calibration approach to allow simultaneous calibration against multiple responses, including intermediate response variables, 2) we identified empirical spatiotemporal datasets to use in our comparison, and 3) we compared functional patterns in landuse-nutrient relationships between SWAT and empirical data. Comparing monthly SWAT-simulated runoff against USGS data produced satisfactory median Nash-Sutcliffe Efficiencies of 0.83 and 0.72 for calibration and validation periods, respectively. SWAT-simulated water quality responses (sediment, TP, TN, and inorganic N) reproduced the seasonal patterns found in LOADEST data. SWAT-simulated spatial TN loadings were significantly correlated with empirical SPARROW estimates. The spatial correlation analyses indicated that SWAT-modeled runoff was primarily controlled by precipitation; sedimentation was controlled by topography; and NO₃ and soluble P were highly influenced by land management, particularly the proportion of agricultural lands in a subbasin
44. Yue Hu, YueHan Lu, Jennifer W Edmonds, Chuankun Liu, Sai Wang, Oindrila Das, Jie Liu, Chunmiao Zheng: Hydrological and Land Use Control of Watershed Exports of DOM in a Large Arid River Basin in Northwestern China 2016-01-20T23:35:29.035424-05:00 Journal of Geophysical Research: Biogeosciences+
    
    We evaluated variation in DOM export as a function of hydrology and land use from a large arid river basin in northwestern China. Two soil-derived, humic-like (C1, C2) and three protein-like fluorescence components (C3, C4, C5) were identified. During high discharges, river water DOM had higher values of DOC concentration, percent humic fluorescence, and humification index, but lower values of fluorescence index and percent protein fluorescence than found at base flow, suggesting that flow paths shifted to shallower depths flushing out topsoil OM. Loading of DOC and soil-derived humic fluorescence were driven largely by discharge, with values over 10 times higher during high discharges than at base flow. Furthermore, both δ13C-DOC and C1 at high flows positively correlated with %agricultural lands within 1 km river buffers, demonstrating that near-river agricultural activities enhanced storm export of soil DOM. At base flow, C4 positively correlated with %agricultural lands, showing stimulation of aquatic bacterial carbon production as a result of elevated nutrient inputs from agricultural lands. Percent contributions of humic fluorescence in groundwater varied with well depths in shallow wells but this pattern was not observed for deeper groundwater, suggesting that humic DOM could serve as a water source tracer indicating deeper aquifers were isolated from river water and shallow groundwater. Together, our data demonstrate that hydrology and land use controlled the sources and amount of riverine DOM in this large agricultural basin, and that regulating storm runoff and near-river agricultural activities should be incorporated in ecosystem-based management of water resources.
45. George L Vourlitis, Cloe S. Hentz: Chronic N addition alters the carbon and nitrogen storage of a post-fire Mediterranean-type shrubland 2016-01-14T17:25:03.58515-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Mediterranean-type shublands are subject to periodic fire and high levels of nitrogen (N) deposition, but little is known how chronic N deposition affects carbon (C) and N storage during succession. We conducted a long-term experiment in Californian chaparral to test the hypothesis that chronic N-enrichment would increase post-fire C and N accumulation. The experimental layout consisted of a randomized design where four-10 x 10 m plots received 5 gN m−2 annually since 2003 and four-10 x 10 m plots served as controls. Aboveground and belowground C and N pools and fluxes were measured seasonally (every 3 months) for a period of 10 years. Added N rapidly increased soil extractable N pools and decreased soil pH; however, total soil C and N storage were not affected. Added N plots initially had significantly lower C and N storage than control plots, presumably because of nutrient losses from leaching and/or higher belowground C allocation. However, rates of aboveground N and C storage became significantly higher in added N plots after 4–5 years of exposure, thus increasing fuel buildup, which has implications for future fire intensity. This recovering chaparral stand is not yet “N-saturated” after 10 years of chronic N input. However, N leaching continues to be higher in added N plots, indicating that post-fire chaparral stands in high-N deposition areas can be important sources of N to groundwater/aquatic systems even if productivity is stimulated by N input.
46. Natalie A. Griffiths, C. Rhett Jackson, Jeffrey J. McDonnell, Julian Klaus, Enhao Du, Menberu M. Bitew: Dual nitrate isotopes clarify the role of biological processing and hydrologic flowpaths on nitrogen cycling in subtropical low-gradient watersheds 2016-01-10T22:55:27.641938-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Nitrogen (N) is an important nutrient as it often limits productivity, but in excess can impair water quality. Most studies on watershed N cycling have occurred in upland forested catchments where snowmelt dominates N export; fewer studies have focused on low-relief watersheds that lack snow. We examined watershed N cycling in three adjacent, low-relief watersheds in the Upper Coastal Plain of the southeastern United States to better understand the role of hydrological flowpaths and biological transformations of N at the watershed scale. Groundwater was the dominant source of nitrified N to stream water in 2 of the 3 watersheds, while atmospheric deposition comprised 28% of stream water nitrate in one watershed. The greater atmospheric contribution may have been due to the larger stream channel area relative to total watershed area or the dominance of shallow subsurface flowpaths contributing to stream flow in this watershed. There was a positive relationship between temperature and stream water ammonium concentrations and a negative relationship between temperature and stream water nitrate concentrations in each watershed suggesting that N cycling processes (i.e., nitrification, denitrification) varied seasonally. However, there were no clear patterns in the importance of denitrification in different water pools possibly because a variety of factors (i.e., assimilatory uptake, dissimilatory uptake, mixing) affected nitrate concentrations. Together, these results highlight the hydrological and biological controls on N cycling in low-gradient watersheds, and variability in N delivery flowpaths among adjacent watersheds with similar physical characteristics.
47. Antonio Mannino, Sergio Signorini, Michael Novak, John Wilkin, Marjorie A. M. Friedrichs, Raymond G. Najjar: Dissolved Organic Carbon Fluxes in the Middle Atlantic Bight: An integrated approach based on satellite data and ocean model products 2015-12-17T19:41:07.851612-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Continental margins play an important role in global carbon cycle, accounting for 15-21% of the global marine primary production. Since carbon fluxes across continental margins from land to the open ocean are not well constrained, we undertook a study to develop satellite algorithms to retrieve dissolved organic carbon (DOC) and combined these satellite data with physical circulation model products to quantify the shelf boundary fluxes of DOC for the U.S. Middle Atlantic Bight (MAB). Satellite DOC was computed through seasonal relationships of DOC with colored dissolved organic matter absorption coefficients, which were derived from an extensive set of in situ measurements. The multi-year time series of satellite-derived DOC stocks (4.9 Teragrams C; Tg) shows that freshwater discharge influences the magnitude and seasonal variability of DOC on the continental shelf. For the 2010–2012 period studied, the average total estuarine export of DOC into the MAB shelf is 0.77 Tg C yr−1 (year). The integrated DOC tracer fluxes across the shelf boundaries are 12.1 Tg C yr−1 entering the MAB from the southwest along-shore boundary, 18.5 Tg C yr−1 entering the MAB from the northeast along-shore boundary, and 29.0 Tg C yr−1 flowing out of the MAB across the entire length of the 100-m isobath. The magnitude of the cross-shelf DOC flux is quite variable in time (monthly) and space (north-to-south). The highly dynamic exchange of water along the shelf boundaries regulates the DOC budget of the MAB at sub-seasonal time scales.
48. E. Carmack, M. Yamamoto-Kawai, T. Haine, S. Bacon, B. Bluhm, C. Lique, H. Melling, I. Polyakov, F. Straneo, M.-L. Timmermans, W. Williams: Fresh water and its role in the Arctic Marine System: sources, disposition, storage, export, and physical and biogeochemical consequences in the Arctic and global oceans 2015-10-11T23:31:02.78218-05:00 Journal of Geophysical Research: Biogeosciences+
    
    The Arctic Ocean is a fundamental node in the global hydrological cycle and the ocean's thermohaline circulation. We here assess the system's key functions and processes: 1) the delivery of fresh and low salinity waters to the Arctic Ocean by river inflow, net precipitation, distillation during the freeze/thaw cycle and Pacific Ocean inflows; 2) the disposition (e.g. sources, pathways and storage) of freshwater components within the Arctic Ocean; and 3) the release and export of freshwater components into the bordering convective domains of the North Atlantic. We then examine physical, chemical or biological processes which are influenced or constrained by the local quantities and geochemical qualities of fresh water; these include: stratification and vertical mixing, ocean heat flux, nutrient supply, primary production, ocean acidification and biogeochemical cycling. Internal to the Arctic the joint effects of sea ice decline and hydrological cycle intensification have strengthened coupling between the ocean and the atmosphere (e.g. wind and ice-drift stresses, solar radiation, heat and moisture exchange), the bordering drainage basins (e.g. river discharge, sediment transport, erosion) and terrestrial ecosystems (e.g. Arctic greening, dissolved and particulate carbon loading, altered phenology of biotic components). External to the Arctic freshwater export acts as both a constraint to and a necessary ingredient for deep convection in the bordering subarctic gyres and thus affects the global thermohaline circulation. Geochemical fingerprints attained within the Arctic Ocean are likewise exported into the neighboring subarctic systems and beyond. Finally, we discuss observed and modelled functions and changes in this system on seasonal, annual and decadal time scales, and discuss mechanisms that link the marine system to atmospheric, terrestrial and cryospheric systems.
49. Yuanyuan Huang, Stefan Gerber: Nitrogen restrictions buffer modeled interactions of water with the carbon cycle 2016-01-23T13:58:26.084883-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Terrestrial carbon and water cycles are coupled at multiple spatiotemporal scales and are crucial to carbon sequestration. Water related climate extremes, such as drought and intense precipitation, can substantially affect the carbon cycle. Meanwhile, nitrogen is a limiting resource to plant and has therefore the potential to alter the coupling of water and carbon cycles on land. Here we assess the effect of nitrogen limitation on the response of the terrestrial carbon cycle to moisture anomalies using Geophysical Fluid Dynamics Laboratory's land surface model LM3V-N. We analyzed the response of three central carbon fluxes: net primary productivity (NPP), heterotrophic respiration (Rh), and net ecosystem productivity (NEP, the difference between NPP and Rh) and how these fluxes were altered under anomalies of the standardized precipitation and evapotranspiration index (SPEI). We found that globally, the correlations between each of the carbon flux and SPEI depended on the timescale and a strong legacy effect of SPEI anomalies on Rh. Consideration of nitrogen constraints reduced anomalies in carbon fluxes in response to extreme dry/wet events. This nitrogen-induced buffer constrained the growth of plants under wet extremes and allowed for enhanced growth during droughts. Extra gain of soil moisture from the downregulation of canopy transpiration by nitrogen limitation and shifts in the relative importance of water and nitrogen limitation during dry/wet extreme events are possible mechanisms contributing to the buffering of modeled NPP and NEP. Responses of Rh to moisture anomalies were much weaker compared to NPP, and N buffering effects were less evident.
50. Bianca Rodríguez-Cardona, Adam S. Wymore, William H. McDowell: DOC:NO3− ratios and NO3− uptake in forested headwater streams 2016-01-21T18:48:13.215348-05:00 Journal of Geophysical Research: Biogeosciences+
    
    The underlying mechanisms driving the coupled interactions between inorganic nitrogen uptake and dissolved organic matter are not well understood, particularly in surface waters. To determine the relationship between dissolved organic carbon (DOC) quantity and nitrate (NO3−) uptake kinetics in streams, we performed a series of NO3− Tracer Additions for Spiraling Curve Characterization experiments in four streams within the Lamprey River Watershed, New Hampshire, across a range in background DOC concentrations (1–8 mg C/L). Experiments were performed throughout the 2013 and 2014 growing seasons. Across streams and experimental dates, ambient uptake velocity (Vf) correlated positively with increasing DOC concentrations and DOC:NO3− ratios but was only weakly negatively associated with NO3− concentrations. Ambient NO3− Vf was unrelated to pH, light, temperature, dissolved oxygen, and Specific Ultraviolet Absorbance at 254 nm. Although there were general tendencies across the entire Lamprey River Watershed, individual sites behaved differently in their uptake kinetics. NO3− uptake dynamics in the Lamprey River Watershed are most strongly influenced by DOC concentrations rather than NO3− concentrations or physicochemical parameters, which have been identified as regional- to continental-scale drivers in previous research. Understanding the fundamental relationships between dissolved organic matter and inorganic nutrients will be important as global and climatic changes influence the delivery and production of DOC and NO3− in aquatic ecosystems.
51. G. V. M. Gupta, V. Sudheesh, K. V. Sudharma, N. Saravanane, V. Dhanya, K. R. Dhanya, G. Lakshmi, M. Sudhakar, S. W. A. Naqvi: Evolution to decay of upwelling and associated biogeochemistry over the southeastern Arabian Sea shelf 2016-01-16T22:22:55.226984-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Observations along 10 shelf transects in 2012 near 10°N in the southeastern Arabian Sea revealed the usual warm oligotrophic conditions during the winter monsoon and upwelling of oxygen-deficient, nutrient-rich cool water during the summer monsoon (SM). By changing an oligotrophic to a nutrient-replete condition, the upwelling is the major process that regulates the biogeochemistry of this shelf. Its onset is perceptible at 100 m depth between January and March. The upwelling reaches the surface layer in May and intensifies during June–July but withdraws completely and abruptly by October. Despite the nutrient injection, the primary production during SM, integrated for euphotic zone, is comparable to that during the preceding spring intermonsoon (SIM). Again, as usual, the high oxygen demand coupled with low concentration in the upwelled subsurface waters causes severe oxygen depletion below the shallow pycnocline. The oxygen concentrations/saturations of 2012 on the midshelf are similar from those of mid-1958 to early 1960, except for marginally higher values during the peak upwelling period due to relatively weak upwelling in 2012. This implies little anthropogenic influence on coastal hypoxia unlike many other coastal regions. In 2012, the inner shelf system shifted from net autotrophy in SIM to net heterotrophy in SM but on an annual basis it was net autotrophic (gross primary production to community respiration ratio, GPP/R:1.11 ± 0.84) as organic production exceeded consumption.
52. Peng Wu, Rong Bi, Shanshan Duan, Haiyan Jin, Jianfang Chen, Qiang Hao, Yuming Cai, Xinyan Mao, Meixun Zhao: Spatiotemporal variations of phytoplankton in the East China Sea and the Yellow Sea revealed by lipid biomarkers 2016-01-14T16:13:14.313781-05:00 Journal of Geophysical Research: Biogeosciences+
    
    The East China Sea (ECS) and the Southern Yellow Sea (SYS) ecosystem is undergoing dramatic changes, but the spatiotemporal patterns and forcing mechanisms of phytoplankton variations remain understudied. Phytoplankton lipid biomarkers are useful proxies for productivity and community structure changes, and they were measured in suspended particles of more than 81 sites from spring and summer of 2011 in the ECS and SYS. In spring, the concentrations of brassicasterol (4.7–127 ng L−1) and dinosterol (0.7–37 ng L−1) were markedly higher in the northern and central SYS, while C37 alkenones (0–15 ng L−1) were detected at only seven sites in the ECS. In summer, brassicasterol (25.3–1178 ng L−1) and dinosterol (0–125 ng L−1) showed high values off the Changjiang River Estuary (CRE), while C37 alkenones (0–410 ng L−1) had high values in the northwest and central SYS. The mean concentrations of the three lipid biomarkers in summer were 3 to 61 times higher than those in spring. Spatiotemporal patterns of biomarkers reveal higher ratios of diatom/dinoflagellate and diatom/haptophyte in higher productivity areas, off the CRE in summer and the northern and central SYS in spring. This study validates the applicability of brassicasterol, dinosterol, and alkenones as proxies of productivity and community structure of the three phytoplankton taxa: diatoms, dinoflagellates, and haptophytes. The results indicate that nutrients (in summer) and turbidity-induced photosynthetic available radiation (in spring) play important roles in regulating spatiotemporal variations of phytoplankton in the ECS and SYS.
53. Marina Frants, Mark Holzer, Timothy DeVries, Richard Matear: Constraints on the global marine iron cycle from a simple inverse model 2016-01-09T04:24:05.249239-05:00 Journal of Geophysical Research: Biogeosciences+
    
    A simple model of the global marine iron cycle is used to constrain the sources, sinks, and biological cycling of iron. The iron model is embedded in a data-assimilated steady state circulation, with biological cycling driven by a prescribed, data-constrained phosphate cycle. Biogeochemical parameters are determined by minimizing a suitably weighted quadratic mismatch with available dissolved iron (dFe) observations, including GEOTRACES transects. Because the effective iron sources and sinks overlap, current dFe observations cannot constrain sources and sinks independently. We therefore determine a family of optimal solutions for a range of the aeolian source strength σA from 0.3 to 6.1 Gmol/yr. We find that the dFe observations constrain the maximum Fe:P uptake ratio R0 to be proportional to σA, with a range that spans most available measurements. Thus, with either R0 or σA specified, a unique solution is determined. Global inventories of total and free iron are well constrained at (7.4 ± 0.2) × 1011 and (1.39 ± 0.05) × 1010 mol Fe, respectively. The dFe distributions are very similar across the family of solutions, with iron limitation in the known high-nutrient low-chlorophyll regions. Hydrothermal source strength ranges from 0.57 to 0.73 Gmol/yr and does not vary systematically with σA suggesting that the hydrothermal and aeolian parts of the iron cycle are largely decoupled. The hydrothermal dFe anomaly in the euphotic zone is ∼10% and concentrated in subpolar regions of iron limitation. Enhanced ligand concentrations in old waters and in hydrothermal plumes are necessary to capture key features of the dFe observations.
54. J. F. Tjiputra, A. Grini, H. Lee: Impact of idealized future stratospheric aerosol injection on the large-scale ocean and land carbon cycles 2016-01-06T15:17:28.890712-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Using an Earth system model, we simulate stratospheric aerosol injection (SAI) on top of the Representative Concentration Pathways 8.5 future scenario. Our idealized method prescribes aerosol concentration, linearly increasing from 2020 to 2100, and thereafter remaining constant until 2200. In the aggressive scenario, the model projects a cooling trend toward 2100 despite warming that persists in the high latitudes. Following SAI termination in 2100, a rapid global warming of 0.35 K yr−1 is simulated in the subsequent 10 years, and the global mean temperature returns to levels close to the reference state, though roughly 0.5 K cooler. In contrast to earlier findings, we show a weak response in the terrestrial carbon sink during SAI implementation in the 21st century, which we attribute to nitrogen limitation. The SAI increases the land carbon uptake in the temperate forest-, grassland-, and shrub-dominated regions. The resultant lower temperatures lead to a reduction in the heterotrophic respiration rate and increase soil carbon retention. Changes in precipitation patterns are key drivers for variability in vegetation carbon. Upon SAI termination, the level of vegetation carbon storage returns to the reference case, whereas the soil carbon remains high. The ocean absorbs nearly 10% more carbon in the geoengineered simulation than in the reference simulation, leading to a ∼15 ppm lower atmospheric CO2 concentration in 2100. The largest enhancement in uptake occurs in the North Atlantic. In both hemispheres' polar regions, SAI delays the sea ice melting and, consequently, export production remains low. In the deep water of North Atlantic, SAI-induced circulation changes accelerate the ocean acidification rate and broaden the affected area.
55. Tao Zhou, Peijun Shi, Gensuo Jia, Yongjiu Dai, Xiang Zhao, Wei Shangguan, Ling Du, Hao Wu, Yiqi Luo: Age-dependent forest carbon sink: Estimation via inverse modeling 2015-12-02T17:15:34.414507-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Forests have been recognized to sequester a substantial amount of carbon (C) from the atmosphere. However, considerable uncertainty remains regarding the magnitude and time course of the C sink. Revealing the intrinsic relationship between forest age and C sink is crucial for reducing uncertainties in prediction of forest C sink potential. In this study, we developed a stepwise data assimilation approach to combine a process-based Terrestrial ECOsystem Regional model, observations from multiple sources, and stochastic sampling to inversely estimate carbon cycle parameters including carbon sink at different forest ages for evergreen needle-leaved forests in China. The new approach is effective to estimate age-dependent parameter of maximal light-use efficiency (R2 = 0.99) and, accordingly, can quantify a relationship between forest age and the vegetation and soil C sinks. The estimated ecosystem C sink increases rapidly with age, peaks at 0.451 kg C m−2 yr−1 at age 22 years (ranging from 0.421 to 0.465 kg C m−2 yr−1), and gradually decreases thereafter. The dynamic patterns of C sinks in vegetation and soil are significantly different. C sink in vegetation first increases rapidly with age and then decreases. C sink in soil, however, increases continuously with age; it acts as a C source when the age is less than 20 years, after which it acts as a sink. For the evergreen needle-leaved forest, the highest C sink efficiency (i.e., C sink per unit net primary productivity) is approximately 60%, with age between 11 and 43 years. Overall, the inverse estimation of carbon cycle parameters can make reasonable estimates of age-dependent C sequestration in forests.
56. Karen L. Vandecar, Christiane W. Runyan, Paolo D'Odorico, Deborah Lawrence, Birgit Schmook, Rishiraj Das: Phosphorus input through fog deposition in a dry tropical forest 2015-12-02T17:32:46.942272-05:00 Journal of Geophysical Research: Biogeosciences+
    
    In many tropical forests, where phosphorus (P) is considered a limiting nutrient, atmospheric deposition can contribute significantly to available P. Previous studies have shown that P inputs from atmospheric deposition are enhanced by plant canopies. This effect is explained as the result of increased deposition of P-rich aerosol particles (dry deposition) and fog droplets (fog or “occult” deposition) onto leaf surfaces. Here we studied the importance of fog as a source of P to a P-limited dry tropical forest. Throughout an 80 day period during the dry season when fog is most common, we sampled fog water and bulk precipitation in a clearing and measured leaf wetness and throughfall in an adjacent secondary and mature forest stand. During the study period, total P (PT) concentrations in fog water ranged from 0.15 to 6.40 mg/L, on average fourteenfold greater than PT concentrations in bulk precipitation (0.011 to 0.451 mg/L), and sixfold and sevenfold greater than throughfall PT concentrations in the secondary and mature forest stands, respectively (0.007 to 1.319 mg/L; 0.009 to 0.443 mg/L). Based on leaf area index, the frequency of fog deposition, and amount of water deposited per fog event, we estimate that fog delivers a maximum of 1.01 kg/ha/yr to secondary forest stands and 1.75 kg/ha/yr to mature forest stands, compared to 0.88 kg/ha/yr to secondary forest stands and 1.98 kg/ha/yr to mature forest stands via throughfall (wet + dry deposition) and stemflow. Thus, fog deposition may contribute substantially to available P in tropical dry forests.
57. Yujie He, Jinyan Yang, Qianlai Zhuang, Jennifer W. Harden, Anthony D. McGuire, Yaling Liu, Gangsheng Wang, Lianhong Gu: Incorporating microbial dormancy dynamics into soil decomposition models to improve quantification of soil carbon dynamics of northern temperate forests 2015-12-22T14:43:34.594535-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Soil carbon dynamics of terrestrial ecosystems play a significant role in the global carbon cycle. Microbial-based decomposition models have seen much growth recently for quantifying this role, yet dormancy as a common strategy used by microorganisms has not usually been represented and tested in these models against field observations. Here we developed an explicit microbial-enzyme decomposition model and examined model performance with and without representation of microbial dormancy at six temperate forest sites of different forest types. We then extrapolated the model to global temperate forest ecosystems to investigate biogeochemical controls on soil heterotrophic respiration and microbial dormancy dynamics at different temporal-spatial scales. The dormancy model consistently produced better match with field-observed heterotrophic soil CO2 efflux (RH) than the no dormancy model. Our regional modeling results further indicated that models with dormancy were able to produce more realistic magnitude of microbial biomass (<2% of soil organic carbon) and soil RH (7.5 ± 2.4 Pg C yr−1). Spatial correlation analysis showed that soil organic carbon content was the dominating factor (correlation coefficient = 0.4–0.6) in the simulated spatial pattern of soil RH with both models. In contrast to strong temporal and local controls of soil temperature and moisture on microbial dormancy, our modeling results showed that soil carbon-to-nitrogen ratio (C:N) was a major regulating factor at regional scales (correlation coefficient = −0.43 to −0.58), indicating scale-dependent biogeochemical controls on microbial dynamics. Our findings suggest that incorporating microbial dormancy could improve the realism of microbial-based decomposition models and enhance the integration of soil experiments and mechanistically based modeling.
58. Zeli Tan, Qianlai Zhuang: Methane emissions from pan-Arctic lakes during the 21st century: An analysis with process-based models of lake evolution and biogeochemistry 2015-12-30T09:26:44.672096-05:00 Journal of Geophysical Research: Biogeosciences+
    
    The importance of methane emissions from pan-Arctic lakes in the global carbon cycle has been suggested by recent studies. These studies indicated that climate change influences this methane source mainly in two ways: the warming of lake sediments and the evolution of thermokarst lakes. Few studies have been conducted to quantify the two impacts together in a unified modeling framework. Here we adapt a region-specific lake evolution model to the pan-Arctic scale and couple it with a lake methane biogeochemical model to quantify the change of this freshwater methane source in the 21st century. Our simulations show that the extent of thaw lakes will increase throughout the 21st century in the northern lowlands of the pan-Arctic where the reworking of epigenetic ice in drained lake basins will continue. The projected methane emissions by 2100 are 28.3 ± 4.5 Tg CH4 yr−1 under a low warming scenario (Representative Concentration Pathways (RCPs) 2.6) and 32.7 ± 5.2 Tg CH4 yr−1 under a high warming scenario (RCP 8.5), which are about 2.5 and 2.9 times the simulated present-day emissions. Most of the emitted methane originates from nonpermafrost carbon stock. For permafrost carbon, the methanogenesis will mineralize a cumulative amount of 3.4 ± 0.8 Pg C under RCP 2.6 and 3.9 ± 0.9 Pg C under RCP 8.5 from 2006 to 2099. The projected emissions could increase atmospheric methane concentrations by 55.0–69.3 ppb. This study further indicates that the warming of lake sediments dominates the increase of methane emissions from pan-Arctic lakes in the future.
59. : Constraining a land surface model with multiple observations by application of the MPI-Carbon Cycle Data Assimilation System Tue, 19 Jan 2016 16:42:35 +0100 Geoscientific Model Development+
    
    Constraining a land surface model with multiple observations by application of the MPI-Carbon Cycle Data Assimilation System
    G. J. Schürmann, T. Kaminski, C. Köstler, N. Carvalhais, M. Voßbeck, J. Kattge, R. Giering, C. Rödenbeck, M. Heimann, and S. Zaehle
    Geosci. Model Dev. Discuss., doi:10.5194/gmd-2015-263,2016
    Manuscript under review for GMD (discussion: open, 0 comments)
    We describe the Max Planck Institute Carbon Cycle Data Assimilation System (MPI-CCDAS). The system improves the modelled carbon cycle of the terrestrial biosphere by systematically confronting (or assimilating) the model with observations of atmospheric CO₂ and fraction of absorbed photosynthetically active radiation. Jointly assimilating both data streams outperforms the single-data stream experiments thus showing the value of a multi-data stream assimilation.
60. Jeanine L. Olsen: The genome of the seagrass Zostera marina reveals angiosperm adaptation to the sea 2016-01-27 Nature+
    
    Seagrasses colonized the sea on at least three independent occasions to form the basis of one of the most productive and widespread coastal ecosystems on the planet. Here we report the genome of Zostera marina (L.), the first, to our knowledge, marine angiosperm to be fully sequenced. This reveals unique insights into the genomic losses and gains involved in achieving the structural and physiological adaptations required for its marine lifestyle, arguably the most severe habitat shift ever accomplished by flowering plants. Key angiosperm innovations that were lost include the entire repertoire of stomatal genes, genes involved in the synthesis of terpenoids and ethylene signalling, and genes for ultraviolet protection and phytochromes for far-red sensing. Seagrasses have also regained functions enabling them to adjust to full salinity. Their cell walls contain all of the polysaccharides typical of land plants, but also contain polyanionic, low-methylated pectins and sulfated galactans, a feature shared with the cell walls of all macroalgae and that is important for ion homoeostasis, nutrient uptake and O2/CO2 exchange through leaf epidermal cells. The Z. marina genome resource will markedly advance a wide range of functional ecological studies from adaptation of marine ecosystems under climate warming, to unravelling the mechanisms of osmoregulation under high salinities that may further inform our understanding of the evolution of salt tolerance in crop plants.
61. Jae Man Lee: Nutrient-sensing nuclear receptors coordinate autophagy 2014-11-12 Nature+
    
    Autophagy is an evolutionarily conserved catabolic process that recycles nutrients upon starvation and maintains cellular energy homeostasis. Its acute regulation by nutrient-sensing signalling pathways is well described, but its longer-term transcriptional regulation is not. The nuclear receptors peroxisome proliferator-activated receptor-α (PPARα) and farnesoid X receptor (FXR) are activated in the fasted and fed liver, respectively. Here we show that both PPARα and FXR regulate hepatic autophagy in mice. Pharmacological activation of PPARα reverses the normal suppression of autophagy in the fed state, inducing autophagic lipid degradation, or lipophagy. This response is lost in PPARα knockout (Ppara−/−, also known as Nr1c1−/−) mice, which are partially defective in the induction of autophagy by fasting. Pharmacological activation of the bile acid receptor FXR strongly suppresses the induction of autophagy in the fasting state, and this response is absent in FXR knockout (Fxr−/−, also known as Nr1h4−/−) mice, which show a partial defect in suppression of hepatic autophagy in the fed state. PPARα and FXR compete for binding to shared sites in autophagic gene promoters, with opposite transcriptional outputs. These results reveal complementary, interlocking mechanisms for regulation of autophagy by nutrient status.
62. Sunmi Seok: Transcriptional regulation of autophagy by an FXR–CREB axis 2014-11-12 Nature+
    
    Lysosomal degradation of cytoplasmic components by autophagy is essential for cellular survival and homeostasis under nutrient-deprived conditions. Acute regulation of autophagy by nutrient-sensing kinases is well defined, but longer-term transcriptional regulation is relatively unknown. Here we show that the fed-state sensing nuclear receptor farnesoid X receptor (FXR) and the fasting transcriptional activator cAMP response element-binding protein (CREB) coordinately regulate the hepatic autophagy gene network. Pharmacological activation of FXR repressed many autophagy genes and inhibited autophagy even in fasted mice, and feeding-mediated inhibition of macroautophagy was attenuated in FXR-knockout mice. From mouse liver chromatin immunoprecipitation and high-throughput sequencing data, FXR and CREB binding peaks were detected at 178 and 112 genes, respectively, out of 230 autophagy-related genes, and 78 genes showed shared binding, mostly in their promoter regions. CREB promoted autophagic degradation of lipids, or lipophagy, under nutrient-deprived conditions, and FXR inhibited this response. Mechanistically, CREB upregulated autophagy genes, including Atg7, Ulk1 and Tfeb, by recruiting the coactivator CRTC2. After feeding or pharmacological activation, FXR trans-repressed these genes by disrupting the functional CREB–CRTC2 complex. This study identifies the new FXR–CREB axis as a key physiological switch regulating autophagy, resulting in sustained nutrient regulation of autophagy during feeding/fasting cycles.
63. Nuno Carvalhais: Global covariation of carbon turnover times with climate in terrestrial ecosystems 2014-09-24 Nature+
    
    The response of the terrestrial carbon cycle to climate change is among the largest uncertainties affecting future climate change projections. The feedback between the terrestrial carbon cycle and climate is partly determined by changes in the turnover time of carbon in land ecosystems, which in turn is an ecosystem property that emerges from the interplay between climate, soil and vegetation type. Here we present a global, spatially explicit and observation-based assessment of whole-ecosystem carbon turnover times that combines new estimates of vegetation and soil organic carbon stocks and fluxes. We find that the overall mean global carbon turnover time is  years (95 per cent confidence interval). On average, carbon resides in the vegetation and soil near the Equator for a shorter time than at latitudes north of 75° north (mean turnover times of 15 and 255 years, respectively). We identify a clear dependence of the turnover time on temperature, as expected from our present understanding of temperature controls on ecosystem dynamics. Surprisingly, our analysis also reveals a similarly strong association between turnover time and precipitation. Moreover, we find that the ecosystem carbon turnover times simulated by state-of-the-art coupled climate/carbon-cycle models vary widely and that numerical simulations, on average, tend to underestimate the global carbon turnover time by 36 per cent. The models show stronger spatial relationships with temperature than do observation-based estimates, but generally do not reproduce the strong relationships with precipitation and predict faster carbon turnover in many semi-arid regions. Our findings suggest that future climate/carbon-cycle feedbacks may depend more strongly on changes in the hydrological cycle than is expected at present and is considered in Earth system models.
64. Randall M. Chin: The metabolite α-ketoglutarate extends lifespan by inhibiting ATP synthase and TOR 2014-05-14 Nature+
    
    Metabolism and ageing are intimately linked. Compared with ad libitum feeding, dietary restriction consistently extends lifespan and delays age-related diseases in evolutionarily diverse organisms. Similar conditions of nutrient limitation and genetic or pharmacological perturbations of nutrient or energy metabolism also have longevity benefits. Recently, several metabolites have been identified that modulate ageing; however, the molecular mechanisms underlying this are largely undefined. Here we show that α-ketoglutarate (α-KG), a tricarboxylic acid cycle intermediate, extends the lifespan of adult Caenorhabditis elegans. ATP synthase subunit β is identified as a novel binding protein of α-KG using a small-molecule target identification strategy termed drug affinity responsive target stability (DARTS). The ATP synthase, also known as complex V of the mitochondrial electron transport chain, is the main cellular energy-generating machinery and is highly conserved throughout evolution. Although complete loss of mitochondrial function is detrimental, partial suppression of the electron transport chain has been shown to extend C. elegans lifespan. We show that α-KG inhibits ATP synthase and, similar to ATP synthase knockdown, inhibition by α-KG leads to reduced ATP content, decreased oxygen consumption, and increased autophagy in both C. elegans and mammalian cells. We provide evidence that the lifespan increase by α-KG requires ATP synthase subunit β and is dependent on target of rapamycin (TOR) downstream. Endogenous α-KG levels are increased on starvation and α-KG does not extend the lifespan of dietary-restricted animals, indicating that α-KG is a key metabolite that mediates longevity by dietary restriction. Our analyses uncover new molecular links between a common metabolite, a universal cellular energy generator and dietary restriction in the regulation of organismal lifespan, thus suggesting new strategies for the prevention and treatment of ageing and age-related diseases.
65. Carina Bunse: Response of marine bacterioplankton pH homeostasis gene expression to elevated CO2 2016-01-11 Nature: Climate Change+
    
    How marine bacteria respond to ocean acidification was investigated by metatranscriptome analysis of mesocosm experiments. Bacteria in low-nutrient sea water had enhanced gene expression under elevated CO2 levels, in order to adapt to environmental stress.
66. Michael Behrenfeld: Marine biogeochemistry: Phytoplankton in a witch's brew 2016-01-25 Nature: Geoscience+
    
    Natural seafloor hydrocarbon seeps are responsible for roughly half of the oil released into the ocean. As these oils and gases rise to the surface, they transport nutrients upwards, benefiting phytoplankton in the upper sunlit layer.
67. Luis P. A. M. Duprat: Enhanced Southern Ocean marine productivity due to fertilization by giant icebergs 2016-01-11 Nature: Geoscience+
    
    Nutrient input from icebergs can fertilize productivity in the ocean. Ten years of satellite measurements reveal that giant icebergs could be responsible for up to 20% of carbon export to depth in the Southern Ocean.
68. Elisabetta Dejana: [Perspective] Oligodendrocytes follow blood vessel trails in the brain 2016-01-22 Science+
    
    The mammalian brain is probably the most complex organ generated by evolution so far. It is composed of more distinct cell types than any other organ (1), including those that protrude myriads of processes to make the connections that define brain function. Brain development requires that differentiated cells become dispersed and positioned correctly, which demands that progenitor cells migrate—often over very long distances—from sites of origin in the early neural tube to final sites of differentiation in the multilayered brain tissue. Oligodendrocytes, the cells that myelinate axons, are perhaps the most migratory of all the brain's cell types, but their paths have remained ill defined (2). On page 379 of this issue, Tsai et al. (3) report that oligodendrocyte precursor cells (OPCs) migrate along blood vessels, and define a signaling pathway involved in the process. The findings are exciting not only for their pathological implications, but because they add to the emerging picture that blood vessels do much more than provide oxygen and nutrients to the developing or regenerating tissue—a concept sometimes referred to as angiocrine signaling (4, 5). Authors: Elisabetta Dejana, Christer Betsholtz
69. Wei Xuan: [Report] Cyclic programmed cell death stimulates hormone signaling and root development in Arabidopsis 2016-01-22 Science+
    
    The plant root cap, surrounding the very tip of the growing root, perceives and transmits environmental signals to the inner root tissues. In Arabidopsis thaliana, auxin released by the root cap contributes to the regular spacing of lateral organs along the primary root axis. Here, we show that the periodicity of lateral organ induction is driven by recurrent programmed cell death at the most distal edge of the root cap. We suggest that synchronous bursts of cell death in lateral root cap cells release pulses of auxin to surrounding root tissues, establishing the pattern for lateral root formation. The dynamics of root cap turnover may therefore coordinate primary root growth with root branching in order to optimize the uptake of water and nutrients from the soil. Authors: Wei Xuan, Leah R. Band, Robert P. Kumpf, Daniël Van Damme, Boris Parizot, Gieljan De Rop, Davy Opdenacker, Barbara K. Möller, Noemi Skorzinski, Maria F. Njo, Bert De Rybel, Dominique Audenaert, Moritz K. Nowack, Steffen Vanneste, Tom Beeckman

Soil Carbon

1. J.A.C. Davies, E. Tipping, E.C. Rowe, J.F. Boyle, E. Graf Pannatier, V. Martinsen: Long-term P weathering and recent N deposition control contemporary plant-soil C, N and P 2016-01-06T08:19:51.461438-05:00 Global Biogeochemical Cycles+
   
   Models are needed to understand how plant-soil nutrient stores and fluxes have responded to the last two centuries of widespread anthropogenic nutrient pollution and predict future change. These models need to integrate across carbon, nitrogen and phosphorus (C, N, & P) cycles and simulate changes over suitable timescales using available driving data. It is also vital that they are constrainable against observed data to provide confidence in their outputs. To date, no models address all of these requirements. To meet this need, a new model, N14CP, is introduced, which is initially applied to Northern hemisphere temperate and boreal ecosystems over the Holocene. N14CP is parameterized and tested using 88 northern Europe plot-scale studies, providing the most robust test of such a model to date. The model simulates long-term P weathering, based on the assumption of a starting pool of weatherable P (Pweath0, g m−2), which is gradually transformed into organic and sorbed pools. Nitrogen fixation (and consequently primary production) is made dependent on available P. In the absence of knowledge about the spatial variability of Pweath0, N14CP produces good average soil and plant variables, but cannot simulate variations among sites. Allowing Pweath0 to vary between sites improves soil C, N and P results greatly, suggesting contemporary soil C, N and P are sensitive to long-term P weathering. Most sites were found to be N limited. Anthropogenic N deposition since 1800 was calculated to have increased plant biomass substantially, in agreement with observations, and consequently increased soil carbon pools.
2. Yiqi Luo, Anders Ahlström, Steven D. Allison, Niels H. Batjes, Victor Brovkin, Nuno Carvalhais, Adrian Chappell, Philippe Ciais, Eric A. Davidson, Adien Finzi, Katerina Georgiou, Bertrand Guenet, Oleksandra Hararuk, Jennifer W. Harden, Yujie He, Francesca Hopkins, Lifen Jiang, Charlie Koven, Robert B. Jackson, Chris D. Jones, Mark J. Lara, Junyi Liang, A. David McGuire, William Parton, Changhui Peng, James T. Randerson, Alejandro Salazar, Carlos A. Sierra, Matthew J. Smith, Hanqin Tian, Katherine E. O. Todd-Brown, Margaret Torn, Kees Jan Groenigen, Ying Ping Wang, Tristram O. West, Yaxing Wei, William R. Wieder, Jianyang Xia, Xia Xu, Xiaofeng Xu, Tao Zhou: Toward more realistic projections of soil carbon dynamics by Earth system models 2016-01-21T17:40:19.692208-05:00 Global Biogeochemical Cycles+
   
   Soil carbon (C) is a critical component of Earth system models (ESMs), and its diverse representations are a major source of the large spread across models in the terrestrial C sink from the third to fifth assessment reports of the Intergovernmental Panel on Climate Change (IPCC). Improving soil C projections is of a high priority for Earth system modeling in the future IPCC and other assessments. To achieve this goal, we suggest that (1) model structures should reflect real-world processes, (2) parameters should be calibrated to match model outputs with observations, and (3) external forcing variables should accurately prescribe the environmental conditions that soils experience. First, most soil C cycle models simulate C input from litter production and C release through decomposition. The latter process has traditionally been represented by first-order decay functions, regulated primarily by temperature, moisture, litter quality, and soil texture. While this formulation well captures macroscopic soil organic C (SOC) dynamics, better understanding is needed of their underlying mechanisms as related to microbial processes, depth-dependent environmental controls, and other processes that strongly affect soil C dynamics. Second, incomplete use of observations in model parameterization is a major cause of bias in soil C projections from ESMs. Optimal parameter calibration with both pool- and flux-based data sets through data assimilation is among the highest priorities for near-term research to reduce biases among ESMs. Third, external variables are represented inconsistently among ESMs, leading to differences in modeled soil C dynamics. We recommend the implementation of traceability analyses to identify how external variables and model parameterizations influence SOC dynamics in different ESMs. Overall, projections of the terrestrial C sink can be substantially improved when reliable data sets are available to select the most representative model structure, constrain parameters, and prescribe forcing fields.
3. KS Chin, J Lento, JM Culp, D Lacelle, SV Kokelj: Permafrost thaw and intense thermokarst activity decreases abundance of stream benthic macroinvertebrates 2016-01-14T11:28:35.247479-05:00 Global Change Biology+
   
   Intensification of permafrost thaw has increased the frequency and magnitude of large permafrost slope disturbances (mega slumps) in glaciogenic terrain of northwestern Canada. Individual thermokarst disturbances up to 40 ha in area have made large volumes of previously frozen, highly weatherable fine-grained sediments available for leaching and transport to adjacent streams, significantly increasing sediment and solute loads in these systems. To test the effects of this climate-sensitive disturbance regime on the ecology of Arctic streams, we explored the relationship between physical and chemical variables and benthic macroinvertebrate communities in disturbed and undisturbed stream reaches in the Peel Plateau in the Northwest Territories, Canada. Highly disturbed and undisturbed stream reaches differed with respect to taxonomic composition and invertebrate abundance. Minimally disturbed reaches were not differentiated by these variables but rather were distributed along a disturbance gradient between highly disturbed and undisturbed sites. In particular, there was evidence of a strong negative relationship between macroinvertebrate abundance and total suspended solids, and a positive relationship between abundance and the distance from the disturbance. Increases in both sediments and nutrients appear to be the proximate cause of community differences in highly disturbed streams. Declines in macroinvertebrate abundance in response to slump activity have implications for the food webs of these systems, potentially leading to negative impacts on higher trophic levels, such as fish. Further, the disturbance impacts on stream health can be expected to intensify as climate change increases the frequency and magnitude of thermokarst. This article is protected by copyright. All rights reserved.
4. Colin Averill, Bonnie G. Waring, Christine V. Hawkes: Historical precipitation predictably alters the shape and magnitude of microbial functional response to soil moisture 2016-01-08T07:54:53.626548-05:00 Global Change Biology+
   
   Soil moisture constrains the activity of decomposer soil microorganisms, and in turn the rate at which soil carbon returns to the atmosphere. While increases in soil moisture are generally associated with increased microbial activity, historical climate may constrain current microbial responses to moisture. However, it is not known if variation in the shape and magnitude of microbial functional responses to soil moisture can be predicted from historical climate at regional scales. To address this problem, we measured soil enzyme activity at 12 sites across a broad climate gradient spanning 442 – 887 mm mean annual precipitation. Measurements were made eight times over 21 months to maximize sampling during different moisture conditions. We then fit saturating functions of enzyme activity to soil moisture and extracted half saturation and maximum activity parameter values from model fits. We found that 50% of the variation in maximum activity parameters across sites could be predicted by 30-year mean annual precipitation, an indicator of historical climate, and that the effect is independent of variation in temperature, soil texture, or soil carbon concentration. Based on this finding, we suggest that variation in the shape and magnitude of soil microbial response to soil moisture due to historical climate may be remarkably predictable at regional scales, and this approach may extend to other systems. If historical contingencies on microbial activities prove to be persistent in the face of environmental change, this approach also provides a framework for incorporating historical climate effects into biogeochemical models simulating future global change scenarios. This article is protected by copyright. All rights reserved.
5. Tom N. Walker, Mark H. Garnett, Susan E. Ward, Simon Oakley, Richard D. Bardgett, Nicholas J. Ostle: Vascular plants promote ancient peatland carbon loss with climate warming 2016-01-04T02:22:38.523425-05:00 Global Change Biology+
   
   Northern peatlands have accumulated one third of the Earth's soil carbon stock since the last Ice Age. Rapid warming across northern biomes threatens to accelerate rates of peatland ecosystem respiration. Despite compensatory increases in net primary production, greater ecosystem respiration could signal the release of ancient, century- to millennia-old carbon from the peatland organic matter stock. Warming has already been shown to promote ancient peatland carbon release, but, despite the key role of vegetation in carbon dynamics, little is known about how plants influence the source of peatland ecosystem respiration. Here, we address this issue using in situ 14C measurements of ecosystem respiration on an established peatland warming and vegetation manipulation experiment. Results show that warming of approximately 1 °C promotes respiration of ancient peatland carbon (up to 2100 years old) when dwarf-shrubs or graminoids are present, an effect not observed when only bryophytes are present. We demonstrate that warming likely promotes ancient peatland carbon release via its control over organic inputs from vascular plants. Our findings suggest that dwarf-shrubs and graminoids prime microbial decomposition of previously ‘locked-up’ organic matter from potentially deep in the peat profile, facilitating liberation of ancient carbon as CO2. Furthermore, such plant-induced peat respiration could contribute up to 40% of ecosystem CO2 emissions. If consistent across other sub-arctic and arctic ecosystems, this represents a considerable fraction of ecosystem respiration that is currently not acknowledged by global carbon cycle models. Ultimately, greater contribution of ancient carbon to ecosystem respiration may signal the loss of a previously stable peatland carbon pool, creating potential feedbacks to future climate change. This article is protected by copyright. All rights reserved.
6. Verity G. Salmon, Patrick Soucy, Marguerite Mauritz, Gerardo Celis, Susan M. Natali, Michelle C. Mack, Edward A. G. Schuur: Nitrogen availability increases in a tundra ecosystem during five years of experimental permafrost thaw 2015-12-31T02:32:42.133267-05:00 Global Change Biology+
   
   Perennially frozen soil in high latitude ecosystems (permafrost) currently stores 1330-1580 Pg of carbon (C). As these ecosystems warm, the thaw and decomposition of permafrost is expected to release large amounts of C to the atmosphere. Fortunately, losses from the permafrost C pool will be partially offset by increased plant productivity. The degree to which plants are able to sequester C, however, will be determined by changing nitrogen (N) availability in these thawing soil profiles. N availability currently limits plant productivity in tundra ecosystems but plant access to N is expected improve as decomposition increases in speed and extends to deeper soil horizons. In order to evaluate the relationship between permafrost thaw and N availability, we monitored N cycling during five years of experimentally induced permafrost thaw at the Carbon in Permafrost Experimental Heating Research project (CiPEHR). Inorganic N availability increased significantly in response to deeper thaw and greater soil moisture induced by Soil warming. This treatment also prompted a 23% increase in aboveground biomass and a 49% increase in foliar N pools. The sedge Eriophorum vaginatum responded most strongly to warming: this species explained 91% of the change in aboveground biomass during the five year period. Air warming had little impact when applied alone, but when applied in combination with Soil warming, growing season soil inorganic N availability was significantly reduced. These results demonstrate that there is a strong positive relationship between the depth of permafrost thaw and N availability in tundra ecosystems but that this relationship can be diminished by interactions between increased thaw, warmer air temperatures and higher levels of soil moisture. Within five years of permafrost thaw, plants actively incorporate newly available N into biomass but C storage in live vascular plant biomass is unlikely to be greater than losses from deep soil C pools. This article is protected by copyright. All rights reserved.
7. Emanuele Lugato, Keith Paustian, Panos Panagos, Arwyn Jones, Pasquale Borrelli: Quantifying the erosion effect on current carbon budget of European agricultural soils at high spatial resolution 2015-12-18T01:14:15.989629-05:00 Global Change Biology+
   
   The idea of offsetting anthropogenic CO2 emissions by increasing global soil organic carbon (SOC), as recently proposed by French authorities ahead of COP21 in the ‘four per mil’ initiative, is notable. However, a high uncertainty still exits on land C balance components. In particular, the role of erosion in the global C cycle is not totally disentangled, leading to disagreement whether this process induces lands to be a source or sink of CO2. To investigate this issue, we coupled soil erosion into a biogeochemistry model, running at 1 km2 resolution across the agricultural soils of the European Union (EU). Based on data-driven assumptions, the simulation took into account also soil deposition within grid cells and the potential C export to riverine systems, in a way to be conservative in a mass balance. We estimated that 143 out of 187 Mha have C erosion rates <0.05 Mg C ha−1 yr−1, although some hot-spot areas showed eroded SOC >0.45 Mg C ha−1 yr−1. In comparison with a baseline without erosion, the model suggested an erosion-induced sink of atmospheric C consistent with previous empirical-based studies. Integrating all C fluxes for the EU agricultural soils, we estimated a net C loss or gain of -2.28 and +0.79 Tg yr−1 of CO2eq, respectively, depending on the value for the short-term enhancement of soil C mineralization due to soil disruption and displacement/transport with erosion. We concluded that erosion fluxes were in the same order of current carbon gains from improved management. Even if erosion could potentially induce a sink for atmospheric CO2, strong agricultural policies are needed to prevent or reduce soil erosion, in order to maintain soil health and productivity. This article is protected by copyright. All rights reserved.
8. Pete Smith: Soil carbon sequestration and biochar as negative emission technologies 2016-01-06T01:12:40.397463-05:00 Global Change Biology+
   
   Despite 20 years of effort to curb emissions, greenhouse gas (GHG) emissions grew faster during the 2000s than in the 1990s, which presents a major challenge for meeting the international goal of limiting warming to <2 °C relative to the preindustrial era. Most recent scenarios from integrated assessment models require large-scale deployment of negative emissions technologies (NETs) to reach the 2 °C target. A recent analysis of NETs, including direct air capture, enhanced weathering, bioenergy with carbon capture and storage and afforestation/deforestation, showed that all NETs have significant limits to implementation, including economic cost, energy requirements, land use, and water use. In this paper, I assess the potential for negative emissions from soil carbon sequestration and biochar addition to land, and also the potential global impacts on land use, water, nutrients, albedo, energy and cost. Results indicate that soil carbon sequestration and biochar have useful negative emission potential (each 0.7 GtCeq. yr−1) and that they potentially have lower impact on land, water use, nutrients, albedo, energy requirement and cost, so have fewer disadvantages than many NETs. Limitations of soil carbon sequestration as a NET centre around issues of sink saturation and reversibility. Biochar could be implemented in combination with bioenergy with carbon capture and storage. Current integrated assessment models do not represent soil carbon sequestration or biochar. Given the negative emission potential of SCS and biochar and their potential advantages compared to other NETs, efforts should be made to include these options within IAMs, so that their potential can be explored further in comparison with other NETs for climate stabilization.
9. Mark J. Lara, Hélène Genet, Anthony D. McGuire, Eugénie S. Euskirchen, Yujin Zhang, Dana R. N. Brown, Mark T. Jorgenson, Vladimir Romanovsky, Amy Breen, William R. Bolton: Thermokarst rates intensify due to climate change and forest fragmentation in an Alaskan boreal forest lowland 2016-01-06T06:51:23.049768-05:00 Global Change Biology+
   
   Lowland boreal forest ecosystems in Alaska are dominated by wetlands comprised of a complex mosaic of fens, collapse-scar bogs, low shrub/scrub, and forests growing on elevated ice-rich permafrost soils. Thermokarst has affected the lowlands of the Tanana Flats in central Alaska for centuries, as thawing permafrost collapses forests that transition to wetlands. Located within the discontinuous permafrost zone, this region has significantly warmed over the past half-century, and much of these carbon-rich permafrost soils are now within ~0.5 °C of thawing. Increased permafrost thaw in lowland boreal forests in response to warming may have consequences for the climate system. This study evaluates the trajectories and potential drivers of 60 years of forest change in a landscape subjected to permafrost thaw in unburned dominant forest types (paper birch and black spruce) associated with location on elevated permafrost plateau and across multiple time periods (1949, 1978, 1986, 1998, and 2009) using historical and contemporary aerial and satellite images for change detection. We developed (i) a deterministic statistical model to evaluate the potential climatic controls on forest change using gradient boosting and regression tree analysis, and (ii) a 30 × 30 m land cover map of the Tanana Flats to estimate the potential landscape-level losses of forest area due to thermokarst from 1949 to 2009. Over the 60-year period, we observed a nonlinear loss of birch forests and a relatively continuous gain of spruce forest associated with thermokarst and forest succession, while gradient boosting/regression tree models identify precipitation and forest fragmentation as the primary factors controlling birch and spruce forest change, respectively. Between 1950 and 2009, landscape-level analysis estimates a transition of ~15 km² or ~7% of birch forests to wetlands, where the greatest change followed warm periods. This work highlights that the vulnerability and resilience of lowland ice-rich permafrost ecosystems to climate changes depend on forest type.
10. Choimaa Dulamsuren, Michael Klinge, Jan Degener, Mookhor Khishigjargal, Tselmeg Chenlemuge, Banzragch Bat-Enerel, Yolk Yeruult, Davaadorj Saindovdon, Kherlenchimeg Ganbaatar, Jamsran Tsogtbaatar, Christoph Leuschner, Markus Hauck: Carbon pool densities and a first estimate of the total carbon pool in the Mongolian forest-steppe 2016-01-27T00:08:40.033365-05:00 Global Change Biology+
    
    The boreal forest biome represents one of the most important terrestrial carbon stores, which gave reason to intensive research on carbon stock densities. However, such an analysis does not yet exist for the southernmost Eurosiberian boreal forests in Inner Asia. Most of these forests are located in the Mongolian forest-steppe, which is largely dominated by Larix sibirica. We quantified the carbon stock density and total carbon pool of Mongolia's boreal forests and adjacent grasslands and draw conclusions on possible future change. Mean aboveground carbon stock density in the interior of L. sibirica forests was 66 Mg C ha−1, which is in the upper range of values reported from boreal forests and probably due to the comparably long growing season. The density of soil organic carbon (SOC, 108 Mg C ha−1) and total belowground carbon density (149 Mg C ha−1) are at the lower end of the range known from boreal forests, which might be the result of higher soil temperatures and a thinner permafrost layer than in the central and northern boreal forest belt. Land use effects are especially relevant at forest edges, where mean carbon stock density was 188 Mg C ha−1, compared with 215 Mg C ha−1 in the forest interior. Carbon stock density in grasslands was 144 Mg C ha−1. Analysis of satellite imagery of the highly fragmented forest area in the forest-steppe zone showed that Mongolia's total boreal forest area is currently 73 818 km2, and 22% of this area refers to forest edges (defined as the first 30 m from the edge). The total forest carbon pool of Mongolia was estimated at ~ 1.5−1.7 Pg C, a value which is likely to decrease in future with increasing deforestation and fire frequency, and global warming.
11. Markus Janout, Jens Hölemann, Bennet Juhls, Thomas Krumpen, Benjamin Rabe, Dorothea Bauch, Carolyn Wegner, Heidemarie Kassens, Leonid Timokhov: Episodic warming of near-bottom waters under the Arctic sea ice on the central Laptev Sea shelf 2016-01-14T15:55:33.017973-05:00 Geophysical Research Letters+
    
    A multiyear mooring record (2007–2014) and satellite imagery highlight the strong temperature variability and unique hydrographic nature of the Laptev Sea. This Arctic shelf is a key region for river discharge and sea ice formation and export and includes submarine permafrost and methane deposits, which emphasizes the need to understand the thermal variability near the seafloor. Recent years were characterized by early ice retreat and a warming near-shore environment. However, warming was not observed on the deeper shelf until year-round under-ice measurements recorded unprecedented warm near-bottom waters of +0.6°C in winter 2012/2013, just after the Arctic sea ice extent featured a record minimum. In the Laptev Sea, early ice retreat in 2012 combined with Lena River heat and solar radiation produced anomalously warm summer surface waters, which were vertically mixed, trapped in the pycnocline, and subsequently transferred toward the bottom until the water column cooled when brine rejection eroded stratification.
12. Yeyi Liu, Dabang Jiang: Mid-Holocene permafrost: Results from CMIP5 simulations 2016-01-14T11:29:47.068964-05:00 Journal of Geophysical Research: Atmospheres+
    
    Distribution of frozen ground and active layer thickness in the Northern Hemisphere during the mid-Holocene (MH) and differences with respect to the preindustrial (PI) were investigated here using the Coupled Model Intercomparison Project Phase 5 (CMIP5) models. Two typical diagnostic methods, respectively, based on soil temperature (Ts based; a direct method) and air temperature (Ta based; an indirect method) were employed to classify categories and extents of frozen ground. In relation to orbitally induced changes in climate and in turn freezing and thawing indices, the MH permafrost extent was 20.5% (1.8%) smaller than the PI, whereas seasonally frozen ground increased by 9.2% (0.8%) in the Northern Hemisphere according to the Ts-based (Ta-based) method. Active layer thickness became larger, but by ≤ 1.0 m in most of permafrost areas during the MH. Intermodel disagreement remains within areas of permafrost boundary by both the Ts-based and Ta-based results, with the former demonstrating less agreement among the CMIP5 models because of larger variation in land model abilities to represent permafrost processes. However, both the methods were able to reproduce the MH relatively degenerated permafrost and increased active layer thickness (although with smaller magnitudes) as observed in data reconstruction. Disparity between simulation and reconstruction was mainly found in the seasonally frozen ground regions at low to middle latitudes, where the reconstruction suggested a reduction of seasonally frozen ground extent to the north, whereas the simulation demonstrated a slightly expansion to the south for the MH compared to the PI.
13. Tamara K. Harms, Jennifer W. Edmonds, Hélène Genet, Irena F. Creed, David Aldred, Andrew Balser, Jeremy B. Jones: Catchment influence on nitrate and dissolved organic matter in Alaskan streams across a latitudinal gradient 2016-01-10T22:41:44.579039-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Spatial patterns in carbon (C) and nitrogen (N) cycles of high-latitude catchments have been linked to climate and permafrost, and used to infer potential changes in biogeochemical cycles under climate warming. However, inconsistent spatial patterns across regions indicate that factors in addition to permafrost and regional climate may shape responses of C and N cycles to climate change. We hypothesized that physical attributes of catchments modify responses of C and N cycles to climate and permafrost. We measured dissolved organic C (DOC) and nitrate (NO3–) concentrations, and composition of dissolved organic matter (DOM) in 21 streams spanning boreal to arctic Alaska, and assessed permafrost, topography, and attributes of soils and vegetation as predictors of stream chemistry. Multiple regression analyses indicated that catchment slope is a primary driver, with lower DOC and higher NO3– concentration in streams draining steeper catchments, respectively. Depth of the active layer explained additional variation in concentration of DOC and NO3–. Vegetation type explained regional variation in concentration and composition of DOM, which was characterized by optical methods. Composition of DOM was further correlated with attributes of soils, including moisture, temperature, and thickness of the organic layer. Regional patterns of DOC and NO3–concentrations in boreal to arctic Alaska were driven primarily by catchment topography and modified by permafrost, whereas composition of DOM was driven by attributes of soils and vegetation, suggesting that predicting changes to C and N cycling from permafrost-influenced regions should consider catchment setting in addition to dynamics of climate and permafrost.
14. Elizabeth E. Webb, Edward A.G. Schuur, Susan M. Natali, Kiva L. Oken, Rosvel Bracho, John P. Krapek, David Risk, Nick R. Nickerson: Increased wintertime CO2 loss as a result of sustained tundra warming 2016-01-04T05:15:43.704417-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Permafrost soils currently store approximately 1672 Pg of carbon (C), but as high latitudes warm, this temperature-protected C reservoir will become vulnerable to higher rates of decomposition. In recent decades, air temperatures in the high latitudes have warmed more than any other region globally, particularly during the winter. Over the coming century, the arctic winter is also expected to experience the most warming of any region or season, yet it is notably understudied. Here we present non-summer season (NSS) CO2 flux data from the Carbon in Permafrost Experimental Heating Research (CiPEHR) project, an ecosystem warming experiment of moist acidic tussock tundra in interior Alaska. Our goals were to quantify the relationship between environmental variables and winter CO2 production, account for subnivean photosynthesis and late fall plant C uptake in our estimate of NSS CO2 exchange, constrain NSS CO2 loss estimates using multiple methods of measuring winter CO2 flux, and quantify the effect of winter soil warming on total NSS CO2 balance. We measured CO2 flux using four methods: two chamber techniques (the snow pit method and one where a chamber is left under the snow for the entire season), eddy covariance, and soda lime adsorption, and found that NSS CO2 loss varied up to 4 fold, depending on the method used. CO2 production was dependent on soil temperature and day of season but atmospheric pressure and air temperature were also important in explaining CO2 diffusion out of the soil. Warming stimulated both ecosystem respiration and productivity during the NSS and increased overall CO2 loss during this period by 14% (this effect varied by year, ranging from 7 to 24%). When combined with the summertime CO2 fluxes from the same site, our results suggest that this sub-arctic tundra ecosystem is shifting away from its historical function as a C sink to a C source.
15. Qi Deng, Dafeng Hui, Junming Wang, Chih-Li Yu, Changsheng Li, Chandra Reddy, Sam Dennis: Assessing the impacts of tillage and fertilization management on nitrous oxide emissions in a cornfield using the DNDC model 2016-01-04T03:22:31.021201-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Quantification and prediction of N2O emissions from croplands under different agricultural management practices are vital for sustainable agriculture and climate change mitigation. We simulated N2O emissions under tillage and no-tillage, and different nitrogen (N) fertilizer types and application methods (i.e. nitrification inhibitor, chicken manure and split applications) in a cornfield using the DeNitrification-DeComposition (DNDC) model. The model was parameterized with field experimental data collected in Nashville, Tennessee under various agricultural management treatments and run for a short term (3 yrs) and a long term (100 yrs). Results showed that the DNDC model could adequately simulate N2O emissions as well as soil properties under different agricultural management practices. The modeled emissions of N2O significantly increased by 35% with tillage, and decreased by 24% with the use of nitrification inhibitor, compared with no-tillage and normal N fertilization. Chicken manure amendment and split applications of N fertilizer had minor impact on N2O emission in a short term, but over a long term (100 yrs) the treatments significantly altered N2O emission (+35%, −10%, respectively). Sensitivity analysis showed that N2O emission was sensitive to mean annual precipitation, mean annual temperature, soil organic carbon, and the amount of total N fertilizer application. Our model results provide valuable information for determining agricultural best management practice to maintain highly productive corn yield while reducing greenhouse gas emissions.
16. Xiaodong Wu, Lin Zhao, Hongbing Fang, Yuguo Zhao, Joseph M. Smoak, Qiangqiang Pang, Yongjian Ding: Environmental controls on soil organic carbon and nitrogen stocks in the high-altitude arid western Qinghai-Tibetan Plateau permafrost region 2016-01-16T21:36:53.61712-05:00 Journal of Geophysical Research: Biogeosciences+
    
    While permafrost in the circum-Artic has great influence on soil organic carbon (SOC) and total nitrogen (TN) stocks, this might not be the case in low-latitude arid permafrost regions. We test this hypothesis in the western Qinghai-Tibetan Plateau (QTP) permafrost region. Fifty-nine soil profiles were analyzed to examine the SOC and TN distribution and the controlling factors in western QTP, which is a desert steppe ecoregion. Mean stocks of SOC (5.29 kg m−2) and TN (0.56 kg m−2) for the top 200 cm in this area were lower than those of the east QTP and circum-Arctic regions. The SOC and TN stocks under vegetative cover with permafrost conditions were significantly higher than those of desert conditions. The SOC and TN stocks for the layers of different depths were related to the content of clay, silt, and moisture. Although the active layer thickness (ALT) had a significant negative correlation to soil moisture, the ALT explained little or no variance in the SOC and TN stocks. The results showed that in the vast permafrost regions of the western QTP, the SOC and TN stocks are very low, and the main controlling factors for the SOC and TN are soil texture, moisture, and vegetation type. The SOC pool in this area may not be as vulnerable to degradation associated with climate warming and thus not emit greenhouse gases at the same rate as other permafrost regions. The different response of the SOC in this region should be considered in carbon cycling models.
17. Mattias Winterdahl, Hjalmar Laudon, Steve W. Lyon, Charlotta Pers, Kevin Bishop: Sensitivity of stream dissolved organic carbon to temperature and discharge: Implications of future climates 2016-01-15T15:20:03.896397-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Dissolved organic carbon (DOC) is a significant constituent in aquatic ecosystems with concentrations in streams influenced by both temperature and water flow pathway dynamics associated with changes in discharge (streamflow). We investigated the sensitivity of DOC concentrations in 12 high-latitude headwater streams to changes in temperature and discharge using a mathematical model. The implications of differences in sensitivities were explored by using downscaled projections of air temperature and discharge to simulate possible trajectories of DOC concentrations in a changing climate. We found two distinct responses: (i) catchments where stream DOC sensitivity was high to temperature but low to discharge and (ii) catchments where stream DOC sensitivity was low to temperature but high to discharge. Streams with strong seasonal DOC dynamics were more sensitive to temperature changes than nonseasonal systems. In addition, stream DOC sensitivity to discharge was strongly correlated with vertical soil water DOC differences in the near-stream zone. Simulations of possible future changes in DOC concentrations indicated median increases of about 4–24% compared to current levels when using projections of air temperature and discharge but even larger increases were observed for base flow concentrations (13–42%). Streams with high-temperature sensitivity showed the largest increases in DOC concentrations. Our results suggest that future climatic changes could bring significant increases in surface water DOC concentrations in boreal and hemiboreal areas but that the response ultimately is dependent on vertical soil solution DOC differences and soil organic carbon distribution.
18. C. C. Treat, M. C. Jones, P. Camill, A. Gallego-Sala, M. Garneau, J. W. Harden, G. Hugelius, E. S. Klein, U. Kokfelt, P. Kuhry, J. Loisel, P. J. H. Mathijssen, J. A. O'Donnell, P. O. Oksanen, T. M. Ronkainen, A. B. K. Sannel, J. Talbot, C. Tarnocai, M. Väliranta: Effects of permafrost aggradation on peat properties as determined from a pan-Arctic synthesis of plant macrofossils 2016-01-14T16:06:45.912977-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Permafrost dynamics play an important role in high-latitude peatland carbon balance and are key to understanding the future response of soil carbon stocks. Permafrost aggradation can control the magnitude of the carbon feedback in peatlands through effects on peat properties. We compiled peatland plant macrofossil records for the northern permafrost zone (515 cores from 280 sites) and classified samples by vegetation type and environmental class (fen, bog, tundra and boreal permafrost, and thawed permafrost). We examined differences in peat properties (bulk density, carbon (C), nitrogen (N) and organic matter content, and C/N ratio) and C accumulation rates among vegetation types and environmental classes. Consequences of permafrost aggradation differed between boreal and tundra biomes, including differences in vegetation composition, C/N ratios, and N content. The vegetation composition of tundra permafrost peatlands was similar to permafrost-free fens, while boreal permafrost peatlands more closely resembled permafrost-free bogs. Nitrogen content in boreal permafrost and thawed permafrost peatlands was significantly lower than in permafrost-free bogs despite similar vegetation types (0.9% versus 1.5% N). Median long-term C accumulation rates were higher in fens (23 g C m−2 yr−1) than in permafrost-free bogs (18 g C m−2 yr−1) and were lowest in boreal permafrost peatlands (14 g C m−2 yr−1). The plant macrofossil record demonstrated transitions from fens to bogs to permafrost peatlands, bogs to fens, permafrost aggradation within fens, and permafrost thaw and reaggradation. Using data synthesis, we have identified predominant peatland successional pathways, changes in vegetation type, peat properties, and C accumulation rates associated with permafrost aggradation.
19. J. F. Tjiputra, A. Grini, H. Lee: Impact of idealized future stratospheric aerosol injection on the large-scale ocean and land carbon cycles 2016-01-06T15:17:28.890712-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Using an Earth system model, we simulate stratospheric aerosol injection (SAI) on top of the Representative Concentration Pathways 8.5 future scenario. Our idealized method prescribes aerosol concentration, linearly increasing from 2020 to 2100, and thereafter remaining constant until 2200. In the aggressive scenario, the model projects a cooling trend toward 2100 despite warming that persists in the high latitudes. Following SAI termination in 2100, a rapid global warming of 0.35 K yr−1 is simulated in the subsequent 10 years, and the global mean temperature returns to levels close to the reference state, though roughly 0.5 K cooler. In contrast to earlier findings, we show a weak response in the terrestrial carbon sink during SAI implementation in the 21st century, which we attribute to nitrogen limitation. The SAI increases the land carbon uptake in the temperate forest-, grassland-, and shrub-dominated regions. The resultant lower temperatures lead to a reduction in the heterotrophic respiration rate and increase soil carbon retention. Changes in precipitation patterns are key drivers for variability in vegetation carbon. Upon SAI termination, the level of vegetation carbon storage returns to the reference case, whereas the soil carbon remains high. The ocean absorbs nearly 10% more carbon in the geoengineered simulation than in the reference simulation, leading to a ∼15 ppm lower atmospheric CO2 concentration in 2100. The largest enhancement in uptake occurs in the North Atlantic. In both hemispheres' polar regions, SAI delays the sea ice melting and, consequently, export production remains low. In the deep water of North Atlantic, SAI-induced circulation changes accelerate the ocean acidification rate and broaden the affected area.
20. Christian Knoblauch, Oliver Spott, Svetlana Evgrafova, Lars Kutzbach, Eva-Maria Pfeiffer: Regulation of methane production, oxidation, and emission by vascular plants and bryophytes in ponds of the northeast Siberian polygonal tundra 2015-12-09T09:12:59.839332-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Methane (CH4) production, oxidation, and emission were studied in ponds of the permafrost-affected polygonal tundra in northeast Siberia. Microbial degradation of organic matter in water-saturated soils is the most important source for the climate-relevant trace gas CH4. Although ponds and lakes cover a substantial fraction of the land surface of northern Siberia, data on CH4 fluxes from these water bodies are scarce. Summer CH4 fluxes were measured with closed chambers at the margins of ponds vegetated by vascular plants and in their centers without vascular plants. Furthermore, CH4 and oxygen concentration gradients, stable carbon isotope signatures of dissolved and emitted CH4, and microbial CH4 production and CH4 oxidation were determined. Mean summer fluxes were significantly higher at the margins of the ponds (46.1 ± 15.4 mg CH4 m−2 d−1) than at the centers (5.9 ± 8.2 mg CH4 m−2 d−1). CH4 transport was dominated by diffusion in most open water sites, but substantial ebullitive fluxes (12.0 ± 8.1 mg CH4 m−2 d−1) were detected in one pond. Plant-mediated transport accounted for 70 to 90% of total CH4 fluxes above emerged vegetation. In the absence of vascular plants, 61 to 99% of the CH4 produced in the anoxic bottom soil was consumed in a layer of the submerged moss Scorpidium scorpioides, which covered the bottoms of the ponds. The fraction of CH4 oxidized was lower at sites with vascular plants since CH4 was predominantly transported through their aerenchyma, thereby bypassing the CH4 oxidation zone in the moss layer. These results emphasize the importance of moss-associated CH4 oxidation causing low CH4 fluxes from the studied Siberian ponds.
21. Yujie He, Jinyan Yang, Qianlai Zhuang, Jennifer W. Harden, Anthony D. McGuire, Yaling Liu, Gangsheng Wang, Lianhong Gu: Incorporating microbial dormancy dynamics into soil decomposition models to improve quantification of soil carbon dynamics of northern temperate forests 2015-12-22T14:43:34.594535-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Soil carbon dynamics of terrestrial ecosystems play a significant role in the global carbon cycle. Microbial-based decomposition models have seen much growth recently for quantifying this role, yet dormancy as a common strategy used by microorganisms has not usually been represented and tested in these models against field observations. Here we developed an explicit microbial-enzyme decomposition model and examined model performance with and without representation of microbial dormancy at six temperate forest sites of different forest types. We then extrapolated the model to global temperate forest ecosystems to investigate biogeochemical controls on soil heterotrophic respiration and microbial dormancy dynamics at different temporal-spatial scales. The dormancy model consistently produced better match with field-observed heterotrophic soil CO2 efflux (RH) than the no dormancy model. Our regional modeling results further indicated that models with dormancy were able to produce more realistic magnitude of microbial biomass (<2% of soil organic carbon) and soil RH (7.5 ± 2.4 Pg C yr−1). Spatial correlation analysis showed that soil organic carbon content was the dominating factor (correlation coefficient = 0.4–0.6) in the simulated spatial pattern of soil RH with both models. In contrast to strong temporal and local controls of soil temperature and moisture on microbial dormancy, our modeling results showed that soil carbon-to-nitrogen ratio (C:N) was a major regulating factor at regional scales (correlation coefficient = −0.43 to −0.58), indicating scale-dependent biogeochemical controls on microbial dynamics. Our findings suggest that incorporating microbial dormancy could improve the realism of microbial-based decomposition models and enhance the integration of soil experiments and mechanistically based modeling.
22. Zeli Tan, Qianlai Zhuang: Methane emissions from pan-Arctic lakes during the 21st century: An analysis with process-based models of lake evolution and biogeochemistry 2015-12-30T09:26:44.672096-05:00 Journal of Geophysical Research: Biogeosciences+
    
    The importance of methane emissions from pan-Arctic lakes in the global carbon cycle has been suggested by recent studies. These studies indicated that climate change influences this methane source mainly in two ways: the warming of lake sediments and the evolution of thermokarst lakes. Few studies have been conducted to quantify the two impacts together in a unified modeling framework. Here we adapt a region-specific lake evolution model to the pan-Arctic scale and couple it with a lake methane biogeochemical model to quantify the change of this freshwater methane source in the 21st century. Our simulations show that the extent of thaw lakes will increase throughout the 21st century in the northern lowlands of the pan-Arctic where the reworking of epigenetic ice in drained lake basins will continue. The projected methane emissions by 2100 are 28.3 ± 4.5 Tg CH4 yr−1 under a low warming scenario (Representative Concentration Pathways (RCPs) 2.6) and 32.7 ± 5.2 Tg CH4 yr−1 under a high warming scenario (RCP 8.5), which are about 2.5 and 2.9 times the simulated present-day emissions. Most of the emitted methane originates from nonpermafrost carbon stock. For permafrost carbon, the methanogenesis will mineralize a cumulative amount of 3.4 ± 0.8 Pg C under RCP 2.6 and 3.9 ± 0.9 Pg C under RCP 8.5 from 2006 to 2099. The projected emissions could increase atmospheric methane concentrations by 55.0–69.3 ppb. This study further indicates that the warming of lake sediments dominates the increase of methane emissions from pan-Arctic lakes in the future.
23. Nuno Carvalhais: Global covariation of carbon turnover times with climate in terrestrial ecosystems 2014-09-24 Nature+
    
    The response of the terrestrial carbon cycle to climate change is among the largest uncertainties affecting future climate change projections. The feedback between the terrestrial carbon cycle and climate is partly determined by changes in the turnover time of carbon in land ecosystems, which in turn is an ecosystem property that emerges from the interplay between climate, soil and vegetation type. Here we present a global, spatially explicit and observation-based assessment of whole-ecosystem carbon turnover times that combines new estimates of vegetation and soil organic carbon stocks and fluxes. We find that the overall mean global carbon turnover time is  years (95 per cent confidence interval). On average, carbon resides in the vegetation and soil near the Equator for a shorter time than at latitudes north of 75° north (mean turnover times of 15 and 255 years, respectively). We identify a clear dependence of the turnover time on temperature, as expected from our present understanding of temperature controls on ecosystem dynamics. Surprisingly, our analysis also reveals a similarly strong association between turnover time and precipitation. Moreover, we find that the ecosystem carbon turnover times simulated by state-of-the-art coupled climate/carbon-cycle models vary widely and that numerical simulations, on average, tend to underestimate the global carbon turnover time by 36 per cent. The models show stronger spatial relationships with temperature than do observation-based estimates, but generally do not reproduce the strong relationships with precipitation and predict faster carbon turnover in many semi-arid regions. Our findings suggest that future climate/carbon-cycle feedbacks may depend more strongly on changes in the hydrological cycle than is expected at present and is considered in Earth system models.
24. R. de Oliveira Silva: Increasing beef production could lower greenhouse gas emissions in Brazil if decoupled from deforestation 2016-01-18 Nature: Climate Change+
    
    Modelling shows that increased beef production in the Brazilian Cerrado could lower greenhouse gas emissions and increase soil organic carbon stocks, provided that it is decoupled from deforestation.

Decomposition

1. : Dissolved organic carbon lability and stable isotope shifts during microbial decomposition in a tropical river system Tue, 26 Jan 2016 23:20:26 +0100 Biogeosciences+
   
   Dissolved organic carbon lability and stable isotope shifts during microbial decomposition in a tropical river system
   N. Geeraert, F. O. Omengo, G. Govers, and S. Bouillon
   Biogeosciences, 13, 517-525, doi:10.5194/bg-13-517-2016, 2016
   Rivers transport a large amount of carbon as dissolved organic carbon (DOC). Our incubation experiments on water of the Tana River, Kenya, showed that microbial decomposition of 10–60 % of the initial DOC occurred within the first 24–48 h. Simultaneously, there was a decrease in isotopic composition, indicating that DOC derived from C₄ vegetation is preferentially decomposed. This has implications for the assessment of vegetation in a catchment based on isotope signatures of riverine carbon.
2. : Habitat associations drive species vulnerability to climate change in boreal forests 2016-01-12 Climatic Change+
   

   Abstract

   Species climate change vulnerability, their predisposition to be adversely affected, has been assessed for a limited portion of biodiversity. Our knowledge of climate change impacts is often based only on exposure, the magnitude of climatic variation in the area occupied by the species, even if species sensitivity, the species ability to tolerate climatic variations determined by traits, plays a key role in determining vulnerability. We analyse the role of species’ habitat associations, a proxy for sensitivity, in explaining vulnerability for two poorly-known but species-rich taxa in boreal forest, saproxylic beetles and fungi, using three IPCC emissions scenarios. Towards the end of the 21st century we projected an improvement in habitat quality associated with an increase of deadwood, an important resource for species, as a consequence of increased tree growth under high emissions scenarios. However, climate change will potentially reduce habitat suitability for ~9–43 % of the threatened deadwood-associated species. This loss is likely caused by future increase in timber extraction and decomposition rates causing higher deadwood turnover, which have a strong negative effect on boreal forest biodiversity. Our results are species- and scenario-specific. Diversified forest management and restoration ensuring deadwood resources in the landscape would allow the persistence of species whose capacity of delivering important supporting ecosystem services can be undermined by climate change.

3. Yiqi Luo, Anders Ahlström, Steven D. Allison, Niels H. Batjes, Victor Brovkin, Nuno Carvalhais, Adrian Chappell, Philippe Ciais, Eric A. Davidson, Adien Finzi, Katerina Georgiou, Bertrand Guenet, Oleksandra Hararuk, Jennifer W. Harden, Yujie He, Francesca Hopkins, Lifen Jiang, Charlie Koven, Robert B. Jackson, Chris D. Jones, Mark J. Lara, Junyi Liang, A. David McGuire, William Parton, Changhui Peng, James T. Randerson, Alejandro Salazar, Carlos A. Sierra, Matthew J. Smith, Hanqin Tian, Katherine E. O. Todd-Brown, Margaret Torn, Kees Jan Groenigen, Ying Ping Wang, Tristram O. West, Yaxing Wei, William R. Wieder, Jianyang Xia, Xia Xu, Xiaofeng Xu, Tao Zhou: Toward more realistic projections of soil carbon dynamics by Earth system models 2016-01-21T17:40:19.692208-05:00 Global Biogeochemical Cycles+
   
   Soil carbon (C) is a critical component of Earth system models (ESMs), and its diverse representations are a major source of the large spread across models in the terrestrial C sink from the third to fifth assessment reports of the Intergovernmental Panel on Climate Change (IPCC). Improving soil C projections is of a high priority for Earth system modeling in the future IPCC and other assessments. To achieve this goal, we suggest that (1) model structures should reflect real-world processes, (2) parameters should be calibrated to match model outputs with observations, and (3) external forcing variables should accurately prescribe the environmental conditions that soils experience. First, most soil C cycle models simulate C input from litter production and C release through decomposition. The latter process has traditionally been represented by first-order decay functions, regulated primarily by temperature, moisture, litter quality, and soil texture. While this formulation well captures macroscopic soil organic C (SOC) dynamics, better understanding is needed of their underlying mechanisms as related to microbial processes, depth-dependent environmental controls, and other processes that strongly affect soil C dynamics. Second, incomplete use of observations in model parameterization is a major cause of bias in soil C projections from ESMs. Optimal parameter calibration with both pool- and flux-based data sets through data assimilation is among the highest priorities for near-term research to reduce biases among ESMs. Third, external variables are represented inconsistently among ESMs, leading to differences in modeled soil C dynamics. We recommend the implementation of traceability analyses to identify how external variables and model parameterizations influence SOC dynamics in different ESMs. Overall, projections of the terrestrial C sink can be substantially improved when reliable data sets are available to select the most representative model structure, constrain parameters, and prescribe forcing fields.
4. Tom N. Walker, Mark H. Garnett, Susan E. Ward, Simon Oakley, Richard D. Bardgett, Nicholas J. Ostle: Vascular plants promote ancient peatland carbon loss with climate warming 2016-01-04T02:22:38.523425-05:00 Global Change Biology+
   
   Northern peatlands have accumulated one third of the Earth's soil carbon stock since the last Ice Age. Rapid warming across northern biomes threatens to accelerate rates of peatland ecosystem respiration. Despite compensatory increases in net primary production, greater ecosystem respiration could signal the release of ancient, century- to millennia-old carbon from the peatland organic matter stock. Warming has already been shown to promote ancient peatland carbon release, but, despite the key role of vegetation in carbon dynamics, little is known about how plants influence the source of peatland ecosystem respiration. Here, we address this issue using in situ 14C measurements of ecosystem respiration on an established peatland warming and vegetation manipulation experiment. Results show that warming of approximately 1 °C promotes respiration of ancient peatland carbon (up to 2100 years old) when dwarf-shrubs or graminoids are present, an effect not observed when only bryophytes are present. We demonstrate that warming likely promotes ancient peatland carbon release via its control over organic inputs from vascular plants. Our findings suggest that dwarf-shrubs and graminoids prime microbial decomposition of previously ‘locked-up’ organic matter from potentially deep in the peat profile, facilitating liberation of ancient carbon as CO2. Furthermore, such plant-induced peat respiration could contribute up to 40% of ecosystem CO2 emissions. If consistent across other sub-arctic and arctic ecosystems, this represents a considerable fraction of ecosystem respiration that is currently not acknowledged by global carbon cycle models. Ultimately, greater contribution of ancient carbon to ecosystem respiration may signal the loss of a previously stable peatland carbon pool, creating potential feedbacks to future climate change. This article is protected by copyright. All rights reserved.
5. Verity G. Salmon, Patrick Soucy, Marguerite Mauritz, Gerardo Celis, Susan M. Natali, Michelle C. Mack, Edward A. G. Schuur: Nitrogen availability increases in a tundra ecosystem during five years of experimental permafrost thaw 2015-12-31T02:32:42.133267-05:00 Global Change Biology+
   
   Perennially frozen soil in high latitude ecosystems (permafrost) currently stores 1330-1580 Pg of carbon (C). As these ecosystems warm, the thaw and decomposition of permafrost is expected to release large amounts of C to the atmosphere. Fortunately, losses from the permafrost C pool will be partially offset by increased plant productivity. The degree to which plants are able to sequester C, however, will be determined by changing nitrogen (N) availability in these thawing soil profiles. N availability currently limits plant productivity in tundra ecosystems but plant access to N is expected improve as decomposition increases in speed and extends to deeper soil horizons. In order to evaluate the relationship between permafrost thaw and N availability, we monitored N cycling during five years of experimentally induced permafrost thaw at the Carbon in Permafrost Experimental Heating Research project (CiPEHR). Inorganic N availability increased significantly in response to deeper thaw and greater soil moisture induced by Soil warming. This treatment also prompted a 23% increase in aboveground biomass and a 49% increase in foliar N pools. The sedge Eriophorum vaginatum responded most strongly to warming: this species explained 91% of the change in aboveground biomass during the five year period. Air warming had little impact when applied alone, but when applied in combination with Soil warming, growing season soil inorganic N availability was significantly reduced. These results demonstrate that there is a strong positive relationship between the depth of permafrost thaw and N availability in tundra ecosystems but that this relationship can be diminished by interactions between increased thaw, warmer air temperatures and higher levels of soil moisture. Within five years of permafrost thaw, plants actively incorporate newly available N into biomass but C storage in live vascular plant biomass is unlikely to be greater than losses from deep soil C pools. This article is protected by copyright. All rights reserved.
6. Alison R. Marklein, Joy B. Winbourne, Sara K. Enders, David J. X. Gonzalez, Tiff L. Huysen, Jorge E. Izquierdo, Derrick R. Light, Daniel Liptzin, Kimberley E. Miller, Scott L. Morford, Robert A. Norton, Benjamin Z. Houlton: Mineralization ratios of nitrogen and phosphorus from decomposing litter in temperate versus tropical forests 2015-12-23T01:25:37.085976-05:00 Global Ecology and Biogeography+
   
   Aim Terrestrial ecosystems sequester about 25% of anthropogenic CO2 emissions annually; however, nitrogen (N) and phosphorus (P) limitation of plant productivity and microbial functioning could curtail this key ecosystem service in the future. Our aim is to address variations in nutrient resupply during decomposition – especially whether the N:P ratio of nutrient recycling via mineralization varies within and across diverse forest biomes. Location Global forest ecosystems. Methods We compiled data on in situ litter decomposition experiments (leaf, wood and root) from the primary literature to examine the relationships between net N and P mineralization across temperate versus tropical forests world-wide. We define net nutrient mineralization ratios as the average N:P released from decomposing substrates at a given ecosystem site. Results We show that net N and P mineralization are strongly correlated within biomes, suggesting strong coupling between N and P recycling in forest ecosystems. The net N:P of leaf-litter mineralization is higher in tropical forests than in temperate forests, consistent with latitudinal patterns in foliar and leaf-litter N:P. At the global scale, the N:P of net mineralization tracks, but tends to be lower than that of litter N:P, pointing to preferential P (versus N) mineralization in forest ecosystems. Main conclusions Our results do not support the view that there is a single, globally consistent mineralization N:P ratio. Instead, our results show that the N:P of net mineralization can be predicted by the N:P of litter, offering a method for incorporating P into global-scale models of carbon–nutrient–climate interactions. In addition, these results imply that P is scarce relative to microbial decomposer demands in tropical forests, whereas N and P may be more co-limiting when compared with microbial biomass in the temperate zone.
7. Jianhua Lu, Fuchang Wang, Hailong Liu, Pengfei Lin: Stationary mesoscale eddies, upgradient eddy fluxes, and the anisotropy of eddy diffusivity 2016-01-23T16:27:20.846923-05:00 Geophysical Research Letters+
   
   The mesoscale eddies of which parameterization is needed in coarse-resolution ocean models include not only the transient eddies akin to baroclinic instability but also the stationary eddies associated with topography. By applying a modified Lorenz-type decomposition to the eddy-permitting Southern Ocean State Estimate, we show that the stationary mesoscale eddies contribute a significant part to the total eddy kinetic energy, eddy enstrophy, and the total eddy-induced isopycnal thickness and potential vorticity fluxes. We find that beneath middepth (about 1000 m) the upgradient eddy fluxes, or so-called “negative” eddy diffusivities, are mainly attributed to the stationary mesoscale eddies, whereas the remaining transient eddy diffusivity is positive, for which the Gent and McWilliams (1990) parameterization scheme applies well. A quantitative method of measuring the anisotropy of eddy diffusivity is presented. The effect of stationary mesoscale eddies is one of major sources responsible for the anisotropy of eddy diffusivity. We suggest that an independent parameterization scheme for stationary mesoscale eddies may be needed for coarse-resolution ocean models, although the transient eddies remain the predominant part of mesoscale eddies in the oceans.
8. Ruijia Wang, Yu Jeffrey Gu, Ryan Schultz, Ahyi Kim, Gail Atkinson: Source analysis of a potential hydraulic-fracturing-induced earthquake near Fox Creek, Alberta 2016-01-19T18:02:16.073796-05:00 Geophysical Research Letters+
   
   An earthquake with a reported magnitude of 4.4 (ML) was detected on 13 June 2015 in western central Alberta, Canada. This event was the third felt earthquake this year near Fox Creek, a shale gas exploration region. Our results from full moment tensor inversions of regional broadband data show a strong strike-slip mechanism with near-vertical fault plane solutions. The decomposition of the moment tensor solution is overwhelmingly double couple, while only a modest (∼20%) contribution is attributed to compensated-linear-vector-dipole. The depth of this earthquake is 3–4 km, near the base of the sedimentary layer, and the moment magnitude (M = 3.9) of this event is considerably smaller than the initial reported ML value. The hypocenter location, depth, and mechanism are favorable to a possible association between this earthquake and hydraulic fracturing operations within the Duvernay shale.
9. Ruijie Zeng, Ximing Cai: Climatic and terrestrial storage control on evapotranspiration temporal variability: Analysis of river basins around the world 2016-01-09T05:19:33.246407-05:00 Geophysical Research Letters+
   
   Knowledge of the temporal variability of evapotranspiration (ET) is fundamental to a comprehensive understanding of hydroclimatologic processes under a changing climate and anthropogenic interferences. This study applies a variance decomposition framework to assessing ET interannual and intra-annual variance in 32 large river basins. It is found that climate (precipitation and potential ET) and terrestrial storage play different roles in ET variance with different time scales. At the interannual scale, ET variance is primarily controlled by climatic variability and dampened/enhanced by terrestrial storage change. At the intra-annual scale, the sources of ET variance exhibit a geographic pattern: ET variance is controlled by terrestrial storage change in Middle Asia, by seasonality in the Indian monsoon region, by precipitation and terrestrial storage in low altitude arid regions, and by potential ET and terrestrial storage in boreal regions. Quantifying the components of ET variability will help scientists understand the factors on ET processes under various natural and anthropogenic conditions.
10. Zhongming Gao, Heping Liu, Eric S. Russell, Jianping Huang, Thomas Foken, Steven P. Oncley: Large Eddies Modulating Flux Convergence and Divergence in a Disturbed Unstable Atmospheric Surface Layer 2016-01-22T11:14:08.989359-05:00 Journal of Geophysical Research: Atmospheres+
    
    The effects of large eddies on turbulence structures and flux transport were studied using data collected over a flat cotton field during the Energy Balance Experiment (EBEX-2000) in the San Joaquin Valley of California in August 2000. Flux convergence (FC; larger fluxes at 8.7 m than 2.7 m) and divergence (FD) in latent heat flux (LE) were observed in a disturbed, unstable atmospheric surface layer, and their magnitudes largely departed from the prediction of Monin-Obukhov similarity theory. From our wavelet analysis, it was identified that large eddies affected turbulence structures, scalar distribution, and flux transport differently at 8.7 m and 2.7 m under the FC and FD conditions. Using the ensemble empirical mode decomposition (EEMD), time-series data were decomposed into large eddies and small-scale background turbulence, the time-domain characteristics of large eddies were examined, and the flux contribution by large eddies was also determined quantitatively. The results suggest that large eddies over the frequency range of 0.002 Hz
11. John L. Campbell, Joseph B. Fontaine, Daniel C. Donato: Carbon emissions from decomposition of fire-killed trees following a large wildfire in Oregon, United States 2016-01-12T08:26:28.50207-05:00 Journal of Geophysical Research: Biogeosciences+
    
    A key uncertainty concerning the effect of wildfire on carbon dynamics is the rate at which fire-killed biomass (e.g., dead trees) decays and emits carbon to the atmosphere. We used a ground-based approach to compute decomposition of forest biomass killed, but not combusted, in the Biscuit Fire of 2002, an exceptionally large wildfire that burned over 200,000 ha of mixed conifer forest in southwestern Oregon, USA. A combination of federal inventory data and supplementary ground measurements afforded the estimation of fire-caused mortality and subsequent 10-year decomposition for several functionally distinct carbon pools at 180 independent locations in the burn area. Decomposition was highest for fire-killed leaves and fine roots and lowest for large diameter wood. Decomposition rates varied somewhat among tree species and was only 35% lower for trees still standing than for trees fallen at the time of the fire. We estimate a total of 4.7 Tg C was killed but not combusted in the Biscuit Fire, 85% of which remains 10 years after. Biogenic carbon emissions from fire-killed necromass were estimated to be 1.0, 0.6, and 0.4 Mg C ha-1 yr-1 at 1, 10, and 50 years after the fire, respectively; compared to the one-time pyrogenic emission of nearly 17 Mg C ha-1.
12. Elizabeth E. Webb, Edward A.G. Schuur, Susan M. Natali, Kiva L. Oken, Rosvel Bracho, John P. Krapek, David Risk, Nick R. Nickerson: Increased wintertime CO2 loss as a result of sustained tundra warming 2016-01-04T05:15:43.704417-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Permafrost soils currently store approximately 1672 Pg of carbon (C), but as high latitudes warm, this temperature-protected C reservoir will become vulnerable to higher rates of decomposition. In recent decades, air temperatures in the high latitudes have warmed more than any other region globally, particularly during the winter. Over the coming century, the arctic winter is also expected to experience the most warming of any region or season, yet it is notably understudied. Here we present non-summer season (NSS) CO2 flux data from the Carbon in Permafrost Experimental Heating Research (CiPEHR) project, an ecosystem warming experiment of moist acidic tussock tundra in interior Alaska. Our goals were to quantify the relationship between environmental variables and winter CO2 production, account for subnivean photosynthesis and late fall plant C uptake in our estimate of NSS CO2 exchange, constrain NSS CO2 loss estimates using multiple methods of measuring winter CO2 flux, and quantify the effect of winter soil warming on total NSS CO2 balance. We measured CO2 flux using four methods: two chamber techniques (the snow pit method and one where a chamber is left under the snow for the entire season), eddy covariance, and soda lime adsorption, and found that NSS CO2 loss varied up to 4 fold, depending on the method used. CO2 production was dependent on soil temperature and day of season but atmospheric pressure and air temperature were also important in explaining CO2 diffusion out of the soil. Warming stimulated both ecosystem respiration and productivity during the NSS and increased overall CO2 loss during this period by 14% (this effect varied by year, ranging from 7 to 24%). When combined with the summertime CO2 fluxes from the same site, our results suggest that this sub-arctic tundra ecosystem is shifting away from its historical function as a C sink to a C source.
13. Qi Deng, Dafeng Hui, Junming Wang, Chih-Li Yu, Changsheng Li, Chandra Reddy, Sam Dennis: Assessing the impacts of tillage and fertilization management on nitrous oxide emissions in a cornfield using the DNDC model 2016-01-04T03:22:31.021201-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Quantification and prediction of N2O emissions from croplands under different agricultural management practices are vital for sustainable agriculture and climate change mitigation. We simulated N2O emissions under tillage and no-tillage, and different nitrogen (N) fertilizer types and application methods (i.e. nitrification inhibitor, chicken manure and split applications) in a cornfield using the DeNitrification-DeComposition (DNDC) model. The model was parameterized with field experimental data collected in Nashville, Tennessee under various agricultural management treatments and run for a short term (3 yrs) and a long term (100 yrs). Results showed that the DNDC model could adequately simulate N2O emissions as well as soil properties under different agricultural management practices. The modeled emissions of N2O significantly increased by 35% with tillage, and decreased by 24% with the use of nitrification inhibitor, compared with no-tillage and normal N fertilization. Chicken manure amendment and split applications of N fertilizer had minor impact on N2O emission in a short term, but over a long term (100 yrs) the treatments significantly altered N2O emission (+35%, −10%, respectively). Sensitivity analysis showed that N2O emission was sensitive to mean annual precipitation, mean annual temperature, soil organic carbon, and the amount of total N fertilizer application. Our model results provide valuable information for determining agricultural best management practice to maintain highly productive corn yield while reducing greenhouse gas emissions.
14. Yujie He, Jinyan Yang, Qianlai Zhuang, Jennifer W. Harden, Anthony D. McGuire, Yaling Liu, Gangsheng Wang, Lianhong Gu: Incorporating microbial dormancy dynamics into soil decomposition models to improve quantification of soil carbon dynamics of northern temperate forests 2015-12-22T14:43:34.594535-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Soil carbon dynamics of terrestrial ecosystems play a significant role in the global carbon cycle. Microbial-based decomposition models have seen much growth recently for quantifying this role, yet dormancy as a common strategy used by microorganisms has not usually been represented and tested in these models against field observations. Here we developed an explicit microbial-enzyme decomposition model and examined model performance with and without representation of microbial dormancy at six temperate forest sites of different forest types. We then extrapolated the model to global temperate forest ecosystems to investigate biogeochemical controls on soil heterotrophic respiration and microbial dormancy dynamics at different temporal-spatial scales. The dormancy model consistently produced better match with field-observed heterotrophic soil CO2 efflux (RH) than the no dormancy model. Our regional modeling results further indicated that models with dormancy were able to produce more realistic magnitude of microbial biomass (<2% of soil organic carbon) and soil RH (7.5 ± 2.4 Pg C yr−1). Spatial correlation analysis showed that soil organic carbon content was the dominating factor (correlation coefficient = 0.4–0.6) in the simulated spatial pattern of soil RH with both models. In contrast to strong temporal and local controls of soil temperature and moisture on microbial dormancy, our modeling results showed that soil carbon-to-nitrogen ratio (C:N) was a major regulating factor at regional scales (correlation coefficient = −0.43 to −0.58), indicating scale-dependent biogeochemical controls on microbial dynamics. Our findings suggest that incorporating microbial dormancy could improve the realism of microbial-based decomposition models and enhance the integration of soil experiments and mechanistically based modeling.

CMIP5

1. : The status and challenge of global fire modelling Mon, 25 Jan 2016 23:20:26 +0100 Biogeosciences+
   
   The status and challenge of global fire modelling
   S. Hantson, A. Arneth, S. P. Harrison, D. I. Kelley, I. C. Prentice, S. S. Rabin, S. Archibald, F. Mouillot, S. R. Arnold, P. Artaxo, D. Bachelet, P. Ciais, M. Forrest, P. Friedlingstein, T. Hickler, J. O. Kaplan, S. Kloster, W. Knorr, G. Lasslop, F. Li, S. Mangeon, J. R. Melton, A. Meyn, S. Sitch, A. Spessa, G. R. van der Werf, A. Voulgarakis, and C. Yue
   Biogeosciences Discuss., doi:10.5194/bg-2016-17,2016
   Manuscript under review for BG (discussion: open, 0 comments)
   Our ability to predict the magnitude and geographic pattern of past and future fire impacts rests on our ability to model fire regimes. A large variety of models exist and it is unclear which type of model or degree of complexity is required to model fire adequately at regional to global scales. In this paper we summarise the current state-of-the-art in fire regime modelling and model evaluation, and outline what lessons may be learned from the Fire Model Intercomparison Project – FireMIP.
2. : Rainfall in Iberian transnational basins: a drier future for the Douro, Tagus and Guadiana? 2016-01-16 Climatic Change+
   

   Abstract

   Water scarcity is critical in both Portugal and Spain; therefore, assessing future changes in rainfall for this region is vital. We analyse rainfall projections from climate models in the CMIP5 ensemble for the transnational basins of the Douro, Tagus and Guadiana with the aim of estimating future impacts on water resources. Two downscaling methods (change factor and a variation of empirical quantile mapping) and two ways of analysing future rainfall changes (differences between 30 years periods and trends in transient rainfall) are used. For the 2050s, most models project a reduction in rainfall for all months and for both methods, but there is significant spread between models. Almost all significant seasonal trends identified from 1961 to 2100 are negative. For annual rainfall, only 3 (2) models show no significant trends for the Douro/Tagus (Guadiana), while the rest show negative trends up to −6 % per decade. Reductions in rainfall are projected for spring and autumn by almost all models, both downscaling methods and both ways of analysing changes. This increases the confidence in the projection of the lengthening of the dry season which could have serious impacts for agriculture, water supply and forest fires in the region. A considerable part of the climate model disagreement in the projection of future rainfall changes for the 2050s is shown to be due to the use of 30 year intervals, leading to the conclusion that such intervals are too short to be used under conditions of high inter-annual variability as found in the Iberian Peninsula.

3. Stefanos Mystakidis, Edouard L. Davin, Nicolas Gruber, Sonia I. Seneviratne: Constraining future terrestrial carbon cycle projections using observation-based water and carbon flux estimates 2016-01-06T01:47:17.097707-05:00 Global Change Biology+
   
   The terrestrial biosphere is currently acting as a sink for about a third of the total anthropogenic CO2 emissions. However, the future fate of this sink in the coming decades is very uncertain, as current Earth System Models (ESMs) simulate diverging responses of the terrestrial carbon cycle to upcoming climate change. Here, we use observation-based constraints of water and carbon fluxes to reduce uncertainties in the projected terrestrial carbon cycle response derived from simulations of ESMs conducted as part of the 5th phase of the Coupled Model Intercomparison Project (CMIP5). We find in the ESMs a clear linear relationship between present-day Evapotranspiration (ET) and Gross Primary Productivity (GPP), as well as between these present-day fluxes and projected changes in GPP, thus providing an emergent constraint on projected GPP. Constraining the ESMs based on their ability to simulate present-day ET and GPP leads to a substantial decrease of the projected GPP and to a ca. 50% reduction of the associated model spread in GPP by the end of the century. Given the strong correlation between projected changes in GPP and in NBP in the ESMs, applying the constraints on Net Biome Productivity (NBP) reduces the model spread in the projected land sink by more than 30% by 2100. Moreover, the projected decline in the land sink is at least doubled in the constrained ensembles and the probability that the terrestrial biosphere is turned into a net carbon source by the end of the century is strongly increased. This indicates that the decline in the future land carbon uptake might be stronger than previously thought, which would have important implications for the rate of increase of the atmospheric CO2 concentration and for future climate change. This article is protected by copyright. All rights reserved.
4. Philip C. Reid, Renata E. Hari, Grégory Beaugrand, David M. Livingstone, Christoph Marty, Dietmar Straile, Jonathan Barichivich, Eric Goberville, Rita Adrian, Yasuyuki Aono, Ross Brown, James Foster, Pavel Groisman, Pierre Hélaouët, Huang-Hsiung Hsu, Richard Kirby, Jeff Knight, Alexandra Kraberg, Jianping Li, Tzu-Ting Lo, Ranga B. Myneni, Ryan P. North, J. Alan Pounds, Tim Sparks, René Stübi, Yongjun Tian, Karen H. Wiltshire, Dong Xiao, Zaichun Zhu: Global impacts of the 1980s regime shift 2015-11-23T23:32:53.046168-05:00 Global Change Biology+
   
   Despite evidence from a number of Earth systems that abrupt temporal changes known as regime shifts are important, their nature, scale and mechanisms remain poorly documented and understood. Applying principal component analysis, change-point analysis and a sequential t-test analysis of regime shifts to 72 time series, we confirm that the 1980s regime shift represented a major change in the Earth's biophysical systems from the upper atmosphere to the depths of the ocean and from the Arctic to the Antarctic, and occurred at slightly different times around the world. Using historical climate model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) and statistical modelling of historical temperatures, we then demonstrate that this event was triggered by rapid global warming from anthropogenic plus natural forcing, the latter associated with the recovery from the El Chichón volcanic eruption. The shift in temperature that occurred at this time is hypothesized as the main forcing for a cascade of abrupt environmental changes. Within the context of the last century or more, the 1980s event was unique in terms of its global scope and scale; our observed consequences imply that if unavoidable natural events such as major volcanic eruptions interact with anthropogenic warming unforeseen multiplier effects may occur.
5. Cheng Sun, Jianping Li, Ruiqiang Ding: Strengthening relationship between ENSO and western Russian summer surface temperature 2016-01-21T17:59:41.800607-05:00 Geophysical Research Letters+
   
   Western Russia (WR) experienced an extremely hot summer in 2010 that caused tremendous social and economic losses. The WR summer surface temperature (WRST) in the observational record is characterized by substantial interannual variability superimposed on the secular warming trend. Analysis of the 130 year observational record reveals that a strong and significant inverse relationship between WRST interannual variability and the tropical El Niño–Southern Oscillation (ENSO) has emerged during the past three decades. The ENSO influence on the summer extratropical atmospheric circulation was weak before 1980 but became strong and significant afterward, showing a structure similar to the East Atlantic/WR teleconnection pattern. This pattern is associated with rising/falling upper level geopotential height over WR, which leads to the warming/cooling of surface and tropospheric air temperatures. Numerical simulations from a theoretical linear baroclinic model and Atmospheric Model Intercomparison Project models further suggest that the enhancement of the ENSO teleconnection to WR may be attributable to a change in the ENSO-related tropical thermal forcing. A tripole-type rainfall anomaly pattern over tropical Pacific and Atlantic is found to be associated with ENSO in the past three decades. The tripole heating pattern can excite a Rossby wave that extends northwestward reaching WR and is necessary for the strong influence of ENSO on WR summer climate.
6. Claudia Timmreck, Holger Pohlmann, Sebastian Illing, Christopher Kadow: The impact of stratospheric volcanic aerosol on decadal-scale climate predictions 2016-01-19T17:44:26.65607-05:00 Geophysical Research Letters+
   
   To understand the impact of volcanic aerosol on multiyear seasonal and decadal climate predictions, we performed Coupled Model Intercomparison Project Phase 5-type hindcasts without volcanic aerosol using the German Mittelfristige Klimaprognosen prediction system and compared them to the corresponding simulations including aerosols. Our results show that volcanic aerosol significantly affects the prediction skill for global mean surface air temperature in the first five years after strong volcanic eruptions. Also, on the regional scale a volcanic imprint on decadal-scale variability is detectable. Neglecting volcanic aerosol leads to a reduced prediction skill over the tropical and subtropical Atlantic, Indic, and west Pacific but to an improvement over the tropical east Pacific, where the model has in general no skill. Multiseasonal differences in the skill for seasonal mean temperatures are evident over Continental Europe with significant skill loss due to neglection of volcanic aerosol in boreal winter over central Europe, Scandinavia and over southeastern Europe, and the East Mediterranean in boreal summer.
7. N. Herold, L. V. Alexander, M. G. Donat, S. Contractor, A. Becker: How much does it rain over land? 2016-01-09T22:26:11.29921-05:00 Geophysical Research Letters+
   
   Despite the availability of several observationally constrained data sets of daily precipitation based on rain gauge measurements, remote sensing, and/or reanalyses, we demonstrate a large disparity in the quasi-global land mean of daily precipitation intensity. Surprisingly, the magnitude of this spread is similar to that found in the Coupled Model Intercomparison Project Phase 5 (CMIP5). A weakness of reanalyses and CMIP5 models is their tendency to over simulate wet days, consistent with previous studies. However, there is no clear agreement within and between rain gauge and remotely sensed data sets either. This large discrepancy highlights a shortcoming in our ability to characterize not only modeled daily precipitation intensities but even observed precipitation intensities. This shortcoming is partially reconciled by an appreciation of the different spatial scales represented in gridded data sets of in situ precipitation intensities and intensities calculated from gridded precipitation. Unfortunately, the spread in intensities remains large enough to prevent us from satisfactorily determining how much it rains over land.
8. J. Scott Hosking, Andrew Orr, Thomas J. Bracegirdle, John Turner: Future circulation changes off West Antarctica: Sensitivity of the Amundsen Sea Low to projected anthropogenic forcing 2016-01-09T22:32:24.349324-05:00 Geophysical Research Letters+
   
   The Amundsen Sea Low (ASL) is a major driver of West Antarctic climate variability, with the potential to accelerate the loss of glacial ice. Using the 11 global climate models which most reliably simulate the seasonality in ASL location, we assess the ASL sensitivity to projected future changes using the CMIP5 historical (1951–2000) and representative concentration pathway experiment RCP8.5 (2051–2100). For the first time, we show that the future ASL will likely migrate poleward in summer (December, January, and February) and autumn (March, April, and May), and eastward in autumn and winter (June, July, and August). The autumn-winter changes drive colder southerly winds over the Ross Sea and warmer northerly winds toward the Antarctic Peninsula. This is consistent with recent trends in ERA-Interim reanalysis meridional winds (1979–2014) and reconstructed temperature (1957–2006), suggesting that the impact of anthropogenic forcing on the ASL is likely to play an important role on both past and future patterns of West Antarctic climate variability.
9. Margot Bador, Laurent Terray, Julien Boé: Emergence of human influence on summer record-breaking temperatures over Europe 2016-01-14T15:54:35.41235-05:00 Geophysical Research Letters+
   
   Observational analysis of Europe summer record-breaking temperatures suggests that their occurrence differs from that expected in a stationary climate since the late 1980s. The observed cold and warm record evolution is well simulated by the ensemble mean of 27 coupled models from the Coupled Model Intercomparison Project phase 5 (CMIP5). We find that this evolution is still today within the range of internal variability derived from CMIP5 preindustrial simulations. We then estimate a time of emergence of the summer record anthropogenic influence in a world under a business as usual greenhouse gas emission scenario. We suggest a time of emergence around 2020 for the cold records and 2030 for the warm ones with an uncertainty of ± 20 years. By 2100, the multimodel ensemble mean indicates a tenfold increase of the number of warm records compared to the first half of the twentieth century and the quasi-disappearance of cold records.
10. Yoo-Geun Ham, Jong-Seong Kug: ENSO amplitude changes due to greenhouse warming in CMIP5: Role of mean tropical precipitation in the twentieth century 2016-01-14T16:02:48.034174-05:00 Geophysical Research Letters+
    
    This study examines the relationship between the intermodel diversities of the present climate climatology and those of El Niño–Southern Oscillation (ENSO) amplitude change under global warming in the Coupled Model Intercomparison Project Phase 5 (CMIP5) models. The models with increased ENSO amplitude under greenhouse warming (i.e., “ENSO-amplified models”) tend to simulate a twentieth century stronger climatological Intertropical Convergence Zone and South Pacific Convergence Zone over the central-eastern Pacific that are located farther away from the equator during boreal spring. Moisture budget analysis indicates that those climatological differences lead to stronger positive climatological precipitation change over the off-equatorial central-eastern Pacific under greenhouse warming. The stronger positive climatological precipitation change enhances the air-sea coupling strength over the central-eastern Pacific, which leads to increase the ENSO amplitude.
11. Jinyoung Rhee et al: Future Changes in Drought Characteristics: Regional Analysis for South Korea under CMIP5 Projections Thu, 31 Dec 2015 20:27:57 GMT Journal of Hydrometeorology+
    
    Journal of Hydrometeorology, Volume 17, Issue 1, Page 437-451, January 2016.
    
12. João A. Santos, Margarida Belo-Pereira, Helder Fraga, Joaquim G. Pinto: Understanding climate change projections for precipitation over Western Europe with a weather typing approach 2016-01-11T18:02:13.752837-05:00 Journal of Geophysical Research: Atmospheres+
    
    Precipitation over Western Europe (WE) is projected to increase (decrease) roughly northward (equatorward) of 50°N during the twenty first century. These changes are generally attributed to alterations in the regional large-scale circulation, e.g. jet stream, cyclone activity and blocking frequencies. A novel weather typing within the sector (30°W–10°E, 25–70°N) is used for a more comprehensive dynamical interpretation of precipitation changes. A k-means clustering on daily mean sea level pressure was undertaken for ERA-Interim reanalysis (1979–2014). Eight weather types are identified: S1, S2, S3 (summertime types), W1, W2, W3 (wintertime types), B1 and B2 (blocking-like types). Their distinctive dynamical characteristics allow identifying the main large-scale precipitation-driving mechanisms. Simulations with 22 CMIP5 models for recent climate conditions show biases in reproducing the observed seasonality of weather types. In particular, an overestimation of weather type frequencies associated with zonal airflow is identified. Considering projections following the RCP8.5 scenario over 2071–2100, the frequencies of the three driest types (S1, B2 and W3) are projected to increase (mainly S1, +4%) in detriment of the rainiest types, particularly W1 (-3%). These changes explain most of the precipitation projections over WE. However, a weather type-independent background signal is identified (increase/decrease in precipitation over northern/southern WE), suggesting modifications in precipitation-generating processes and/or model inability to accurately simulate these processes. Despite these caveats in the precipitation scenarios for WE, which must be duly taken into account, our approach permits a better understanding of the projected trends for precipitation over WE.
13. Libo Wang, Jason N. S. Cole, Paul Bartlett, Diana Verseghy, Chris Derksen, Ross Brown, Knut Salzen: Investigating the spread in surface albedo for snow covered forests in CMIP5 models 2016-01-07T06:55:24.318643-05:00 Journal of Geophysical Research: Atmospheres+
    
    This study investigates the role of leaf/plant area index (LAI/PAI) specification on the large spread of winter albedo simulated by climate models. To examine the sensitivity of winter albedo to LAI, we perform a sensitivity analysis using two methods commonly used to compute albedo in snow-covered forests as well as diagnostic calculations within version 4.2 of the Canadian Atmospheric Model for which PAI is systematically varied. The results show that the simulated albedo is very sensitive to negative PAI biases, especially for smaller PAI values. The LAI and surface albedo of boreal forests in the presence of snow simulated by the Coupled Model Intercomparison Project Phase 5 (CMIP5) models are evaluated using satellite observations. The evaluation of CMIP5 models suggest that inaccurate tree cover fraction due to improper plant functional type specification or erroneous LAI parameterization in some models explains, in part, an observed positive bias in winter albedo over boreal forest regions of the Northern Hemisphere. This contributes to a large intermodel spread in simulated surface albedo in the presence of snow over these regions and is largely responsible for uncertainties in simulated snow-albedo feedback strength. Errors are largest (+20-40 %) in models with large underestimation of LAI but are typically within ±15% when simulated LAI is within the observed range. This study underscores the importance of accurate representation of vegetation distribution and parameters in realistic simulation of surface albedo.
14. Jung Choi, Jian Lu, Seok-Woo Son, Dargan M. W. Frierson, Jin-Ho Yoon: Uncertainty in future projections of the North Pacific subtropical high and its implication for California winter precipitation change 2016-01-27T11:06:58.563957-05:00 Journal of Geophysical Research: Atmospheres+
    
    This study examines future projections of sea level pressure change in the North Pacific and its impact on winter precipitation changes in California. The multimodel analysis, based on the Coupled Model Intercomparison Project phase 5 models under the Representative Concentration Pathway 8.5 scenario, shows a robust sea level pressure change in the late 21st century over the western North Pacific in which both the Aleutian Low and the North Pacific subtropical high (NPSH) shift poleward in concert with a widening of the Hadley cell. This change is partly explained by a systematic increase of static stability in the subtropics. Despite its robustness, the projected NPSH changes over the eastern North Pacific exhibit a substantial intermodel spread, contributing as a cause for uncertain projections of precipitation changes in California. This intermodel spread in the eastern North Pacific is associated with a Pacific Decadal Oscillation-like surface temperature change in the western North Pacific and the resulting meridional temperature gradient change. This study points to a major source of uncertainty for the response of winter precipitation to global warming over the West Coast of North America: atmosphere-ocean coupling in the North Pacific.
15. Ruth Lorenz, Daniel Argüeso, Markus G. Donat, Andrew J. Pitman, Bart van den Hurk, Alexis Berg, David M. Lawrence, Frédérique Chéruy, Agnès Ducharne, Stefan Hagemann, Arndt Meier, P. C. D. Milly, Sonia I. Seneviratne: Influence of land-atmosphere feedbacks on temperature and precipitation extremes in the GLACE-CMIP5 ensemble 2016-01-19T11:47:42.565278-05:00 Journal of Geophysical Research: Atmospheres+
    
    We examine how soil moisture variability and trends affect the simulation of temperature and precipitation extremes in six global climate models using the experimental protocol of the Global Land-Atmosphere Coupling Experiment of the Coupled Model Intercomparison Project, Phase 5 (GLACE-CMIP5). This protocol enables separate examinations of the influences of soil moisture variability and trends on the intensity, frequency, and duration of climate extremes by the end of the 21st century under a business-as-usual (Representative Concentration Pathway 8.5) emission scenario. Removing soil moisture variability significantly reduces temperature extremes over most continental surfaces, while wet precipitation extremes are enhanced in the tropics. Projected drying trends in soil moisture lead to increases in intensity, frequency, and duration of temperature extremes by the end of the 21st century. Wet precipitation extremes are decreased in the tropics with soil moisture trends in the simulations, while dry extremes are enhanced in some regions, in particular the Mediterranean and Australia. However, the ensemble results mask considerable differences in the soil moisture trends simulated by the six climate models. We find that the large differences between the models in soil moisture trends, which are related to an unknown combination of differences in atmospheric forcing (precipitation, net radiation), flux partitioning at the land surface, and how soil moisture is parameterized, imply considerable uncertainty in future changes in climate extremes.
16. Yeyi Liu, Dabang Jiang: Mid-Holocene permafrost: Results from CMIP5 simulations 2016-01-14T11:29:47.068964-05:00 Journal of Geophysical Research: Atmospheres+
    
    Distribution of frozen ground and active layer thickness in the Northern Hemisphere during the mid-Holocene (MH) and differences with respect to the preindustrial (PI) were investigated here using the Coupled Model Intercomparison Project Phase 5 (CMIP5) models. Two typical diagnostic methods, respectively, based on soil temperature (Ts based; a direct method) and air temperature (Ta based; an indirect method) were employed to classify categories and extents of frozen ground. In relation to orbitally induced changes in climate and in turn freezing and thawing indices, the MH permafrost extent was 20.5% (1.8%) smaller than the PI, whereas seasonally frozen ground increased by 9.2% (0.8%) in the Northern Hemisphere according to the Ts-based (Ta-based) method. Active layer thickness became larger, but by ≤ 1.0 m in most of permafrost areas during the MH. Intermodel disagreement remains within areas of permafrost boundary by both the Ts-based and Ta-based results, with the former demonstrating less agreement among the CMIP5 models because of larger variation in land model abilities to represent permafrost processes. However, both the methods were able to reproduce the MH relatively degenerated permafrost and increased active layer thickness (although with smaller magnitudes) as observed in data reconstruction. Disparity between simulation and reconstruction was mainly found in the seasonally frozen ground regions at low to middle latitudes, where the reconstruction suggested a reduction of seasonally frozen ground extent to the north, whereas the simulation demonstrated a slightly expansion to the south for the MH compared to the PI.
17. Noel R. Aloysius, Justin Sheffield, James E. Saiers, Haibin Li, Eric F. Wood: Evaluation of historical and future simulations of precipitation and temperature in central Africa from CMIP5 climate models 2016-01-08T17:32:24.495144-05:00 Journal of Geophysical Research: Atmospheres+
    
    Global and regional climate change assessments rely heavily on the general circulation model (GCM) outputs such as provided by the Coupled Model Intercomparison Project phase 5 (CMIP5). Here we evaluate the ability of 25 CMIP5 GCMs to simulate historical precipitation and temperature over central Africa and assess their future projections in the context of historical performance and intermodel and future emission scenario uncertainties. We then apply a statistical bias correction technique to the monthly climate fields and develop monthly downscaled fields for the period of 1948–2099. The bias-corrected and downscaled data set is constructed by combining a suite of global observation and reanalysis-based data sets, with the monthly GCM outputs for the 20th century, and 21st century projections for the medium mitigation (representative concentration pathway (RCP)45) and high emission (RCP85) scenarios. Overall, the CMIP5 models simulate temperature better than precipitation, but substantial spatial heterogeneity exists. Many models show limited skill in simulating the seasonality, spatial patterns, and magnitude of precipitation. Temperature projections by the end of the 21st century (2070–2099) show a robust warming between 2 and 4°C across models, whereas precipitation projections vary across models in the sign and magnitude of change (−9% to 27%). Projected increase in precipitation for a subset of models (single model ensemble (SME)) identified based on performance metrics and causal mechanisms are slightly higher compared to the full multimodel ensemble (MME) mean; however, temperature projections are similar between the two ensemble means. For the near-term (2021–2050), neither the historical performance nor choice of models is related to the precipitation projections, indicating that natural variability dominated any signal. With fewer models, the “blind” MME approach will have larger uncertainties in future precipitation projections compared to projections by the SME models. We propose the latter a better approach in regions that lack quality climate observations. Our analyses also show that the choice of model and emission scenario dominate the uncertainty in precipitation projections, whereas the emission scenario dominates the temperature projections. Although our analyses are done for central Africa, the final Bias-Corrected Spatially Downscaled data set is available for global land areas. The framework for climate change assessment and the data will be useful for a variety of climate assessment, impact, and adaptation studies.
18. J.-L. F. Li, Wei-Liang Lee, Jia-Yuh Yu, Glynn Hulley, Eric Fetzer, Yi-Chun Chen, Yi-Hui Wang: The impacts of precipitating hydrometeors radiative effects on land surface temperature in contemporary GCMs using satellite observations 2016-01-07T15:47:55.910684-05:00 Journal of Geophysical Research: Atmospheres+
    
    An accurate representation of the land surface temperature (LST) climatology of the coupled land-atmosphere system has strong implications for the reliability of projected land surface processes and their variability inferred by the global climate models (GCMs) contributed to the Intergovernmental Panel on Climate Change CMIP5. We have identified a substantial underestimation of the total ice water path and biases of surface radiation budget commonly seen in the CMIP models which are highly correlated to the biases of LST over land. One of the potential causes of the CMIP model biases is the missing representation of large frozen precipitating hydrometeors and their radiative effects (i.e., snow) in all CMIP3 and most CMIP5 models. We examine the impacts of snow on the radiation, all-sky and clear-sky LST, and air-land heat fluxes to explore the implications to the common biases in CMIP models by performing sensitivity experiments with and without snow radiation effects using the National Center for Atmospheric Research Community Earth System Model version 1. It is found that an exclusion of the snow radiative effects the CESM1 generates the LST biases (up to 2–3 K) in the midlatitude and high latitude, in particular, in December, January, and February (DJF). All-sky and clear-sky LST in model simulations are found to be too cold and are mainly due to underestimated downward surface (longwave) LW radiation in DJF, which is consistent with those in CMIP models. The correlation between the changes of the LST and downward surface LW radiation is very high both in summer and winter seasons.
19. F. M. Woldemeskel, A. Sharma, B. Sivakumar, R. Mehrotra: Quantification of precipitation and temperature uncertainties simulated by CMIP3 and CMIP5 models 2016-01-05T11:43:22.678142-05:00 Journal of Geophysical Research: Atmospheres+
    
    Assessment of climate change impacts on water resources is extremely challenging, due to the inherent uncertainties in climate projections using global climate models (GCMs). Three main sources of uncertainties can be identified in GCMs, i.e., model structure, emission scenario, and natural variability. The recently released fifth phase of the Coupled Model Intercomparison Project (CMIP5) includes a number of advances relative to its predecessor (CMIP3), in terms of the spatial resolution of models, list of variables, and concept of specifying future radiative forcing, among others. The question, however, is do these modifications indeed reduce the uncertainty in the projected climate at global and/or regional scales? We address this question by quantifying and comparing uncertainty in precipitation and temperature from 6 CMIP3 and 13 CMIP5 models. Uncertainty is quantified using the square root of error variance, which specifies uncertainty as a function of time and space, and decomposes the total uncertainty into its three constituents. The results indicate a visible reduction in the uncertainty of CMIP5 precipitation relative to CMIP3 but no significant change for temperature. For precipitation, the GCM uncertainty is found to be larger in regions of the world that receive heavy rainfall, as well as mountainous and coastal areas. For temperature, however, uncertainty is larger in extratropical cold regions and lower elevation areas.

Productivity

1. : Sources, cycling and export of nitrogen on the Greenland Ice Sheet Mon, 25 Jan 2016 23:20:26 +0100 Biogeosciences+
   
   Sources, cycling and export of nitrogen on the Greenland Ice Sheet
   J. L. Wadham, J. Hawkings, J. Telling, D. Chandler, J. Alcock, E. Lawson, P. Kaur, E. A. Bagshaw, M. Tranter, A. Tedstone, and P. Nienow
   Biogeosciences Discuss., doi:10.5194/bg-2015-484,2016
   Manuscript under review for BG (discussion: open, 0 comments)
   Fjord and continental shelf environments in the Polar Regions are host to some of the planet’s most productive ecosystems, and support economically important fisheries. Their productivity, however, is often critically dependent upon nutrient supply from up-stream terrestrial environments delivered via river systems. One of the most extensive glacially-fed coastal ecosystems is that bordering the Greenland Ice Sheet. The future primary productivity of this marine ecosystem, however, is uncertain. A potential increase in primary productivity driven by reduced sea ice extent and associated increased light levels may be curtailed by insufficient nutrient supply, and specifically nitrogen. Research on small valley glaciers indicates that glaciers are important sources of nitrogen to downstream environments. However, no data exists from ice sheet systems such as Greenland. Time series of nitrogen concentrations in runoff are documented from a large Greenland glacier, demonstrating seasonally elevated fluxes to the ocean. Fluxes are highest in mid-summer, when nitrogen limitation is commonly reported in coastal waters. It is estimated that approximately half of the glacially-exported nitrogen is sourced from microbial activity within glacial sediments at the surface and bed of the ice sheet, doubling nitrogen fluxes in runoff. Summer dissolved inorganic nitrogen fluxes from the Greenland Ice Sheet (30–40 Gg) are a similar order of magnitude to those from a large Arctic river (40 Gg, Holmes et al., 2012). Nitrogen yields from the ice sheet (100–160 kg TDN km⁻² a⁻¹), however, are approximately double those from Arctic riverine catchments. We assert that this ice sheet nitrogen subsidy to Arctic coastal ecosystems may be important for understanding coastal biodiversity, productivity and fisheries, and should be considered in future biogeochemical modelling studies of coastal marine productivity in the Arctic regions.
2. : Degradation of net primary production in a semi-arid rangeland Thu, 21 Jan 2016 23:20:26 +0100 Biogeosciences+
   
   Degradation of net primary production in a semi-arid rangeland
   H. Jackson and S. D. Prince
   Biogeosciences Discuss., doi:10.5194/bg-2015-634,2016
   Manuscript under review for BG (discussion: open, 0 comments)
   Anthropogenic land degradation affects many terrestrial processes including reductions of net primary productivity. This study identifies degradation through spatial comparison of areas with similar growth potentials in six semi-arid river basins in Australia using satellite data from 2000 to 2013. Varying severities and rates of degradation were detected across the basins, most linked to indirect management. Evidence of permanent degradation was found despite an overall trend of greening.
3. : Climate risk management requires explicit representation of societal trade-offs 2016-01-22 Climatic Change+
   

   Abstract

   Strategies for managing climate-change risks impact diverse stakeholder groups that possess potentially conflicting preferences. Basic physics and economics suggest that reconciling all of these preference conflicts may not be possible. Moreover, different climate risk management strategies can yield diverse and potentially severe impacts across different global stakeholders. These preference conflicts and their uncertain impacts require an explicit understanding of the trade-offs that emerge across different risk management strategies. Traditionally, integrated assessment models (IAMs) typically aggregate the stakeholders’ preferences across the entire globe into a single, a priori defined utility function. This framing hides climate risk management trade-offs as well as the inherent stakeholder compromises implicit to the resulting single “optimal” expected utility solution. Here, we analyze a simple IAM to quantify and visualize the multidimensional trade-offs among four objectives representing global concerns: (i) global economic productivity, (ii) reliable temperature stabilization, (iii) climate damages, and (iv) abatement costs. We quantify and visualize the trade-offs across these objectives and demonstrate how a traditional optimal expected utility policy implicitly eliminates many relevant policy pathways. Explicit trade-off analysis provides a richer context for exploring conflicting global policy preferences and clarifies the implications of alternative climate risk mitigation policies to better inform negotiated compromises.

4. Andrea D. Almeida Castanho, David Galbraith, Ke Zhang, Michael T. Coe, Marcos H. Costa, Paul Moorcroft: Changing Amazon biomass and the role of atmospheric CO2 concentration, climate, and land use 2016-01-19T18:44:00.211132-05:00 Global Biogeochemical Cycles+
   
   The Amazon tropical evergreen forest is an important component of the global carbon budget. Its forest floristic composition, structure, and function are sensitive to changes in climate, atmospheric composition, and land use. In this study biomass and productivity simulated by three dynamic global vegetation models (Integrated Biosphere Simulator, Ecosystem Demography Biosphere Model, and Joint UK Land Environment Simulator) for the period 1970–2008 are compared with observations from forest plots (Rede Amazónica de Inventarios Forestales). The spatial variability in biomass and productivity simulated by the DGVMs is low in comparison to the field observations in part because of poor representation of the heterogeneity of vegetation traits within the models. We find that over the last four decades the CO2 fertilization effect dominates a long-term increase in simulated biomass in undisturbed Amazonian forests, while land use change in the south and southeastern Amazonia dominates a reduction in Amazon aboveground biomass, of similar magnitude to the CO2 biomass gain. Climate extremes exert a strong effect on the observed biomass on short time scales, but the models are incapable of reproducing the observed impacts of extreme drought on forest biomass. We find that future improvements in the accuracy of DGVM predictions will require improved representation of four key elements: (1) spatially variable plant traits, (2) soil and nutrients mediated processes, (3) extreme event mortality, and (4) sensitivity to climatic variability. Finally, continued long-term observations and ecosystem-scale experiments (e.g. Free-Air CO2 Enrichment experiments) are essential for a better understanding of the changing dynamics of tropical forests.
5. David W. Crawford, Shea N. Wyatt, Ian A. Wrohan, Adrián O. Cefarelli, Karina E. Giesbrecht, Brianne Kelly, Diana E. Varela: Low particulate carbon to nitrogen ratios in marine surface waters of the Arctic 2015-12-05T10:13:50.40207-05:00 Global Biogeochemical Cycles+
   
   During the Canada's Three Oceans and Joint Ocean Ice Study projects in the summers of 2007 and 2008, we measured particulate organic carbon to nitrogen ratios (POC:PON) throughout the euphotic zone in Subarctic and Arctic waters. Average depth-integrated values (2.65 ± 0.19) in the Beaufort Sea and Canada Basin (BS-CB domain) were much lower than both the Redfield ratio (6.6) and the average ratios (3.9 to 5.6) measured across other Arctic-Subarctic domains. Average uptake ratios of C and N (ρC:ρN) were also lower (0.87 ± 0.14) in BS-CB than in the other four domains (2.10 to 3.51). Decreasing POC:PON ratios were associated with low concentrations of phytoplankton C, reduced abundance of biogenic silica (bSiO2), a smaller relative contribution of the >5 µm fraction to total chlorophyll a and a larger relative contribution of small flagellates (<8 µm) to total phytoplankton C. In the subsurface chlorophyll a maximum (SCM) within the BS-CB domain, phytoplankton C represented only ~13% of POC; and therefore, the presence of heterotrophic microbes may have decreased POC:PON. These ratios are supported by data obtained during other Arctic programs in 2006, 2008, and 2009. Previous work has suggested a link between freshening of surface waters and increasing dominance of picophytoplankton and bacterioplankton in the Canada Basin, and the low POC:PON ratios measured during this study may be a consequence of this shift. Our results have ramifications for the conversion between C- and N-based estimates of primary productivity, and for biogeochemical modeling of marine Arctic waters.
6. Kristina J. Anderson-Teixeira, Maria M. H. Wang, Jennifer C. McGarvey, David S. LeBauer: Carbon dynamics of mature and regrowth tropical forests derived from a pantropical database (TropForC-db) 2016-01-21T03:19:04.279301-05:00 Global Change Biology+
   
   Tropical forests play a critical role in the global carbon (C) cycle, storing ~45% of terrestrial C and constituting the largest component of the terrestrial C sink. Despite their central importance to the global C cycle, their ecosystem-level C cycles are not as well characterized as those of extra-tropical forests, and knowledge gaps hamper efforts to quantify C budgets across the tropics and to model tropical forest- climate interactions. To advance understanding of C dynamics of pantropical forests, we compiled a new database, the Tropical Forest C database (TropForC-db), which contains data on ground-based measurements of ecosystem-level C stocks and annual fluxes along with disturbance history. This database currently contains 3,568 records from 845 plots in 178 geographically distinct areas, making it the largest and most comprehensive database of its type. Using TropForC-db, we characterized C stocks and fluxes for young, intermediate-aged, and mature forests. Relative to existing C budgets of extra-tropical forests, mature tropical broadleaf evergreen forests had substantially higher gross primary productivity (GPP) and ecosystem respiration (Reco), their autotropic respiration (Ra) consumed a larger proportion (~67%) of GPP, and their woody stem growth (ANPPstem) represented a smaller proportion of net primary productivity (NPP, ~32%) or GPP (~9%). In regrowth stands, aboveground biomass increased rapidly during the first 20 years following stand-clearing disturbance, with slower accumulation following agriculture and in deciduous forests, and continued to accumulate at a slower pace in forests aged 20-100 years. Most other C stocks likewise increased with stand age, while potential to describe age trends in C fluxes was generally data-limited. We expect that TropForC-db will prove useful for model evaluation and for quantifying the contribution of forests to the global C cycle. The database version associated with this publication is archived in Dryad (DOI:10.5061/dryad.t516f) and a dynamic version is maintained at https://github. com/forc-db. This article is protected by copyright. All rights reserved.
7. S. Uthicke, T. Ebert, M. Liddy, C. Johansson, K. E. Fabricius, M. Lamare: Echinometra sea urchins acclimatised to elevated pCO2 at volcanic vents outperform those under present-day pCO2 conditions 2016-01-13T07:05:31.184249-05:00 Global Change Biology+
   
   Rising atmospheric CO2 concentrations will significantly reduce ocean pH during the 21st century (ocean acidification, OA). This may hamper calcification in marine organisms such as corals and echinoderms, as shown in many laboratory-based experiments. Sea urchins are considered highly vulnerable to OA. We studied an Echinometra species on natural volcanic CO2 vents in Papua New Guinea, where they are CO2-acclimatized and also subjected to secondary ecological changes from elevated CO2. Near the vent site, the urchins experienced large daily variations in pH (> 1 unit) and pCO2 (> 2000 ppm) and average pH values (pHT 7.73) much below those expected under the most pessimistic future emission scenarios. Growth was measured over a 17-month period using tetracycline tagging of the calcareous feeding lanterns. Average-sized urchins grew more than twice as fast at the vent compared with those at an adjacent control site, and assumed larger sizes at the vent compared to the control site and two other sites at another reef near-by. A small reduction in gonad weight was detected at the vents, but no differences in mortality, respiration, or degree of test calcification were detected between urchins from vent and control populations. Thus, urchins did not only persist but actually ‘thrived’ under extreme CO2 conditions. We suggest an ecological basis for this response: increased algal productivity under increased pCO2 provided more food at the vent, resulting in higher growth rates. The wider implication of our observation is that laboratory studies on non-acclimatized specimens, which typically do not consider ecological changes, can lead to erroneous conclusions on responses to global change. This article is protected by copyright. All rights reserved.
8. Stefanos Mystakidis, Edouard L. Davin, Nicolas Gruber, Sonia I. Seneviratne: Constraining future terrestrial carbon cycle projections using observation-based water and carbon flux estimates 2016-01-06T01:47:17.097707-05:00 Global Change Biology+
   
   The terrestrial biosphere is currently acting as a sink for about a third of the total anthropogenic CO2 emissions. However, the future fate of this sink in the coming decades is very uncertain, as current Earth System Models (ESMs) simulate diverging responses of the terrestrial carbon cycle to upcoming climate change. Here, we use observation-based constraints of water and carbon fluxes to reduce uncertainties in the projected terrestrial carbon cycle response derived from simulations of ESMs conducted as part of the 5th phase of the Coupled Model Intercomparison Project (CMIP5). We find in the ESMs a clear linear relationship between present-day Evapotranspiration (ET) and Gross Primary Productivity (GPP), as well as between these present-day fluxes and projected changes in GPP, thus providing an emergent constraint on projected GPP. Constraining the ESMs based on their ability to simulate present-day ET and GPP leads to a substantial decrease of the projected GPP and to a ca. 50% reduction of the associated model spread in GPP by the end of the century. Given the strong correlation between projected changes in GPP and in NBP in the ESMs, applying the constraints on Net Biome Productivity (NBP) reduces the model spread in the projected land sink by more than 30% by 2100. Moreover, the projected decline in the land sink is at least doubled in the constrained ensembles and the probability that the terrestrial biosphere is turned into a net carbon source by the end of the century is strongly increased. This indicates that the decline in the future land carbon uptake might be stronger than previously thought, which would have important implications for the rate of increase of the atmospheric CO2 concentration and for future climate change. This article is protected by copyright. All rights reserved.
9. Bing Liu, Senthold Asseng, Leilei Liu, Liang Tang, Weixing Cao, Yan Zhu: Testing the responses of four wheat crop models to heat stress at anthesis and grain filling 2016-01-04T02:22:51.958177-05:00 Global Change Biology+
   
   Higher temperatures caused by future climate change will bring more frequent heat stress events and pose an increasing risk to global wheat production. Crop models have been widely used to simulate future crop productivity but are rarely tested with observed heat stress experimental datasets. Four wheat models (DSSAT-CERES-Wheat, DSSAT-Nwheat, APSIM-Wheat, and WheatGrow) were evaluated with four years of environment-controlled phytotron experimental datasets with two wheat cultivars under heat stress at anthesis and grain filling stage. Heat stress at anthesis reduced observed grain numbers per unit area and individual grain size, while heat stress during grain filling mainly decreased the size of the individual grains. The observed impact of heat stress on grain filling duration, total aboveground biomass, grain yield and grain protein concentration varied depending on cultivar and accumulated heat stress. For every unit increase of heat degree days (HDD, degree days over 30°C), grain filling duration was reduced by 0.30% to 0.60%, total aboveground biomass was reduced by 0.37% to 0.43%, and grain yield was reduced by 1.0% to 1.6%, but grain protein concentration was increased by 0.50% for cv Yangmai16 and 0.80% for cv Xumai30. The tested crop simulation models could reproduce some of the observed reductions in grain filling duration, final total aboveground biomass, and grain yield, as well as the observed increase in grain protein concentration due to heat stress. Most of the crop models tended to reproduce heat stress impacts better during grain filling than at anthesis. Some of the tested models require improvements in the response to heat stress during grain filling, but all models need improvements in simulating heat stress effects on grain set during anthesis. The observed significant genetic variability in the response of wheat to heat stress needs to be considered through cultivar parameters in future simulation studies. This article is protected by copyright. All rights reserved.
10. Tanner J. Williamson, Wyatt F. Cross, Jonathan P. Benstead, Gísli M. Gíslason, James M. Hood, Alexander D. Huryn, Philip W. Johnson, Jill R. Welter: Warming alters coupled carbon and nutrient cycles in experimental streams 2015-12-31T02:32:55.744079-05:00 Global Change Biology+
    
    Although much effort has been devoted to quantifying how warming alters carbon cycling across diverse ecosystems, less is known about how these changes are linked to the cycling of bioavailable nitrogen and phosphorus. In freshwater ecosystems, benthic biofilms (i.e. thin films of algae, bacteria, fungi and detrital matter) act as biogeochemical hotspots by controlling important fluxes of energy and material. Understanding how biofilms respond to warming is thus critical for predicting responses of coupled elemental cycles in freshwater systems. We developed biofilm communities in experimental streamside channels along a gradient of mean water temperatures (7.5-23.6°C), while closely maintaining natural diel and seasonal temperature variation and a common water and propagule source. Both structural (i.e. biomass, stoichiometry, assemblage structure) and functional (i.e. metabolism, N2-fixation, nutrient uptake) attributes of biofilms were measured on multiple dates to link changes in carbon flow explicitly to the dynamics of nitrogen and phosphorus. Temperature had strong positive effects on biofilm biomass (2.8- to 24-fold variation) and net ecosystem productivity (44- to 317-fold variation), despite extremely low concentrations of limiting dissolved nitrogen. Temperature had surprisingly minimal effects on biofilm stoichiometry: carbon:nitrogen (C:N) ratios were temperature-invariant, while carbon:phosphorus (C:P) ratios declined slightly with increasing temperature. Biofilm communities were dominated by cyanobacteria at all temperatures (>91% of total biovolume) and N2-fixation rates increased up to 120-fold between the coldest and warmest treatments. Although ammonium-N uptake increased with temperature (2.8- to 6.8-fold variation), the much higher N2-fixation rates supplied the majority of N to the ecosystem at higher temperatures. Our results demonstrate that temperature can alter how carbon is cycled and coupled to nitrogen and phosphorus. The uncoupling of C fixation from dissolved inorganic nitrogen supply produced large unexpected changes in biofilm development, elemental cycling, and likely downstream exports of nutrients and organic matter. This article is protected by copyright. All rights reserved.
11. Verity G. Salmon, Patrick Soucy, Marguerite Mauritz, Gerardo Celis, Susan M. Natali, Michelle C. Mack, Edward A. G. Schuur: Nitrogen availability increases in a tundra ecosystem during five years of experimental permafrost thaw 2015-12-31T02:32:42.133267-05:00 Global Change Biology+
    
    Perennially frozen soil in high latitude ecosystems (permafrost) currently stores 1330-1580 Pg of carbon (C). As these ecosystems warm, the thaw and decomposition of permafrost is expected to release large amounts of C to the atmosphere. Fortunately, losses from the permafrost C pool will be partially offset by increased plant productivity. The degree to which plants are able to sequester C, however, will be determined by changing nitrogen (N) availability in these thawing soil profiles. N availability currently limits plant productivity in tundra ecosystems but plant access to N is expected improve as decomposition increases in speed and extends to deeper soil horizons. In order to evaluate the relationship between permafrost thaw and N availability, we monitored N cycling during five years of experimentally induced permafrost thaw at the Carbon in Permafrost Experimental Heating Research project (CiPEHR). Inorganic N availability increased significantly in response to deeper thaw and greater soil moisture induced by Soil warming. This treatment also prompted a 23% increase in aboveground biomass and a 49% increase in foliar N pools. The sedge Eriophorum vaginatum responded most strongly to warming: this species explained 91% of the change in aboveground biomass during the five year period. Air warming had little impact when applied alone, but when applied in combination with Soil warming, growing season soil inorganic N availability was significantly reduced. These results demonstrate that there is a strong positive relationship between the depth of permafrost thaw and N availability in tundra ecosystems but that this relationship can be diminished by interactions between increased thaw, warmer air temperatures and higher levels of soil moisture. Within five years of permafrost thaw, plants actively incorporate newly available N into biomass but C storage in live vascular plant biomass is unlikely to be greater than losses from deep soil C pools. This article is protected by copyright. All rights reserved.
12. Emanuele Lugato, Keith Paustian, Panos Panagos, Arwyn Jones, Pasquale Borrelli: Quantifying the erosion effect on current carbon budget of European agricultural soils at high spatial resolution 2015-12-18T01:14:15.989629-05:00 Global Change Biology+
    
    The idea of offsetting anthropogenic CO2 emissions by increasing global soil organic carbon (SOC), as recently proposed by French authorities ahead of COP21 in the ‘four per mil’ initiative, is notable. However, a high uncertainty still exits on land C balance components. In particular, the role of erosion in the global C cycle is not totally disentangled, leading to disagreement whether this process induces lands to be a source or sink of CO2. To investigate this issue, we coupled soil erosion into a biogeochemistry model, running at 1 km2 resolution across the agricultural soils of the European Union (EU). Based on data-driven assumptions, the simulation took into account also soil deposition within grid cells and the potential C export to riverine systems, in a way to be conservative in a mass balance. We estimated that 143 out of 187 Mha have C erosion rates <0.05 Mg C ha−1 yr−1, although some hot-spot areas showed eroded SOC >0.45 Mg C ha−1 yr−1. In comparison with a baseline without erosion, the model suggested an erosion-induced sink of atmospheric C consistent with previous empirical-based studies. Integrating all C fluxes for the EU agricultural soils, we estimated a net C loss or gain of -2.28 and +0.79 Tg yr−1 of CO2eq, respectively, depending on the value for the short-term enhancement of soil C mineralization due to soil disruption and displacement/transport with erosion. We concluded that erosion fluxes were in the same order of current carbon gains from improved management. Even if erosion could potentially induce a sink for atmospheric CO2, strong agricultural policies are needed to prevent or reduce soil erosion, in order to maintain soil health and productivity. This article is protected by copyright. All rights reserved.
13. Marko J. Spasojevic, Christie A. Bahlai, Bethany A. Bradley, Bradley J. Butterfield, Mao-Ning Tuanmu, Seeta Sistla, Ruscena Wiederholt, Katharine N. Suding: Scaling up the diversity-resilience relationship with trait databases and remote sensing data: the recovery of productivity after wildfire 2015-11-24T08:48:58.463959-05:00 Global Change Biology+
    
    Understanding the mechanisms underlying ecosystem resilience - why some systems have an irreversible response to disturbances while others recover - is critical for conserving biodiversity and ecosystem function in the face of global change. Despite the widespread acceptance of a positive relationship between biodiversity and resilience, empirical evidence for this relationship remains fairly limited in scope and localized in scale. Assessing resilience at the large landscape and regional scales most relevant to land management and conservation practices has been limited by the ability to measure both diversity and resilience over large spatial scales. Here, we combined tools used in large scale studies of biodiversity (remote sensing, trait databases) with theoretical advances developed from small scale experiments to ask if the functional diversity within a range of woodland and forest ecosystems influences the recovery of productivity after wildfires across the four-corners region of the United States. We additionally asked how environmental variation (topography, macroclimate) across this geographic region influences such resilience, either directly or indirectly via changes in functional diversity. Using structural equation modeling, we found that functional diversity in regeneration traits (fire tolerance, fire resistance, resprout ability) was a stronger predictor of the recovery of productivity after wildfire than the functional diversity of seed mass or species richness. Moreover, slope, elevation and aspect either directly or indirectly influenced the recovery of productivity, likely via their effect on microclimate, while macroclimate had no direct or indirect effects. Our study provides some of the first direct empirical evidence for functional diversity increasing resilience at large spatial scales. Our approach highlights the power of combining theory based on local scale studies with tools used in studies at large spatial scales and trait databases to understand pressing environmental issues. This article is protected by copyright. All rights reserved.
14. Mingjie Shi, Joshua B. Fisher, Edward R. Brzostek, Richard P. Phillips: Carbon cost of plant nitrogen acquisition: global carbon cycle impact from an improved plant nitrogen cycle in the Community Land Model 2016-01-06T06:58:46.125704-05:00 Global Change Biology+
    
    Plants typically expend a significant portion of their available carbon (C) on nutrient acquisition – C that could otherwise support growth. However, given that most global terrestrial biosphere models (TBMs) do not include the C cost of nutrient acquisition, these models fail to represent current and future constraints to the land C sink. Here, we integrated a plant productivity-optimized nutrient acquisition model – the Fixation and Uptake of Nitrogen Model – into one of the most widely used TBMs, the Community Land Model. Global plant nitrogen (N) uptake is dynamically simulated in the coupled model based on the C costs of N acquisition from mycorrhizal roots, nonmycorrhizal roots, N-fixing microbes, and retranslocation (from senescing leaves). We find that at the global scale, plants spend 2.4 Pg C yr−1 to acquire 1.0 Pg N yr−1, and that the C cost of N acquisition leads to a downregulation of global net primary production (NPP) by 13%. Mycorrhizal uptake represented the dominant pathway by which N is acquired, accounting for ~66% of the N uptake by plants. Notably, roots associating with arbuscular mycorrhizal (AM) fungi – generally considered for their role in phosphorus (P) acquisition – are estimated to be the primary source of global plant N uptake owing to the dominance of AM-associated plants in mid- and low-latitude biomes. Overall, our coupled model improves the representations of NPP downregulation globally and generates spatially explicit patterns of belowground C allocation, soil N uptake, and N retranslocation at the global scale. Such model improvements are critical for predicting how plant responses to altered N availability (owing to N deposition, rising atmospheric CO2, and warming temperatures) may impact the land C sink.
15. Craig A. Emmerton, Vincent L. St. Louis, Elyn R. Humphreys, John A. Gamon, Joel D. Barker, Gilberto Z. Pastorello: Net ecosystem exchange of CO2 with rapidly changing high Arctic landscapes 2015-12-26T08:55:04.517149-05:00 Global Change Biology+
    
    High Arctic landscapes are expansive and changing rapidly. However, our understanding of their functional responses and potential to mitigate or enhance anthropogenic climate change is limited by few measurements. We collected eddy covariance measurements to quantify the net ecosystem exchange (NEE) of CO2 with polar semidesert and meadow wetland landscapes at the highest latitude location measured to date (82°N). We coupled these rare data with ground and satellite vegetation production measurements (Normalized Difference Vegetation Index; NDVI) to evaluate the effectiveness of upscaling local to regional NEE. During the growing season, the dry polar semidesert landscape was a near-zero sink of atmospheric CO2 (NEE: −0.3 ± 13.5 g C m−2). A nearby meadow wetland accumulated over 300 times more carbon (NEE: −79.3 ± 20.0 g C m−2) than the polar semidesert landscape, and was similar to meadow wetland NEE at much more southerly latitudes. Polar semidesert NEE was most influenced by moisture, with wetter surface soils resulting in greater soil respiration and CO2 emissions. At the meadow wetland, soil heating enhanced plant growth, which in turn increased CO2 uptake. Our upscaling assessment found that polar semidesert NDVI measured on-site was low (mean: 0.120–0.157) and similar to satellite measurements (mean: 0.155–0.163). However, weak plant growth resulted in poor satellite NDVI–NEE relationships and created challenges for remotely detecting changes in the cycling of carbon on the polar semidesert landscape. The meadow wetland appeared more suitable to assess plant production and NEE via remote sensing; however, high Arctic wetland extent is constrained by topography to small areas that may be difficult to resolve with large satellite pixels. We predict that until summer precipitation and humidity increases enough to offset poor soil moisture retention, climate-related changes to productivity on polar semideserts may be restricted.
16. Jagdish Kumar Ladha, Adusumilli Narayana Rao, Anitha K. Raman, Agnes Tirol Padre, Achim Dobermann, Mahesh Gathala, Virender Kumar, Yashpal Saharawat, Sheetal Sharma, Hans Peter Piepho, Md Mursedul Alam, Ranjan Liak, Ramasamy Rajendran, Chinnagangannagari Kesava Reddy, Rajender Parsad, Parbodh C. Sharma, Sati shankar Singh, Abhijit Saha, Shamsoon Noor: Agronomic improvements can make future cereal systems in South Asia far more productive and result in a lower environmental footprint 2015-12-14T07:19:51.549651-05:00 Global Change Biology+
    
    South Asian countries will have to double their food production by 2050 while using resources more efficiently and minimizing environmental problems. Transformative management approaches and technology solutions will be required in the major grain-producing areas that provide the basis for future food and nutrition security. This study was conducted in four locations representing major food production systems of densely populated regions of South Asia. Novel production-scale research platforms were established to assess and optimize three futuristic cropping systems and management scenarios (S2, S3, S4) in comparison with current management (S1). With best agronomic management practices (BMPs), including conservation agriculture (CA) and cropping system diversification, the productivity of rice- and wheat-based cropping systems of South Asia increased substantially, whereas the global warming potential intensity (GWPi) decreased. Positive economic returns and less use of water, labor, nitrogen, and fossil fuel energy per unit food produced were achieved. In comparison with S1, S4, in which BMPs, CA and crop diversification were implemented in the most integrated manner, achieved 54% higher grain energy yield with a 104% increase in economic returns, 35% lower total water input, and a 43% lower GWPi. Conservation agriculture practices were most suitable for intensifying as well as diversifying wheat–rice rotations, but less so for rice–rice systems. This finding also highlights the need for characterizing areas suitable for CA and subsequent technology targeting. A comprehensive baseline dataset generated in this study will allow the prediction of extending benefits to a larger scale.
17. Peter Søgaard Jørgensen, Katrin Böhning-Gaese, Kasper Thorup, Anders P. Tøttrup, Przemysław Chylarecki, Frédéric Jiguet, Aleksi Lehikoinen, David G. Noble, Jiri Reif, Hans Schmid, Chris Turnhout, Ian J. Burfield, Ruud Foppen, Petr Voříšek, Arco Strien, Richard D. Gregory, Carsten Rahbek: Continent-scale global change attribution in European birds - combining annual and decadal time scales 2015-10-21T00:12:53.107923-05:00 Global Change Biology+
    
    Species attributes are commonly used to infer impacts of environmental change on multiyear species trends, e.g. decadal changes in population size. However, by themselves attributes are of limited value in global change attribution since they do not measure the changing environment. A broader foundation for attributing species responses to global change may be achieved by complementing an attributes-based approach by one estimating the relationship between repeated measures of organismal and environmental changes over short time scales. To assess the benefit of this multiscale perspective, we investigate the recent impact of multiple environmental changes on European farmland birds, here focusing on climate change and land use change. We analyze more than 800 time series from 18 countries spanning the past two decades. Analysis of long-term population growth rates documents simultaneous responses that can be attributed to both climate change and land-use change, including long-term increases in populations of hot-dwelling species and declines in long-distance migrants and farmland specialists. In contrast, analysis of annual growth rates yield novel insights into the potential mechanisms driving long-term climate induced change. In particular, we find that birds are affected by winter, spring, and summer conditions depending on the distinct breeding phenology that corresponds to their migratory strategy. Birds in general benefit from higher temperatures or higher primary productivity early on or in the peak of the breeding season with the largest effect sizes observed in cooler parts of species' climatic ranges. Our results document the potential of combining time scales and integrating both species attributes and environmental variables for global change attribution. We suggest such an approach will be of general use when high-resolution time series are available in large-scale biodiversity surveys.
18. Martin P. Girardin, Edward H. Hogg, Pierre Y. Bernier, Werner A. Kurz, Xiao Jing Guo, Guillaume Cyr: Negative impacts of high temperatures on growth of black spruce forests intensify with the anticipated climate warming 2015-10-28T01:16:17.4325-05:00 Global Change Biology+
    
    An increasing number of studies conclude that water limitations and heat stress may hinder the capacity of black spruce (Picea mariana (Mill.) B.S.P.) trees, a dominant species of Canada's boreal forests, to grow and assimilate atmospheric carbon. However, there is currently no scientific consensus on the future of these forests over the next century in the context of widespread climate warming. The large spatial extent of black spruce forests across the Canadian boreal forest and associated variability in climate, demography, and site conditions pose challenges for projecting future climate change responses. Here we provide an evaluation of the impacts of climate warming and drying, as well as increasing [CO2], on the aboveground productivity of black spruce forests across Canada south of 60°N for the period 1971 to 2100. We use a new extensive network of tree-ring data obtained from Canada's National Forest Inventory, spatially explicit simulations of net primary productivity (NPP) and its drivers, and multivariate statistical modeling. We found that soil water availability is a significant driver of black spruce interannual variability in productivity across broad areas of the western to eastern Canadian boreal forest. Interannual variability in productivity was also found to be driven by autotrophic respiration in the warmest regions. In most regions, the impacts of soil water availability and respiration on interannual variability in productivity occurred during the phase of carbohydrate accumulation the year preceding tree-ring formation. Results from projections suggest an increase in the importance of soil water availability and respiration as limiting factors on NPP over the next century due to warming, but this response may vary to the extent that other factors such as carbon dioxide fertilization, and respiration acclimation to high temperature, contribute to dampening these limitations.
19. Philip C. Reid, Renata E. Hari, Grégory Beaugrand, David M. Livingstone, Christoph Marty, Dietmar Straile, Jonathan Barichivich, Eric Goberville, Rita Adrian, Yasuyuki Aono, Ross Brown, James Foster, Pavel Groisman, Pierre Hélaouët, Huang-Hsiung Hsu, Richard Kirby, Jeff Knight, Alexandra Kraberg, Jianping Li, Tzu-Ting Lo, Ranga B. Myneni, Ryan P. North, J. Alan Pounds, Tim Sparks, René Stübi, Yongjun Tian, Karen H. Wiltshire, Dong Xiao, Zaichun Zhu: Global impacts of the 1980s regime shift 2015-11-23T23:32:53.046168-05:00 Global Change Biology+
    
    Despite evidence from a number of Earth systems that abrupt temporal changes known as regime shifts are important, their nature, scale and mechanisms remain poorly documented and understood. Applying principal component analysis, change-point analysis and a sequential t-test analysis of regime shifts to 72 time series, we confirm that the 1980s regime shift represented a major change in the Earth's biophysical systems from the upper atmosphere to the depths of the ocean and from the Arctic to the Antarctic, and occurred at slightly different times around the world. Using historical climate model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) and statistical modelling of historical temperatures, we then demonstrate that this event was triggered by rapid global warming from anthropogenic plus natural forcing, the latter associated with the recovery from the El Chichón volcanic eruption. The shift in temperature that occurred at this time is hypothesized as the main forcing for a cascade of abrupt environmental changes. Within the context of the last century or more, the 1980s event was unique in terms of its global scope and scale; our observed consequences imply that if unavoidable natural events such as major volcanic eruptions interact with anthropogenic warming unforeseen multiplier effects may occur.
20. Kaiyu Guan, Joseph A. Berry, Yongguang Zhang, Joanna Joiner, Luis Guanter, Grayson Badgley, David B. Lobell: Improving the monitoring of crop productivity using spaceborne solar-induced fluorescence 2015-11-10T08:35:43.546558-05:00 Global Change Biology+
    
    Large-scale monitoring of crop growth and yield has important value for forecasting food production and prices and ensuring regional food security. A newly emerging satellite retrieval, solar-induced fluorescence (SIF) of chlorophyll, provides for the first time a direct measurement related to plant photosynthetic activity (i.e. electron transport rate). Here, we provide a framework to link SIF retrievals and crop yield, accounting for stoichiometry, photosynthetic pathways, and respiration losses. We apply this framework to estimate United States crop productivity for 2007–2012, where we use the spaceborne SIF retrievals from the Global Ozone Monitoring Experiment-2 satellite, benchmarked with county-level crop yield statistics, and compare it with various traditional crop monitoring approaches. We find that a SIF-based approach accounting for photosynthetic pathways (i.e. C3 and C4 crops) provides the best measure of crop productivity among these approaches, despite the fact that SIF sensors are not yet optimized for terrestrial applications. We further show that SIF provides the ability to infer the impacts of environmental stresses on autotrophic respiration and carbon-use-efficiency, with a substantial sensitivity of both to high temperatures. These results indicate new opportunities for improved mechanistic understanding of crop yield responses to climate variability and change.
21. Jane M. Cowles, Peter D. Wragg, Alexandra J. Wright, Jennifer S. Powers, David Tilman: Shifting grassland plant community structure drives positive interactive effects of warming and diversity on aboveground net primary productivity 2016-01-06T06:53:52.48994-05:00 Global Change Biology+
    
    Ecosystems worldwide are increasingly impacted by multiple drivers of environmental change, including climate warming and loss of biodiversity. We show, using a long-term factorial experiment, that plant diversity loss alters the effects of warming on productivity. Aboveground primary productivity was increased by both high plant diversity and warming, and, in concert, warming (≈1.5 °C average above and belowground warming over the growing season) and diversity caused a greater than additive increase in aboveground productivity. The aboveground warming effects increased over time, particularly at higher levels of diversity, perhaps because of warming-induced increases in legume and C4 bunch grass abundances, and facilitative feedbacks of these species on productivity. Moreover, higher plant diversity was associated with the amelioration of warming-induced environmental conditions. This led to cooler temperatures, decreased vapor pressure deficit, and increased surface soil moisture in higher diversity communities. Root biomass (0–30 cm) was likewise consistently greater at higher plant diversity and was greater with warming in monocultures and at intermediate diversity, but at high diversity warming had no detectable effect. This may be because warming increased the abundance of legumes, which have lower root : shoot ratios than the other types of plants. In addition, legumes increase soil nitrogen (N) supply, which could make N less limiting to other species and potentially decrease their investment in roots. The negative warming × diversity interaction on root mass led to an overall negative interactive effect of these two global change factors on the sum of above and belowground biomass, and thus likely on total plant carbon stores. In total, plant diversity increased the effect of warming on aboveground net productivity and moderated the effect on root mass. These divergent effects suggest that warming and changes in plant diversity are likely to have both interactive and divergent impacts on various aspects of ecosystem functioning.
22. David Montwé, Miriam Isaac-Renton, Andreas Hamann, Heinrich Spiecker: Drought tolerance and growth in populations of a wide-ranging tree species indicate climate change risks for the boreal north 2016-01-06T06:51:59.471268-05:00 Global Change Biology+
    
    Choosing drought-tolerant planting stock in reforestation programs may help adapt forests to climate change. To inform such reforestation strategies, we test lodgepole pine (Pinus contorta Doug. ex Loud. var latifolia Englm.) population response to drought and infer potential benefits of a northward transfer of seeds from drier, southern environments. The objective is addressed by combining dendroecological growth analysis with long-term genetic field trials. Over 500 trees originating from 23 populations across western North America were destructively sampled in three experimental sites in southern British Columbia, representing a climate warming scenario. Growth after 32 years from provenances transferred southward or northward over long distances was significantly lower than growth of local populations. All populations were affected by a severe natural drought event in 2002. The provenances from the most southern locations showed the highest drought tolerance but low productivity. Local provenances were productive and drought tolerant. Provenances from the boreal north showed lower productivity and less drought tolerance on southern test sites than all other sources, implying that maladaptation to drought may prevent boreal populations from taking full advantage of more favorable growing conditions under projected climate change.
23. Richard K. F. Nair, Micheal P. Perks, Andrew Weatherall, Elizabeth M. Baggs, Maurizio Mencuccini: Does canopy nitrogen uptake enhance carbon sequestration by trees? 2015-12-14T06:43:34.471036-05:00 Global Change Biology+
    
    Temperate forest 15N isotope trace experiments find nitrogen (N) addition-driven carbon (C) uptake is modest as little additional N is acquired by trees; however, several correlations of ambient N deposition against forest productivity imply a greater effect of atmospheric nitrogen deposition than these studies. We asked whether N deposition experiments adequately represent all processes found in ambient conditions. In particular, experiments typically apply 15N to directly to forest floors, assuming uptake of nitrogen intercepted by canopies (CNU) is minimal. Additionally, conventional 15N additions typically trace mineral 15N additions rather than litter N recycling and may increase total N inputs above ambient levels. To test the importance of CNU and recycled N to tree nutrition, we conducted a mesocosm experiment, applying 54 g N/15N ha−1 yr−1 to Sitka spruce saplings. We compared tree and soil 15N recovery among treatments where enrichment was due to either (1) a 15N-enriched litter layer, or mineral 15N additions to (2) the soil or (3) the canopy. We found that 60% of 15N applied to the canopy was recovered above ground (in needles, stem and branches) while only 21% of 15N applied to the soil was found in these pools. 15N recovery from litter was low and highly variable. 15N partitioning among biomass pools and age classes also differed among treatments, with twice as much 15N found in woody biomass when deposited on the canopy than soil. Stoichiometrically calculated N effect on C uptake from 15N applied to the soil, scaled to real-world conditions, was 43 kg C kg N−1, similar to manipulation studies. The effect from the canopy treatment was 114 kg C kg N−1. Canopy treatments may be critical to accurately represent N deposition in the field and may address the discrepancy between manipulative and correlative studies.
24. Dohyoung Kim, Ram Oren, Song S. Qian: Response to CO2 enrichment of understory vegetation in the shade of forests 2016-01-06T06:50:20.532739-05:00 Global Change Biology+
    
    Responses of forest ecosystems to increased atmospheric CO2 concentration have been studied in few free-air CO2 enrichment (FACE) experiments during last two decades. Most studies focused principally on the overstory trees with little attention given to understory vegetation. Despite its small contribution to total productivity of an ecosystem, understory vegetation plays an important role in predicting successional dynamics and future plant community composition. Thus, the response of understory vegetation in Pinus taeda plantation at the Duke Forest FACE site after 15–17 years of exposure to elevated CO2, 6–13 of which with nitrogen (N) amendment, was examined. Aboveground biomass and density of the understory decreased across all treatments with increasing overstory leaf area index (LAI). However, the CO2 and N treatments had no effect on aboveground biomass, tree density, community composition, and the fraction of shade-tolerant species. The increases of overstory LAI (~28%) under elevated CO2 resulted in a reduction of light available to the understory (~18%) sufficient to nullify the expected growth-enhancing effect of elevated CO2 on understory vegetation.
25. Yan Sun, Shilong Piao, Mengtian Huang, Philippe Ciais, Zhenzhong Zeng, Lei Cheng, Xiran Li, Xinping Zhang, Jiafu Mao, Shushi Peng, Benjamin Poulter, Xiaoying Shi, Xuhui Wang, Ying-Ping Wang, Hui Zeng: Global patterns and climate drivers of water-use efficiency in terrestrial ecosystems deduced from satellite-based datasets and carbon cycle models 2015-12-23T01:40:12.848158-05:00 Global Ecology and Biogeography+
    
    Aim To investigate how ecosystem water-use efficiency (WUE) varies spatially under different climate conditions, and how spatial variations in WUE differ from those of transpiration-based water-use efficiency (WUEt) and transpiration-based inherent water-use efficiency (IWUEt). Location Global terrestrial ecosystems. Methods We investigated spatial patterns of WUE using two datasets of gross primary productivity (GPP) and evapotranspiration (ET) and four biosphere model estimates of GPP and ET. Spatial relationships between WUE and climate variables were further explored through regression analyses. Results Global WUE estimated by two satellite-based datasets is 1.9 ± 0.1 and 1.8 ± 0.6 g C m−2 mm−1 lower than the simulations from four process-based models (2.0 ± 0.3 g C m−2 mm−1) but comparable within the uncertainty of both approaches. In both satellite-based datasets and process models, precipitation is more strongly associated with spatial gradients of WUE for temperate and tropical regions, but temperature dominates north of 50° N. WUE also increases with increasing solar radiation at high latitudes. The values of WUE from datasets and process-based models are systematically higher in wet regions (with higher GPP) than in dry regions. WUEt shows a lower precipitation sensitivity than WUE, which is contrary to leaf- and plant-level observations. IWUEt, the product of WUEt and water vapour deficit, is found to be rather conservative with spatially increasing precipitation, in agreement with leaf- and plant-level measurements. Main conclusions WUE, WUEt and IWUEt produce different spatial relationships with climate variables. In dry ecosystems, water losses from evaporation from bare soil, uncorrelated with productivity, tend to make WUE lower than in wetter regions. Yet canopy conductance is intrinsically efficient in those ecosystems and maintains a higher IWUEt. This suggests that the responses of each component flux of evapotranspiration should be analysed separately when investigating regional gradients in WUE, its temporal variability and its trends.
26. Alison R. Marklein, Joy B. Winbourne, Sara K. Enders, David J. X. Gonzalez, Tiff L. Huysen, Jorge E. Izquierdo, Derrick R. Light, Daniel Liptzin, Kimberley E. Miller, Scott L. Morford, Robert A. Norton, Benjamin Z. Houlton: Mineralization ratios of nitrogen and phosphorus from decomposing litter in temperate versus tropical forests 2015-12-23T01:25:37.085976-05:00 Global Ecology and Biogeography+
    
    Aim Terrestrial ecosystems sequester about 25% of anthropogenic CO2 emissions annually; however, nitrogen (N) and phosphorus (P) limitation of plant productivity and microbial functioning could curtail this key ecosystem service in the future. Our aim is to address variations in nutrient resupply during decomposition – especially whether the N:P ratio of nutrient recycling via mineralization varies within and across diverse forest biomes. Location Global forest ecosystems. Methods We compiled data on in situ litter decomposition experiments (leaf, wood and root) from the primary literature to examine the relationships between net N and P mineralization across temperate versus tropical forests world-wide. We define net nutrient mineralization ratios as the average N:P released from decomposing substrates at a given ecosystem site. Results We show that net N and P mineralization are strongly correlated within biomes, suggesting strong coupling between N and P recycling in forest ecosystems. The net N:P of leaf-litter mineralization is higher in tropical forests than in temperate forests, consistent with latitudinal patterns in foliar and leaf-litter N:P. At the global scale, the N:P of net mineralization tracks, but tends to be lower than that of litter N:P, pointing to preferential P (versus N) mineralization in forest ecosystems. Main conclusions Our results do not support the view that there is a single, globally consistent mineralization N:P ratio. Instead, our results show that the N:P of net mineralization can be predicted by the N:P of litter, offering a method for incorporating P into global-scale models of carbon–nutrient–climate interactions. In addition, these results imply that P is scarce relative to microbial decomposer demands in tropical forests, whereas N and P may be more co-limiting when compared with microbial biomass in the temperate zone.
27. Sophia Ratcliffe, Mario Liebergesell, Paloma Ruiz-Benito, Jaime Madrigal González, Jose M. Muñoz Castañeda, Gerald Kändler, Aleksi Lehtonen, Jonas Dahlgren, Jens Kattge, Josep Peñuelas, Miguel A. Zavala, Christian Wirth: Modes of functional biodiversity control on tree productivity across the European continent 2015-12-11T01:17:40.781152-05:00 Global Ecology and Biogeography+
    
    Aim The relative contribution of community functional diversity and composition to ecosystem functioning is a critical question in ecology in order to enable better predictions of how ecosystems may respond to a changing climate. However, there is little consensus about which modes of functional biodiversity are most important for tree growth at large spatial scales. Here we assessed the relative importance of climate, functional diversity and functional identity (i.e. the community mean values of four key functional traits) for tree growth across the European continent, spanning the northern boreal to the southern Mediterranean forests. Location Finland, Germany, Sweden, Spain and Wallonia (Belgium). Methods Using data from five European national forest inventories we applied a hierarchical linear model to estimate the sensitivity of tree growth to changes in climate, functional diversity and functional identity along a latitudinal gradient. Results Functional diversity was weakly related to tree growth in the temperate and boreal regions and more strongly in the Mediterranean region. In the temperate region, where climate was the most important predictor, functional diversity and identity had a similar importance for tree growth. Functional identity was strongest at the latitudinal extremes of the continent, largely driven by strong changes in the importance of maximum height along the latitudinal gradient. Main conclusions Functional diversity is an important driver of tree growth in the Mediterranean region, providing evidence that niche complementarity may be more important for tree growth in water-limited forests. The strong influence of functional identity at the latitudinal extremes indicates the importance of a particular trait composition for tree growth in harsh climates. Furthermore, we speculate that this functional identity signal may reflect a trait-based differentiation of successional stages rather than abiotic filtering due to water or energy limitation.
28. Tim M. Blackburn, Steven Delean, Petr Pyšek, Phillip Cassey: On the island biogeography of aliens: a global analysis of the richness of plant and bird species on oceanic islands 2015-06-25T02:05:14.53899-05:00 Global Ecology and Biogeography+
    
    Aim (1) To characterize the relationship(s) between species richness and area for alien plant and bird species on islands, and to identify commonalities and differences in those relationships for these different taxa, and between alien and native species; (2) to test whether area per se, native species richness or human factors related to area is the primary determinant of alien species richness; and (3) to explore the effects on alien island biogeography of isolation, productivity and the time since first European landfall. Location Islands around the world. Methods We used structural equation models (SEMs; supported by generalized linear models) to interrogate data on the alien and native species richness of birds and plants on islands. Results Alien plant and bird species richness were both strongly correlated with island area, with similar slopes on logarithmic axes. SEMs for both plants and birds revealed positive direct effects of native species richness and human population size, and positive indirect effects of area, on alien species richness. The models also identified indirect effects of temperature (positive) and isolation (negative) on alien species richness. Native plant and bird species richness were both predicted by direct effects of area (positive), temperature (positive) and isolation (negative). However, native plant richness was the only direct predictor of native bird species richness, and the strongest direct predictor of alien bird species richness, for islands with both plant and bird richness data. Main conclusions Our analyses recover the species–area, species–isolation and productivity relationships in native richness. Alien species richness was most strongly related to native species richness, with additional effects of human population size. Human population size most likely determines the number of alien species that arrive on an island, while the effect of native species richness may be driven by the influence of habitat heterogeneity on the likelihood that those populations persist (establishment success).
29. Yu Zhang, Han Y. H. Chen, Anthony R. Taylor: Aboveground biomass of understorey vegetation has a negligible or negative association with overstorey tree species diversity in natural forests 2015-10-21T23:18:26.507742-05:00 Global Ecology and Biogeography+
    
    Aim The positive relationships between tree species diversity, aboveground biomass and productivity of overstorey tree layers have been widely reported in tropical, temperate and boreal forest ecosystems. However, no consensus has been arrived at on the association between overstorey tree species diversity and the functions of understorey vegetation, such as the biomass of understorey tree, shrub, herb and bryophyte layers, despite their critical contributions to the diversity and functions of natural forests. Location Canadian forest (42°37′ to 68°14′ N; 53°25′ to 134°46′ W; 4 to 2170 m elevation). Methods We employed Canada's National Forest Inventory dataset to evaluate the influences of overstorey tree species diversity on the aboveground biomass of each forest stratum by accounting for the effects of climate, site condition and stand age. Results We found that aboveground biomass of overstorey trees and total aboveground biomass were positively associated with overstorey tree species richness; however, the aboveground biomass of understorey trees, shrubs, herbs and bryophytes were not associated or were negatively associated with overstorey tree species richness, evenness and life-history trait variations. Main conclusions Our results show positive associations between aboveground biomass of the overstorey tree layer and overstorey tree species diversity over a wide range of climate, local site conditions and stand ages in natural forests. However, contrary to previous findings that more tree species result in higher levels of multiple ecosystem functions, including understorey plant functions, our results demonstrate that the aboveground biomass of understorey vegetation has either a negligible or negative association with overstorey tree species diversity. The negative associations between overstorey tree species diversity and understorey biomass possibly resulted from greater resource filtering by overstorey trees in ecosystems with more diverse overstorey tree species.
30. Xiao Liu, Naomi Marcil Levine: Submesoscale frontal dynamics enhances phytoplankton chlorophyll in the North Pacific Subtropical Gyre 2016-01-25T00:42:50.343289-05:00 Geophysical Research Letters+
    
    Subtropical gyres contribute significantly to global ocean productivity. As the climate warms, the strength of these gyres as a biological carbon pump is predicted to diminish due to increased stratification and depleted surface nutrients. We present results suggesting that the impact of submesoscale physics on phytoplankton in the oligotrophic ocean is substantial and may either compensate or exacerbate future changes in carbon cycling. A new statistical tool was developed to quantify surface patchiness from sea surface temperatures. Chlorophyll concentrations in the North Pacific Subtropical Gyre were shown to be enhanced by submesoscale frontal dynamics with an average increase of 38% (max. 83%) during late winter. The magnitude of this enhancement is comparable to the observed decline in chlorophyll due to a warming of ~1.1°C. These results highlight the need for an improved understanding of fine-scale physical variability in order to predict the response of marine ecosystems to projected climate changes.
31. Megumi O. Chikamoto, Axel Timmermann, Masakazu Yoshimori, Flavio Lehner, Audine Laurian, Ayako Abe-Ouchi, Anne Mouchet, Fortunat Joos, Christoph C. Raible, Kim M. Cobb: Volcanic eruptions boost tropical Pacific biological productivity 2016-01-15T16:33:21.578357-05:00 Geophysical Research Letters+
    
    Major volcanic eruptions generate widespread ocean cooling, which reduces upper-ocean stratification. This effect has the potential to increase nutrient delivery into the euphotic zone and boost biological productivity. Using externally forced Last Millennium simulations of three climate/Earth System models (MIROC, CESM and LOVECLIM), we test the hypothesis that large volcanic eruptions intensify nutrient-driven export production. It is found that strong volcanic radiative forcing enhances the likelihood of eastern Pacific El Niño-like warming in CESM and LOVECLIM. This leads to an initial reduction of nutrients and export production in the eastern equatorial Pacific. However, this initial response reverses after about three years in association with La Niña cooling. The resulting delayed enhancement of biological production resembles the multiyear response in MIROC. The model simulations show that volcanic impacts on tropical Pacific dynamics and biogeochemistry persist for several years, thus providing a new source for potential multiyear ecosystem predictability.
32. Claudia Timmreck, Holger Pohlmann, Sebastian Illing, Christopher Kadow: The impact of stratospheric volcanic aerosol on decadal-scale climate predictions 2016-01-19T17:44:26.65607-05:00 Geophysical Research Letters+
    
    To understand the impact of volcanic aerosol on multiyear seasonal and decadal climate predictions, we performed Coupled Model Intercomparison Project Phase 5-type hindcasts without volcanic aerosol using the German Mittelfristige Klimaprognosen prediction system and compared them to the corresponding simulations including aerosols. Our results show that volcanic aerosol significantly affects the prediction skill for global mean surface air temperature in the first five years after strong volcanic eruptions. Also, on the regional scale a volcanic imprint on decadal-scale variability is detectable. Neglecting volcanic aerosol leads to a reduced prediction skill over the tropical and subtropical Atlantic, Indic, and west Pacific but to an improvement over the tropical east Pacific, where the model has in general no skill. Multiseasonal differences in the skill for seasonal mean temperatures are evident over Continental Europe with significant skill loss due to neglection of volcanic aerosol in boreal winter over central Europe, Scandinavia and over southeastern Europe, and the East Mediterranean in boreal summer.
33. Mathew Koll Roxy, Aditi Modi, Raghu Murtugudde, Vinu Valsala, Swapna Panickal, S. Prasanna Kumar, M. Ravichandran, Marcello Vichi, Marina Lévy: A reduction in marine primary productivity driven by rapid warming over the tropical Indian Ocean 2016-01-19T17:33:32.465356-05:00 Geophysical Research Letters+
    
    Among the tropical oceans, the western Indian Ocean hosts one of the largest concentrations of marine phytoplankton blooms in summer. Interestingly, this is also the region with the largest warming trend in sea surface temperatures in the tropics during the past century—although the contribution of such a large warming to productivity changes has remained ambiguous. Earlier studies had described the western Indian Ocean as a region with the largest increase in phytoplankton during the recent decades. On the contrary, the current study points out an alarming decrease of up to 20% in phytoplankton in this region over the past six decades. We find that these trends in chlorophyll are driven by enhanced ocean stratification due to rapid warming in the Indian Ocean, which suppresses nutrient mixing from subsurface layers. Future climate projections suggest that the Indian Ocean will continue to warm, driving this productive region into an ecological desert.
34. N. Herold, L. V. Alexander, M. G. Donat, S. Contractor, A. Becker: How much does it rain over land? 2016-01-09T22:26:11.29921-05:00 Geophysical Research Letters+
    
    Despite the availability of several observationally constrained data sets of daily precipitation based on rain gauge measurements, remote sensing, and/or reanalyses, we demonstrate a large disparity in the quasi-global land mean of daily precipitation intensity. Surprisingly, the magnitude of this spread is similar to that found in the Coupled Model Intercomparison Project Phase 5 (CMIP5). A weakness of reanalyses and CMIP5 models is their tendency to over simulate wet days, consistent with previous studies. However, there is no clear agreement within and between rain gauge and remotely sensed data sets either. This large discrepancy highlights a shortcoming in our ability to characterize not only modeled daily precipitation intensities but even observed precipitation intensities. This shortcoming is partially reconciled by an appreciation of the different spatial scales represented in gridded data sets of in situ precipitation intensities and intensities calculated from gridded precipitation. Unfortunately, the spread in intensities remains large enough to prevent us from satisfactorily determining how much it rains over land.
35. Margot Bador, Laurent Terray, Julien Boé: Emergence of human influence on summer record-breaking temperatures over Europe 2016-01-14T15:54:35.41235-05:00 Geophysical Research Letters+
    
    Observational analysis of Europe summer record-breaking temperatures suggests that their occurrence differs from that expected in a stationary climate since the late 1980s. The observed cold and warm record evolution is well simulated by the ensemble mean of 27 coupled models from the Coupled Model Intercomparison Project phase 5 (CMIP5). We find that this evolution is still today within the range of internal variability derived from CMIP5 preindustrial simulations. We then estimate a time of emergence of the summer record anthropogenic influence in a world under a business as usual greenhouse gas emission scenario. We suggest a time of emergence around 2020 for the cold records and 2030 for the warm ones with an uncertainty of ± 20 years. By 2100, the multimodel ensemble mean indicates a tenfold increase of the number of warm records compared to the first half of the twentieth century and the quasi-disappearance of cold records.
36. Yoo-Geun Ham, Jong-Seong Kug: ENSO amplitude changes due to greenhouse warming in CMIP5: Role of mean tropical precipitation in the twentieth century 2016-01-14T16:02:48.034174-05:00 Geophysical Research Letters+
    
    This study examines the relationship between the intermodel diversities of the present climate climatology and those of El Niño–Southern Oscillation (ENSO) amplitude change under global warming in the Coupled Model Intercomparison Project Phase 5 (CMIP5) models. The models with increased ENSO amplitude under greenhouse warming (i.e., “ENSO-amplified models”) tend to simulate a twentieth century stronger climatological Intertropical Convergence Zone and South Pacific Convergence Zone over the central-eastern Pacific that are located farther away from the equator during boreal spring. Moisture budget analysis indicates that those climatological differences lead to stronger positive climatological precipitation change over the off-equatorial central-eastern Pacific under greenhouse warming. The stronger positive climatological precipitation change enhances the air-sea coupling strength over the central-eastern Pacific, which leads to increase the ENSO amplitude.
37. M. Aydin, J. E. Campbell, T. J. Fudge, K. M. Cuffey, M. R. Nicewonger, K. R. Verhulst, E. S. Saltzman: Changes in atmospheric carbonyl sulfide over the last 54000 years inferred from measurements in Antarctic ice cores 2016-01-27T05:10:52.425031-05:00 Journal of Geophysical Research: Atmospheres+
    
    We measured carbonyl sulfide (COS) in air extracted from ice core samples from the West Antarctic Ice Sheet (WAIS) Divide, Antarctica, with the deepest sample dated to 54,300 years before present. These are the first ice core COS measurements spanning the last glacial maximum (LGM), the last glacial/interglacial transition, and the early Holocene. The WAIS Divide measurements from the LGM and the last transition are the first COS measurements in air extracted from full clathrate (bubble-free) ice. This study also includes new COS measurements from Taylor Dome, Antarctica, including some in bubbly glacial-ice that are concurrent with the WAIS Divide data from clathrate glacial-ice. COS hydrolyzes in ice core air bubbles and the recovery of an atmospheric record requires correcting for this loss. The data presented here suggest that the in situ hydrolysis of COS is significantly slower in clathrate ice than in bubbly ice. The clathrate-ice measurements are corrected for the hydrolysis loss during the time spent as bubbly ice only. The corrected WAIS Divide record indicates that atmospheric COS was 250-300 ppt during the LGM and declined by 80 – 100 ppt during the last glacial/interglacial transition to a minimum at 160-210 ppt at the beginning of the Holocene. This decline was likely caused by an increase in the gross primary productivity of terrestrial plants, with a possible contribution from a reduction in ocean sources. COS levels were above 300 ppt in the late Holocene, indicating that large changes in the COS biogeochemical cycle occurred during the Holocene.
38. Libo Wang, Jason N. S. Cole, Paul Bartlett, Diana Verseghy, Chris Derksen, Ross Brown, Knut Salzen: Investigating the spread in surface albedo for snow covered forests in CMIP5 models 2016-01-07T06:55:24.318643-05:00 Journal of Geophysical Research: Atmospheres+
    
    This study investigates the role of leaf/plant area index (LAI/PAI) specification on the large spread of winter albedo simulated by climate models. To examine the sensitivity of winter albedo to LAI, we perform a sensitivity analysis using two methods commonly used to compute albedo in snow-covered forests as well as diagnostic calculations within version 4.2 of the Canadian Atmospheric Model for which PAI is systematically varied. The results show that the simulated albedo is very sensitive to negative PAI biases, especially for smaller PAI values. The LAI and surface albedo of boreal forests in the presence of snow simulated by the Coupled Model Intercomparison Project Phase 5 (CMIP5) models are evaluated using satellite observations. The evaluation of CMIP5 models suggest that inaccurate tree cover fraction due to improper plant functional type specification or erroneous LAI parameterization in some models explains, in part, an observed positive bias in winter albedo over boreal forest regions of the Northern Hemisphere. This contributes to a large intermodel spread in simulated surface albedo in the presence of snow over these regions and is largely responsible for uncertainties in simulated snow-albedo feedback strength. Errors are largest (+20-40 %) in models with large underestimation of LAI but are typically within ±15% when simulated LAI is within the observed range. This study underscores the importance of accurate representation of vegetation distribution and parameters in realistic simulation of surface albedo.
39. Jung Choi, Jian Lu, Seok-Woo Son, Dargan M. W. Frierson, Jin-Ho Yoon: Uncertainty in future projections of the North Pacific subtropical high and its implication for California winter precipitation change 2016-01-27T11:06:58.563957-05:00 Journal of Geophysical Research: Atmospheres+
    
    This study examines future projections of sea level pressure change in the North Pacific and its impact on winter precipitation changes in California. The multimodel analysis, based on the Coupled Model Intercomparison Project phase 5 models under the Representative Concentration Pathway 8.5 scenario, shows a robust sea level pressure change in the late 21st century over the western North Pacific in which both the Aleutian Low and the North Pacific subtropical high (NPSH) shift poleward in concert with a widening of the Hadley cell. This change is partly explained by a systematic increase of static stability in the subtropics. Despite its robustness, the projected NPSH changes over the eastern North Pacific exhibit a substantial intermodel spread, contributing as a cause for uncertain projections of precipitation changes in California. This intermodel spread in the eastern North Pacific is associated with a Pacific Decadal Oscillation-like surface temperature change in the western North Pacific and the resulting meridional temperature gradient change. This study points to a major source of uncertainty for the response of winter precipitation to global warming over the West Coast of North America: atmosphere-ocean coupling in the North Pacific.
40. Marcin L. Witek, David J. Diner, Michael J. Garay: Satellite assessment of sea spray aerosol productivity: Southern Ocean case study 2016-01-25T11:22:20.2974-05:00 Journal of Geophysical Research: Atmospheres+
    
    Despite many years of observations by multiple sensors, there is still substantial ambiguity regarding aerosol optical depths (AOD) over remote oceans, in particular, over the pristine Southern Ocean. Passive satellite retrievals (e.g., Multiangle Imaging Spectroradiometer (MISR) and Moderate Resolution Imaging Spectroradiometer (MODIS)) and global aerosol transport models show a distinct AOD maximum around the 60°S latitude band. Sun photometer measurements performed by the Maritime Aerosol Network (MAN), on the other hand, indicate no increased AODs over the Southern Ocean. In this study elevated Southern Ocean AODs are examined from the modeling perspective. The primary aerosol component over the Southern Ocean is sea spray aerosol (SSA). Multiple simulations of SSA concentrations and optical depths are carried out using a single modeling framework, the Navy Aerosol Analysis and Prediction System (NAAPS) model. Several SSA emission functions are examined, including recently proposed formulations with sea surface temperature corrections. The differences between NAAPS simulations are primarily due to different SSA emission formulations. The results are compared against satellite-derived AODs from the MISR and MODIS instruments. MISR and MODIS AOD retrievals are further filtered to eliminate retrievals potentially affected by cloud contamination and cloud adjacency effects. The results indicate a very large impact of SSA emission parameterization on the simulated AODs. For some scenarios, the Southern Ocean AOD maximum almost completely disappears. Further MISR and MODIS AOD quality screening substantially improves model/satellite agreement. Based on these comparisons, an optimal SSA emission function for global aerosol transport models is recommended.
41. Ruth Lorenz, Daniel Argüeso, Markus G. Donat, Andrew J. Pitman, Bart van den Hurk, Alexis Berg, David M. Lawrence, Frédérique Chéruy, Agnès Ducharne, Stefan Hagemann, Arndt Meier, P. C. D. Milly, Sonia I. Seneviratne: Influence of land-atmosphere feedbacks on temperature and precipitation extremes in the GLACE-CMIP5 ensemble 2016-01-19T11:47:42.565278-05:00 Journal of Geophysical Research: Atmospheres+
    
    We examine how soil moisture variability and trends affect the simulation of temperature and precipitation extremes in six global climate models using the experimental protocol of the Global Land-Atmosphere Coupling Experiment of the Coupled Model Intercomparison Project, Phase 5 (GLACE-CMIP5). This protocol enables separate examinations of the influences of soil moisture variability and trends on the intensity, frequency, and duration of climate extremes by the end of the 21st century under a business-as-usual (Representative Concentration Pathway 8.5) emission scenario. Removing soil moisture variability significantly reduces temperature extremes over most continental surfaces, while wet precipitation extremes are enhanced in the tropics. Projected drying trends in soil moisture lead to increases in intensity, frequency, and duration of temperature extremes by the end of the 21st century. Wet precipitation extremes are decreased in the tropics with soil moisture trends in the simulations, while dry extremes are enhanced in some regions, in particular the Mediterranean and Australia. However, the ensemble results mask considerable differences in the soil moisture trends simulated by the six climate models. We find that the large differences between the models in soil moisture trends, which are related to an unknown combination of differences in atmospheric forcing (precipitation, net radiation), flux partitioning at the land surface, and how soil moisture is parameterized, imply considerable uncertainty in future changes in climate extremes.
42. Yeyi Liu, Dabang Jiang: Mid-Holocene permafrost: Results from CMIP5 simulations 2016-01-14T11:29:47.068964-05:00 Journal of Geophysical Research: Atmospheres+
    
    Distribution of frozen ground and active layer thickness in the Northern Hemisphere during the mid-Holocene (MH) and differences with respect to the preindustrial (PI) were investigated here using the Coupled Model Intercomparison Project Phase 5 (CMIP5) models. Two typical diagnostic methods, respectively, based on soil temperature (Ts based; a direct method) and air temperature (Ta based; an indirect method) were employed to classify categories and extents of frozen ground. In relation to orbitally induced changes in climate and in turn freezing and thawing indices, the MH permafrost extent was 20.5% (1.8%) smaller than the PI, whereas seasonally frozen ground increased by 9.2% (0.8%) in the Northern Hemisphere according to the Ts-based (Ta-based) method. Active layer thickness became larger, but by ≤ 1.0 m in most of permafrost areas during the MH. Intermodel disagreement remains within areas of permafrost boundary by both the Ts-based and Ta-based results, with the former demonstrating less agreement among the CMIP5 models because of larger variation in land model abilities to represent permafrost processes. However, both the methods were able to reproduce the MH relatively degenerated permafrost and increased active layer thickness (although with smaller magnitudes) as observed in data reconstruction. Disparity between simulation and reconstruction was mainly found in the seasonally frozen ground regions at low to middle latitudes, where the reconstruction suggested a reduction of seasonally frozen ground extent to the north, whereas the simulation demonstrated a slightly expansion to the south for the MH compared to the PI.
43. Noel R. Aloysius, Justin Sheffield, James E. Saiers, Haibin Li, Eric F. Wood: Evaluation of historical and future simulations of precipitation and temperature in central Africa from CMIP5 climate models 2016-01-08T17:32:24.495144-05:00 Journal of Geophysical Research: Atmospheres+
    
    Global and regional climate change assessments rely heavily on the general circulation model (GCM) outputs such as provided by the Coupled Model Intercomparison Project phase 5 (CMIP5). Here we evaluate the ability of 25 CMIP5 GCMs to simulate historical precipitation and temperature over central Africa and assess their future projections in the context of historical performance and intermodel and future emission scenario uncertainties. We then apply a statistical bias correction technique to the monthly climate fields and develop monthly downscaled fields for the period of 1948–2099. The bias-corrected and downscaled data set is constructed by combining a suite of global observation and reanalysis-based data sets, with the monthly GCM outputs for the 20th century, and 21st century projections for the medium mitigation (representative concentration pathway (RCP)45) and high emission (RCP85) scenarios. Overall, the CMIP5 models simulate temperature better than precipitation, but substantial spatial heterogeneity exists. Many models show limited skill in simulating the seasonality, spatial patterns, and magnitude of precipitation. Temperature projections by the end of the 21st century (2070–2099) show a robust warming between 2 and 4°C across models, whereas precipitation projections vary across models in the sign and magnitude of change (−9% to 27%). Projected increase in precipitation for a subset of models (single model ensemble (SME)) identified based on performance metrics and causal mechanisms are slightly higher compared to the full multimodel ensemble (MME) mean; however, temperature projections are similar between the two ensemble means. For the near-term (2021–2050), neither the historical performance nor choice of models is related to the precipitation projections, indicating that natural variability dominated any signal. With fewer models, the “blind” MME approach will have larger uncertainties in future precipitation projections compared to projections by the SME models. We propose the latter a better approach in regions that lack quality climate observations. Our analyses also show that the choice of model and emission scenario dominate the uncertainty in precipitation projections, whereas the emission scenario dominates the temperature projections. Although our analyses are done for central Africa, the final Bias-Corrected Spatially Downscaled data set is available for global land areas. The framework for climate change assessment and the data will be useful for a variety of climate assessment, impact, and adaptation studies.
44. F. M. Woldemeskel, A. Sharma, B. Sivakumar, R. Mehrotra: Quantification of precipitation and temperature uncertainties simulated by CMIP3 and CMIP5 models 2016-01-05T11:43:22.678142-05:00 Journal of Geophysical Research: Atmospheres+
    
    Assessment of climate change impacts on water resources is extremely challenging, due to the inherent uncertainties in climate projections using global climate models (GCMs). Three main sources of uncertainties can be identified in GCMs, i.e., model structure, emission scenario, and natural variability. The recently released fifth phase of the Coupled Model Intercomparison Project (CMIP5) includes a number of advances relative to its predecessor (CMIP3), in terms of the spatial resolution of models, list of variables, and concept of specifying future radiative forcing, among others. The question, however, is do these modifications indeed reduce the uncertainty in the projected climate at global and/or regional scales? We address this question by quantifying and comparing uncertainty in precipitation and temperature from 6 CMIP3 and 13 CMIP5 models. Uncertainty is quantified using the square root of error variance, which specifies uncertainty as a function of time and space, and decomposes the total uncertainty into its three constituents. The results indicate a visible reduction in the uncertainty of CMIP5 precipitation relative to CMIP3 but no significant change for temperature. For precipitation, the GCM uncertainty is found to be larger in regions of the world that receive heavy rainfall, as well as mountainous and coastal areas. For temperature, however, uncertainty is larger in extratropical cold regions and lower elevation areas.
45. George L Vourlitis, Cloe S. Hentz: Chronic N addition alters the carbon and nitrogen storage of a post-fire Mediterranean-type shrubland 2016-01-14T17:25:03.58515-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Mediterranean-type shublands are subject to periodic fire and high levels of nitrogen (N) deposition, but little is known how chronic N deposition affects carbon (C) and N storage during succession. We conducted a long-term experiment in Californian chaparral to test the hypothesis that chronic N-enrichment would increase post-fire C and N accumulation. The experimental layout consisted of a randomized design where four-10 x 10 m plots received 5 gN m−2 annually since 2003 and four-10 x 10 m plots served as controls. Aboveground and belowground C and N pools and fluxes were measured seasonally (every 3 months) for a period of 10 years. Added N rapidly increased soil extractable N pools and decreased soil pH; however, total soil C and N storage were not affected. Added N plots initially had significantly lower C and N storage than control plots, presumably because of nutrient losses from leaching and/or higher belowground C allocation. However, rates of aboveground N and C storage became significantly higher in added N plots after 4–5 years of exposure, thus increasing fuel buildup, which has implications for future fire intensity. This recovering chaparral stand is not yet “N-saturated” after 10 years of chronic N input. However, N leaching continues to be higher in added N plots, indicating that post-fire chaparral stands in high-N deposition areas can be important sources of N to groundwater/aquatic systems even if productivity is stimulated by N input.
46. M. D. Petrie, N. A. Brunsell, R. Vargas, S. L. Collins, L. B. Flanagan, N. P. Hanan, M. E. Litvak, A. E. Suyker: The sensitivity of carbon exchanges in Great Plains grasslands to precipitation variability 2016-01-14T05:22:04.956107-05:00 Journal of Geophysical Research: Biogeosciences+
    
    In the Great Plains, grassland carbon dynamics differ across broad gradients of precipitation and temperature, yet finer-scale variation in these variables may also affect grassland processes. Despite the importance of grasslands, there is little information on how fine-scale relationships compare between them regionally. We compared grassland C exchanges, energy partitioning and precipitation variability in eight sites in the eastern and western Great Plains using eddy covariance and meteorological data. During our study, both eastern and western grasslands varied between an average net carbon sink and a net source. Eastern grasslands had a moderate vapor pressure deficit (VPD = 0.95 kPa) and high growing season gross primary productivity (GPP = 1010 ± 218 g C m−2 m−2 y−1). Western grasslands had a growing season with higher VPD (1.43 kPa) and lower GPP (360 ± 127 g C m−2 m−2 y−1). Western grasslands were sensitive to precipitation at daily timescales, whereas eastern grasslands were sensitive at monthly and seasonal timescales. Our results support the expectation that C exchanges in these grasslands differ as a result of varying precipitation regimes. Because eastern grasslands are less influenced by short-term variability in rainfall than western grasslands, the effects of precipitation change are likely to be more predictable in eastern grasslands because the timescales of variability that must be resolved are relatively longer. We postulate increasing regional heterogeneity in grassland C exchanges in the Great Plains in coming decades.
47. Natalie A. Griffiths, C. Rhett Jackson, Jeffrey J. McDonnell, Julian Klaus, Enhao Du, Menberu M. Bitew: Dual nitrate isotopes clarify the role of biological processing and hydrologic flowpaths on nitrogen cycling in subtropical low-gradient watersheds 2016-01-10T22:55:27.641938-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Nitrogen (N) is an important nutrient as it often limits productivity, but in excess can impair water quality. Most studies on watershed N cycling have occurred in upland forested catchments where snowmelt dominates N export; fewer studies have focused on low-relief watersheds that lack snow. We examined watershed N cycling in three adjacent, low-relief watersheds in the Upper Coastal Plain of the southeastern United States to better understand the role of hydrological flowpaths and biological transformations of N at the watershed scale. Groundwater was the dominant source of nitrified N to stream water in 2 of the 3 watersheds, while atmospheric deposition comprised 28% of stream water nitrate in one watershed. The greater atmospheric contribution may have been due to the larger stream channel area relative to total watershed area or the dominance of shallow subsurface flowpaths contributing to stream flow in this watershed. There was a positive relationship between temperature and stream water ammonium concentrations and a negative relationship between temperature and stream water nitrate concentrations in each watershed suggesting that N cycling processes (i.e., nitrification, denitrification) varied seasonally. However, there were no clear patterns in the importance of denitrification in different water pools possibly because a variety of factors (i.e., assimilatory uptake, dissimilatory uptake, mixing) affected nitrate concentrations. Together, these results highlight the hydrological and biological controls on N cycling in low-gradient watersheds, and variability in N delivery flowpaths among adjacent watersheds with similar physical characteristics.
48. Elizabeth E. Webb, Edward A.G. Schuur, Susan M. Natali, Kiva L. Oken, Rosvel Bracho, John P. Krapek, David Risk, Nick R. Nickerson: Increased wintertime CO2 loss as a result of sustained tundra warming 2016-01-04T05:15:43.704417-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Permafrost soils currently store approximately 1672 Pg of carbon (C), but as high latitudes warm, this temperature-protected C reservoir will become vulnerable to higher rates of decomposition. In recent decades, air temperatures in the high latitudes have warmed more than any other region globally, particularly during the winter. Over the coming century, the arctic winter is also expected to experience the most warming of any region or season, yet it is notably understudied. Here we present non-summer season (NSS) CO2 flux data from the Carbon in Permafrost Experimental Heating Research (CiPEHR) project, an ecosystem warming experiment of moist acidic tussock tundra in interior Alaska. Our goals were to quantify the relationship between environmental variables and winter CO2 production, account for subnivean photosynthesis and late fall plant C uptake in our estimate of NSS CO2 exchange, constrain NSS CO2 loss estimates using multiple methods of measuring winter CO2 flux, and quantify the effect of winter soil warming on total NSS CO2 balance. We measured CO2 flux using four methods: two chamber techniques (the snow pit method and one where a chamber is left under the snow for the entire season), eddy covariance, and soda lime adsorption, and found that NSS CO2 loss varied up to 4 fold, depending on the method used. CO2 production was dependent on soil temperature and day of season but atmospheric pressure and air temperature were also important in explaining CO2 diffusion out of the soil. Warming stimulated both ecosystem respiration and productivity during the NSS and increased overall CO2 loss during this period by 14% (this effect varied by year, ranging from 7 to 24%). When combined with the summertime CO2 fluxes from the same site, our results suggest that this sub-arctic tundra ecosystem is shifting away from its historical function as a C sink to a C source.
49. Yuanyuan Huang, Stefan Gerber: Nitrogen restrictions buffer modeled interactions of water with the carbon cycle 2016-01-23T13:58:26.084883-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Terrestrial carbon and water cycles are coupled at multiple spatiotemporal scales and are crucial to carbon sequestration. Water related climate extremes, such as drought and intense precipitation, can substantially affect the carbon cycle. Meanwhile, nitrogen is a limiting resource to plant and has therefore the potential to alter the coupling of water and carbon cycles on land. Here we assess the effect of nitrogen limitation on the response of the terrestrial carbon cycle to moisture anomalies using Geophysical Fluid Dynamics Laboratory's land surface model LM3V-N. We analyzed the response of three central carbon fluxes: net primary productivity (NPP), heterotrophic respiration (Rh), and net ecosystem productivity (NEP, the difference between NPP and Rh) and how these fluxes were altered under anomalies of the standardized precipitation and evapotranspiration index (SPEI). We found that globally, the correlations between each of the carbon flux and SPEI depended on the timescale and a strong legacy effect of SPEI anomalies on Rh. Consideration of nitrogen constraints reduced anomalies in carbon fluxes in response to extreme dry/wet events. This nitrogen-induced buffer constrained the growth of plants under wet extremes and allowed for enhanced growth during droughts. Extra gain of soil moisture from the downregulation of canopy transpiration by nitrogen limitation and shifts in the relative importance of water and nitrogen limitation during dry/wet extreme events are possible mechanisms contributing to the buffering of modeled NPP and NEP. Responses of Rh to moisture anomalies were much weaker compared to NPP, and N buffering effects were less evident.
50. Peng Wu, Rong Bi, Shanshan Duan, Haiyan Jin, Jianfang Chen, Qiang Hao, Yuming Cai, Xinyan Mao, Meixun Zhao: Spatiotemporal variations of phytoplankton in the East China Sea and the Yellow Sea revealed by lipid biomarkers 2016-01-14T16:13:14.313781-05:00 Journal of Geophysical Research: Biogeosciences+
    
    The East China Sea (ECS) and the Southern Yellow Sea (SYS) ecosystem is undergoing dramatic changes, but the spatiotemporal patterns and forcing mechanisms of phytoplankton variations remain understudied. Phytoplankton lipid biomarkers are useful proxies for productivity and community structure changes, and they were measured in suspended particles of more than 81 sites from spring and summer of 2011 in the ECS and SYS. In spring, the concentrations of brassicasterol (4.7–127 ng L−1) and dinosterol (0.7–37 ng L−1) were markedly higher in the northern and central SYS, while C37 alkenones (0–15 ng L−1) were detected at only seven sites in the ECS. In summer, brassicasterol (25.3–1178 ng L−1) and dinosterol (0–125 ng L−1) showed high values off the Changjiang River Estuary (CRE), while C37 alkenones (0–410 ng L−1) had high values in the northwest and central SYS. The mean concentrations of the three lipid biomarkers in summer were 3 to 61 times higher than those in spring. Spatiotemporal patterns of biomarkers reveal higher ratios of diatom/dinoflagellate and diatom/haptophyte in higher productivity areas, off the CRE in summer and the northern and central SYS in spring. This study validates the applicability of brassicasterol, dinosterol, and alkenones as proxies of productivity and community structure of the three phytoplankton taxa: diatoms, dinoflagellates, and haptophytes. The results indicate that nutrients (in summer) and turbidity-induced photosynthetic available radiation (in spring) play important roles in regulating spatiotemporal variations of phytoplankton in the ECS and SYS.
51. Tao Zhou, Peijun Shi, Gensuo Jia, Yongjiu Dai, Xiang Zhao, Wei Shangguan, Ling Du, Hao Wu, Yiqi Luo: Age-dependent forest carbon sink: Estimation via inverse modeling 2015-12-02T17:15:34.414507-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Forests have been recognized to sequester a substantial amount of carbon (C) from the atmosphere. However, considerable uncertainty remains regarding the magnitude and time course of the C sink. Revealing the intrinsic relationship between forest age and C sink is crucial for reducing uncertainties in prediction of forest C sink potential. In this study, we developed a stepwise data assimilation approach to combine a process-based Terrestrial ECOsystem Regional model, observations from multiple sources, and stochastic sampling to inversely estimate carbon cycle parameters including carbon sink at different forest ages for evergreen needle-leaved forests in China. The new approach is effective to estimate age-dependent parameter of maximal light-use efficiency (R2 = 0.99) and, accordingly, can quantify a relationship between forest age and the vegetation and soil C sinks. The estimated ecosystem C sink increases rapidly with age, peaks at 0.451 kg C m−2 yr−1 at age 22 years (ranging from 0.421 to 0.465 kg C m−2 yr−1), and gradually decreases thereafter. The dynamic patterns of C sinks in vegetation and soil are significantly different. C sink in vegetation first increases rapidly with age and then decreases. C sink in soil, however, increases continuously with age; it acts as a C source when the age is less than 20 years, after which it acts as a sink. For the evergreen needle-leaved forest, the highest C sink efficiency (i.e., C sink per unit net primary productivity) is approximately 60%, with age between 11 and 43 years. Overall, the inverse estimation of carbon cycle parameters can make reasonable estimates of age-dependent C sequestration in forests.
52. K. Fleischer, D. Wårlind, M. K. Molen, K. T. Rebel, A. Arneth, J. W. Erisman, M. J. Wassen, B. Smith, C. M. Gough, H. A. Margolis, A. Cescatti, L. Montagnani, A. Arain, A. J. Dolman: Low historical nitrogen deposition effect on carbon sequestration in the boreal zone 2015-12-18T12:25:56.609768-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Nitrogen (N) cycle dynamics and N deposition play an important role in determining the terrestrial biosphere's carbon (C) balance. We assess global and biome-specific N deposition effects on C sequestration rates with the dynamic global vegetation model LPJ-GUESS. Modeled CN interactions are evaluated by comparing predictions of the C and CN version of the model with direct observations of C fluxes from 68 forest FLUXNET sites. N limitation on C uptake reduced overestimation of gross primary productivity for boreal evergreen needleleaf forests from 56% to 18%, presenting the greatest improvement among forest types. Relative N deposition effects on C sequestration (dC/dN) in boreal, temperate, and tropical sites ranged from 17 to 26 kg C kg N−1 when modeled at site scale and were reduced to 12–22 kg C kg N−1 at global scale. We find that 19% of the recent (1990–2007) and 24% of the historical global C sink (1900–2006) was driven by N deposition effects. While boreal forests exhibit highest dC/dN, their N deposition-induced C sink was relatively low and is suspected to stay low in the future as no major changes in N deposition rates are expected in the boreal zone. N deposition induced a greater C sink in temperate and tropical forests, while predicted C fluxes and N-induced C sink response in tropical forests were associated with greatest uncertainties. Future work should be directed at improving the ability of LPJ-GUESS and other process-based ecosystem models to reproduce C cycle dynamics in the tropics, facilitated by more benchmarking data sets. Furthermore, efforts should aim to improve understanding and model representations of N availability (e.g., N fixation and organic N uptake), N limitation, P cycle dynamics, and effects of anthropogenic land use and land cover changes.
53. Russell L. Scott, Joel A. Biederman, Erik P. Hamerlynck, Greg A. Barron-Gafford: The carbon balance pivot point of southwestern U.S. semiarid ecosystems: Insights from the 21st century drought 2015-12-22T14:43:14.182369-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Global-scale studies indicate that semiarid regions strongly regulate the terrestrial carbon sink. However, we lack understanding of how climatic shifts, such as decadal drought, impact carbon sequestration across the wide range of structural diversity in semiarid ecosystems. Therefore, we used eddy covariance measurements to quantify how net ecosystem production of carbon dioxide (NEP) differed with relative grass and woody plant abundance over the last decade of drought in four Southwest U.S. ecosystems. We identified a precipitation “pivot point” in the carbon balance for each ecosystem where annual NEP switched from negative to positive. Ecosystems with grass had pivot points closer to the drought period precipitation than the predrought average, making them more likely to be carbon sinks (and a grass-free shrubland, a carbon source) during the current drought. One reason for this is that the grassland located closest to the shrubland supported higher leaf area and photosynthesis at the same water availability. Higher leaf area was associated with a greater proportion of evapotranspiration being transpiration (T/ET), and therefore with higher ecosystem water use efficiency (gross ecosystem photosynthesis/ET). Our findings strongly show that water availability is a primary driver of both gross and net semiarid productivity and illustrate that structural differences may contribute to the speed at which ecosystem carbon cycling adjusts to climatic shifts.
54. Sean T. Michaletz: Convergence of terrestrial plant production across global climate gradients 2014-07-20 Nature+
    
    Net primary production is affected by temperature and precipitation, but whether this is a direct kinetic effect on plant metabolism or an indirect ecological effect mediated by changes in plant age, plant biomass or growing season length is unclear — this study develops metabolic scaling theory to be able to answer this question and applies it to a global data set of plant productivity, concluding that it is indirect effects that explain the influence of climate on productivity, which is characterized by a common scaling relationship across climate gradients.
55. W. Kolby Smith: Large divergence of satellite and Earth system model estimates of global terrestrial CO2 fertilization 2015-12-07 Nature: Climate Change+
    
    Satellite-derived estimates of increases in terrestrial net primary productivity are less than half of those derived from Earth system models. This discrepancy is explained by over-sensitivity of Earth system models to atmospheric CO2 concentrations.
56. N. A. D’souza: Elevated surface chlorophyll associated with natural oil seeps in the Gulf of Mexico 2016-01-25 Nature: Geoscience+
    
    Natural hydrocarbon seeps account for up to 47% of the oil released into the oceans. In situ and remote measurements of chlorophyll concentrations suggest that natural hydrocarbons enhance productivity in surface waters in the Gulf of Mexico.
57. Luis P. A. M. Duprat: Enhanced Southern Ocean marine productivity due to fertilization by giant icebergs 2016-01-11 Nature: Geoscience+
    
    Nutrient input from icebergs can fertilize productivity in the ocean. Ten years of satellite measurements reveal that giant icebergs could be responsible for up to 20% of carbon export to depth in the Southern Ocean.
58. Shestakova, T. A., Gutierrez, E., Kirdyanov, A. V., Camarero, J. J., Genova, M., Knorre, A. A., Linares, J. C., Resco de Dios, V., Sanchez–Salguero, R., Voltas, J.: Synchronous tree growth in response to warming [Environmental Sciences] 2016-01-19T11:53:46-08:00 PNAS - Environmental Sciences+
    
    Forests play a key role in the carbon balance of terrestrial ecosystems. One of the main uncertainties in global change predictions lies in how the spatiotemporal dynamics of forest productivity will be affected by climate warming. Here we show an increasing influence of climate on the spatial variability of tree...

Autotrophic Respiration

1. Martin P. Girardin, Edward H. Hogg, Pierre Y. Bernier, Werner A. Kurz, Xiao Jing Guo, Guillaume Cyr: Negative impacts of high temperatures on growth of black spruce forests intensify with the anticipated climate warming 2015-10-28T01:16:17.4325-05:00 Global Change Biology+
   
   An increasing number of studies conclude that water limitations and heat stress may hinder the capacity of black spruce (Picea mariana (Mill.) B.S.P.) trees, a dominant species of Canada's boreal forests, to grow and assimilate atmospheric carbon. However, there is currently no scientific consensus on the future of these forests over the next century in the context of widespread climate warming. The large spatial extent of black spruce forests across the Canadian boreal forest and associated variability in climate, demography, and site conditions pose challenges for projecting future climate change responses. Here we provide an evaluation of the impacts of climate warming and drying, as well as increasing [CO2], on the aboveground productivity of black spruce forests across Canada south of 60°N for the period 1971 to 2100. We use a new extensive network of tree-ring data obtained from Canada's National Forest Inventory, spatially explicit simulations of net primary productivity (NPP) and its drivers, and multivariate statistical modeling. We found that soil water availability is a significant driver of black spruce interannual variability in productivity across broad areas of the western to eastern Canadian boreal forest. Interannual variability in productivity was also found to be driven by autotrophic respiration in the warmest regions. In most regions, the impacts of soil water availability and respiration on interannual variability in productivity occurred during the phase of carbohydrate accumulation the year preceding tree-ring formation. Results from projections suggest an increase in the importance of soil water availability and respiration as limiting factors on NPP over the next century due to warming, but this response may vary to the extent that other factors such as carbon dioxide fertilization, and respiration acclimation to high temperature, contribute to dampening these limitations.
2. Kaiyu Guan, Joseph A. Berry, Yongguang Zhang, Joanna Joiner, Luis Guanter, Grayson Badgley, David B. Lobell: Improving the monitoring of crop productivity using spaceborne solar-induced fluorescence 2015-11-10T08:35:43.546558-05:00 Global Change Biology+
   
   Large-scale monitoring of crop growth and yield has important value for forecasting food production and prices and ensuring regional food security. A newly emerging satellite retrieval, solar-induced fluorescence (SIF) of chlorophyll, provides for the first time a direct measurement related to plant photosynthetic activity (i.e. electron transport rate). Here, we provide a framework to link SIF retrievals and crop yield, accounting for stoichiometry, photosynthetic pathways, and respiration losses. We apply this framework to estimate United States crop productivity for 2007–2012, where we use the spaceborne SIF retrievals from the Global Ozone Monitoring Experiment-2 satellite, benchmarked with county-level crop yield statistics, and compare it with various traditional crop monitoring approaches. We find that a SIF-based approach accounting for photosynthetic pathways (i.e. C3 and C4 crops) provides the best measure of crop productivity among these approaches, despite the fact that SIF sensors are not yet optimized for terrestrial applications. We further show that SIF provides the ability to infer the impacts of environmental stresses on autotrophic respiration and carbon-use-efficiency, with a substantial sensitivity of both to high temperatures. These results indicate new opportunities for improved mechanistic understanding of crop yield responses to climate variability and change.

Drought

1. Andrea D. Almeida Castanho, David Galbraith, Ke Zhang, Michael T. Coe, Marcos H. Costa, Paul Moorcroft: Changing Amazon biomass and the role of atmospheric CO2 concentration, climate, and land use 2016-01-19T18:44:00.211132-05:00 Global Biogeochemical Cycles+
   
   The Amazon tropical evergreen forest is an important component of the global carbon budget. Its forest floristic composition, structure, and function are sensitive to changes in climate, atmospheric composition, and land use. In this study biomass and productivity simulated by three dynamic global vegetation models (Integrated Biosphere Simulator, Ecosystem Demography Biosphere Model, and Joint UK Land Environment Simulator) for the period 1970–2008 are compared with observations from forest plots (Rede Amazónica de Inventarios Forestales). The spatial variability in biomass and productivity simulated by the DGVMs is low in comparison to the field observations in part because of poor representation of the heterogeneity of vegetation traits within the models. We find that over the last four decades the CO2 fertilization effect dominates a long-term increase in simulated biomass in undisturbed Amazonian forests, while land use change in the south and southeastern Amazonia dominates a reduction in Amazon aboveground biomass, of similar magnitude to the CO2 biomass gain. Climate extremes exert a strong effect on the observed biomass on short time scales, but the models are incapable of reproducing the observed impacts of extreme drought on forest biomass. We find that future improvements in the accuracy of DGVM predictions will require improved representation of four key elements: (1) spatially variable plant traits, (2) soil and nutrients mediated processes, (3) extreme event mortality, and (4) sensitivity to climatic variability. Finally, continued long-term observations and ecosystem-scale experiments (e.g. Free-Air CO2 Enrichment experiments) are essential for a better understanding of the changing dynamics of tropical forests.
2. Elena Pellizzari, J. Julio Camarero, Antonio Gazol, Gabriel Sangüesa-Barreda, Marco Carrer: Wood anatomy and carbon-isotope discrimination support long-term hydraulic deterioration as a major cause of drought-induced dieback 2016-01-21T03:18:49.437865-05:00 Global Change Biology+
   
   Hydraulic impairment due to xylem embolism and carbon starvation are the two proposed mechanisms explaining drought-induced forest dieback and tree death. Here we evaluate the relative role played by these two mechanisms in the long-term by quantifying wood-anatomical traits (tracheid size and area of parenchyma rays) and estimating the intrinsic water-use efficiency (iWUE) from carbon isotopic discrimination. We selected silver fir and Scots pine stands in NE Spain with ongoing dieback processes and compared trees showing contrasting vigour (declining vs. non-declining trees). In both species earlywood tracheids in declining trees showed smaller lumen area with thicker cell wall, inducing a lower theoretical hydraulic conductivity. Parenchyma ray area was similar between the two vigour classes. Wet spring and summer conditions promoted the formation of larger lumen areas, particularly in the case of non-declining trees. Declining silver firs presented a lower iWUE than conspecific non-declining trees, but the reverse pattern was observed in Scots pine. The described patterns in wood anatomical traits and iWUE are coherent with a long-lasting deterioration of the hydraulic system in declining trees prior to their dieback. Retrospective quantifications of lumen area permit to forecast dieback in declining trees 2-5 decades before growth decline started. Wood anatomical traits provide a robust tool to reconstruct the long-term capacity of trees to withstand drought-induced dieback. This article is protected by copyright. All rights reserved.
3. Gabriele Manoli, Jean-Christophe Domec, Kimberly Novick, Andrew Christopher Oishi, Asko Noormets, Marco Marani, Gabriel Katul: Soil-Plant-Atmosphere Conditions Regulating Convective Cloud Formation Above Southeastern US Pine Plantations 2016-01-13T05:36:50.089559-05:00 Global Change Biology+
   
   Loblolly pine trees (Pinus taeda L.) occupy more than 20% of the forested area in the Southern United States, represent more than 50% of the standing pine volume in this region, and remove from the atmosphere about 500 g C m−2 per year through net ecosystem exchange. Hence, their significance as a major regional carbon sink can hardly be disputed. What is disputed is whether the proliferation of young plantations replacing old forest in the Southern United States will alter key aspects of the hydrological cycle, including convective rainfall, which is the focus of the present work. Ecosystem fluxes of sensible (Hs) and latent heat (LE) and large-scale, slowlyevolving free atmospheric temperature and water vapor content are known to be first order controls on the formation of convective clouds in the atmospheric boundary layer. These controlling processes are here described by a zero-order analytical model aimed at assessing how plantations of different ages may regulate the persistence and transition of the atmospheric system between cloudy and cloudless conditions. Using the analytical model together with field observations, the roles of ecosystem Hs and LE on convective cloud formation are explored relative to the entrainment of heat and moisture from the free atmosphere. Our results demonstrate that cloudycloudless regimes at the land surface are regulated by a non-linear relation between the Bowen ratio Bo = Hs/LE and root-zone soil water content, suggesting that young/mature pines ecosystems have the ability to recirculate available water (through rainfall predisposition mechanisms). Such non-linearity was not detected in a much older pine stand, suggesting a higher tolerance to drought but a limited control on boundary layer dynamics. These results enable the generation of hypotheses about the impacts on convective cloud formation driven by afforestation/deforestation and groundwater depletion projected to increase following increased human population in the Southeastern United States. This article is protected by copyright. All rights reserved.
4. Chongyang Xu, Hongyan Liu, A. Park Williams, Yi Yin, Xiuchen Wu: Trends toward an earlier peak of the growing season in Northern Hemisphere mid-latitudes 2016-01-11T06:42:04.474098-05:00 Global Change Biology+
   
   Changes in peak photosynthesis timing (PPT) could substantially change the seasonality of the terrestrial carbon cycle. Spring PPT in dry regions has been documented for some individual plant species on a stand scale, but both the spatio-temporal pattern of shifting PPT on a continental scale and its determinants remain unclear. Here we use satellite measurements of vegetation greenness to find that the majority of Northern Hemisphere, mid-latitude vegetated area experienced a trend toward earlier PPT during 1982-2012, with significant trends of approximately 0.61 day·year−1 across 19.4% of areas. These shifts correspond to increased annual accumulation of growing degree days (GDD) due to warming and are most highly concentrated in the eastern United States and Europe. Earlier mean PPT is generally a trait common among areas with summer temperatures higher than 27.62.9°C, summer precipitation lower than 84.241.5 mm, and fraction of pre-growing season precipitation greater than 89.21.5%. The trends toward earlier PPT discovered here have co-occurred with overall increases in vegetation greenness throughout the growing season, suggesting that summer drought is not a dominant driver of these trends. These results imply that continued warming facilitate continued shifts toward earlier PPT and cause these trends to become more pervasive, with important implications for terrestrial carbon, water, nutrient, and energy budgets. This article is protected by copyright. All rights reserved.
5. Jinyan Yang, Yujie He, Doug P. Aubrey, Qianlai Zhuang, Robert O. Teskey: Global patterns and predictors of stem CO2 efflux in forest ecosystems 2015-12-14T23:01:54.618357-05:00 Global Change Biology+
   
   Stem CO2 efflux (ES) plays an important role in the carbon balance of forest ecosystems. However, its primary controls at the global scale are poorly understood and observation-based global estimates are lacking. We synthesized data from 121 published studies across global forest ecosystems and examined the relationships between annual ES and biotic and abiotic factors at individual, biome, and global scales, and developed a global gridded estimate of annual ES. We tested the following hypotheses: (1) Leaf area index (LAI) will be highly correlated with annual ES at biome and global scales; (2) There will be parallel patterns in stem and root CO2 effluxes (RA) in all forests; (3) Annual ES will decline with forest age; and (4) LAI coupled with mean annual temperature (MAT) and mean annual precipitation (MAP) will be sufficient to predict annual ES across forests in different regions. Positive linear relationships were found between ES and LAI, as well as gross primary production (GPP), net primary production (NPP), wood NPP, soil CO2 efflux (RS) and RA. Annual ES was correlated with RA in temperate forests after controlling for GPP and MAT, suggesting other additional factors contributed to the relationship. Annual ES tended to decrease with stand age. Leaf area index, MAT and MAP predicted 74% of variation in ES at global scales. Our statistical model estimated a global annual ES of 6.7 ± 1.1 Pg C yr−1 over the period of 2000-2012 with little interannual variability. Modelled mean annual ES was 89 ± 53, 248 ± 127 and 506 ± 262 g C m−2 yr−1 for boreal, temperate, and tropical forests, respectively. We recommend that future studies report ES at a standardized constant temperature, incorporate more manipulative treatments, such as fertilization and drought, and whenever possible, simultaneously measure both aboveground and belowground CO2 fluxes. This article is protected by copyright. All rights reserved.
6. David L. Hoover, Brendan M. Rogers: Not all droughts are created equal: the impacts of interannual drought pattern and magnitude on grassland carbon cycling 2015-11-14T10:20:52.313403-05:00 Global Change Biology+
   
   Climate extremes, such as drought, may have immediate and potentially prolonged effects on carbon cycling. Grasslands store approximately one third of all terrestrial carbon and may become carbon sources during droughts. However, the magnitude and duration of drought-induced disruptions to the carbon cycle, as well as the mechanisms responsible, remain poorly understood. Over the next century, global climate models predict an increase in two types of drought: chronic but subtle “press-droughts”, and shorter-term but extreme “pulse-droughts”. Much of our current understanding of the ecological impacts of drought comes from experimental rainfall manipulations. These studies have been highly valuable, but are often short-term and rarely quantify carbon feedbacks. To address this knowledge gap, we used the Community Land Model 4.0 to examine the individual and interactive effects of pulse- and press-droughts on carbon cycling in a mesic grassland of the US Great Plains. A series of modeling experiments were imposed by varying drought magnitude (precipitation amount) and interannual pattern (press- vs. pulse-droughts) to examine the effects on carbon storage and cycling at annual to century timescales. We present three main findings. First, a single year pulse-drought had immediate and prolonged effects on carbon storage due to differential sensitivities of ecosystem respiration and gross primary production. Second, short-term pulse-droughts caused greater carbon loss than chronic press-droughts when total precipitation reductions over a twenty-year period were equivalent. Third, combining pulse- and press-droughts had intermediate effects on carbon loss compared to the independent drought types, except at high drought levels. Overall, these results suggest that interannual drought pattern may be as important for carbon dynamics as drought magnitude, and that extreme droughts may have long-lasting carbon feedbacks in grassland ecosystems. This article is protected by copyright. All rights reserved.
7. Paulo M. Brando, Claudinei Oliveria-Santos, Wanderley Rocha, Roberta Cury, Michael T. Coe: Effects of experimental fuel additions on fire intensity and severity: unexpected carbon resilience of a neotropical forest 2016-01-10T23:26:59.646469-05:00 Global Change Biology+
   
   Global changes and associated droughts, heat waves, logging activities, and forest fragmentation may intensify fires in Amazonia by altering forest microclimate and fuel dynamics. To isolate the effects of fuel loads on fire behavior and fire-induced changes in forest carbon cycling, we manipulated fine fuel loads in a fire experiment located in southeast Amazonia. We predicted that a 50% increase in fine fuel loads would disproportionally increase fire intensity and severity (i.e., tree mortality and losses in carbon stocks) due to multiplicative effects of fine fuel loads on the rate of fire spread, fuel consumption, and burned area. The experiment followed a fully replicated randomized block design (N = 6) comprised of unburned control plots and burned plots that were treated with and without fine fuel additions. The fuel addition treatment significantly increased burned area (+22%) and consequently canopy openness (+10%), fine fuel combustion (+5%), and mortality of individuals ≥5 cm in diameter at breast height (dbh; +37%). Surprisingly, we observed nonsignificant effects of the fuel addition treatment on fireline intensity, and no significant differences among the three treatments for (i) mortality of large trees (≥30 cm dbh), (ii) aboveground forest carbon stocks, and (iii) soil respiration. It was also surprising that postfire tree growth and wood increment were higher in the burned plots treated with fuels than in the unburned control. These results suggest that (i) fine fuel load accumulation increases the likelihood of larger understory fires and (ii) single, low-intensity fires weakly influence carbon cycling of this primary neotropical forest, although delayed postfire mortality of large trees may lower carbon stocks over the long term. Overall, our findings indicate that increased fine fuel loads alone are unlikely to create threshold conditions for high-intensity, catastrophic fires during nondrought years.
8. Adam F. A. Pellegrini, Augusto C. Franco, William A. Hoffmann: Shifts in functional traits elevate risk of fire-driven tree dieback in tropical savanna and forest biomes 2016-01-04T04:49:22.197969-05:00 Global Change Biology+
   
   Numerous predictions indicate rising CO2 will accelerate the expansion of forests into savannas. Although encroaching forests can sequester carbon over the short term, increased fires and drought-fire interactions could offset carbon gains, which may be amplified by the shift toward forest plant communities more susceptible to fire-driven dieback. We quantify how bark thickness determines the ability of individual tree species to tolerate fire and subsequently determine the fire sensitivity of ecosystem carbon across 180 plots in savannas and forests throughout the 2.2-million km2 Cerrado region in Brazil. We find that not accounting for variation in bark thickness across tree species underestimated carbon losses in forests by ~50%, totaling 0.22 PgC across the Cerrado region. The lower bark thicknesses of plant species in forests decreased fire tolerance to such an extent that a third of carbon gains during forest encroachment may be at risk of dieback if burned. These results illustrate that consideration of trait-based differences in fire tolerance is critical for determining the climate-carbon-fire feedback in tropical savanna and forest biomes.
9. Leander D. L. Anderegg, Janneke HilleRisLambers: Drought stress limits the geographic ranges of two tree species via different physiological mechanisms 2015-12-10T06:13:52.072515-05:00 Global Change Biology+
   
   Range shifts are among the most ubiquitous ecological responses to anthropogenic climate change and have large consequences for ecosystems. Unfortunately, the ecophysiological forces that constrain range boundaries are poorly understood, making it difficult to mechanistically project range shifts. To explore the physiological mechanisms by which drought stress controls dry range boundaries in trees, we quantified elevational variation in drought tolerance and in drought avoidance-related functional traits of a widespread gymnosperm (ponderosa pine – Pinus ponderosa) and angiosperm (trembling aspen – Populus tremuloides) tree species in the southwestern USA. Specifically, we quantified tree-to-tree variation in growth, water stress (predawn and midday xylem tension), drought avoidance traits (branch conductivity, leaf/needle size, tree height, leaf area-to-sapwood area ratio), and drought tolerance traits (xylem resistance to embolism, hydraulic safety margin, wood density) at the range margins and range center of each species. Although water stress increased and growth declined strongly at lower range margins of both species, ponderosa pine and aspen showed contrasting patterns of clinal trait variation. Trembling aspen increased its drought tolerance at its dry range edge by growing stronger but more carbon dense branch and leaf tissues, implying an increased cost of growth at its range boundary. By contrast, ponderosa pine showed little elevational variation in drought-related traits but avoided drought stress at low elevations by limiting transpiration through stomatal closure, such that its dry range boundary is associated with limited carbon assimilation even in average climatic conditions. Thus, the same climatic factor (drought) may drive range boundaries through different physiological mechanisms – a result that has important implications for process-based modeling approaches to tree biogeography. Further, we show that comparing intraspecific patterns of trait variation across ranges, something rarely done in a range-limit context, helps elucidate a mechanistic understanding of range constraints.
10. William H. Schlesinger, Michael C. Dietze, Robert B. Jackson, Richard P. Phillips, Charles C. Rhoades, Lindsey E. Rustad, James M. Vose: Forest biogeochemistry in response to drought 2015-11-18T06:22:11.226599-05:00 Global Change Biology+
    
    Trees alter their use and allocation of nutrients in response to drought, and changes in soil nutrient cycling and trace gas flux (N2O and CH4) are observed when experimental drought is imposed on forests. In extreme droughts, trees are increasingly susceptible to attack by pests and pathogens, which can lead to major changes in nutrient flux to the soil. Extreme droughts often lead to more common and more intense forest fires, causing dramatic changes in the nutrient storage and loss from forest ecosystems. Changes in the future manifestation of drought will affect carbon uptake and storage in forests, leading to feedbacks to the Earth's climate system. We must improve the recognition of drought in nature, our ability to manage our forests in the face of drought, and the parameterization of drought in earth system models for improved predictions of carbon uptake and storage in the world's forests.
11. Melanie Ruosch, Renato Spahni, Fortunat Joos, Paul D. Henne, Willem O. Knaap, Willy Tinner: Past and future evolution of Abies alba forests in Europe – comparison of a dynamic vegetation model with palaeo data and observations 2016-01-05T06:50:48.455384-05:00 Global Change Biology+
    
    Information on how species distributions and ecosystem services are impacted by anthropogenic climate change is important for adaptation planning. Palaeo data suggest that Abies alba formed forests under significantly warmer-than-present conditions in Europe and might be a native substitute for widespread drought-sensitive temperate and boreal tree species such as beech (Fagus sylvatica) and spruce (Picea abies) under future global warming conditions. Here, we combine pollen and macrofossil data, modern observations, and results from transient simulations with the LPX-Bern dynamic global vegetation model to assess past and future distributions of A. alba in Europe. LPX-Bern is forced with climate anomalies from a run over the past 21 000 years with the Community Earth System Model, modern climatology, and with 21st-century multimodel ensemble results for the high-emission RCP8.5 and the stringent mitigation RCP2.6 pathway. The simulated distribution for present climate encompasses the modern range of A. alba, with the model exceeding the present distribution in north-western and southern Europe. Mid-Holocene pollen data and model results agree for southern Europe, suggesting that at present, human impacts suppress the distribution in southern Europe. Pollen and model results both show range expansion starting during the Bølling–Allerød warm period, interrupted by the Younger Dryas cold, and resuming during the Holocene. The distribution of A. alba expands to the north-east in all future scenarios, whereas the potential (currently unrealized) range would be substantially reduced in southern Europe under RCP8.5. A. alba maintains its current range in central Europe despite competition by other thermophilous tree species. Our combined palaeoecological and model evidence suggest that A. alba may ensure important ecosystem services including stand and slope stability, infrastructure protection, and carbon sequestration under significantly warmer-than-present conditions in central Europe.
12. : Canopy-scale biophysical controls of transpiration and evaporation in the Amazon Basin Wed, 27 Jan 2016 01:59:26 +0100 Hydrology and Earth System Sciences+
    
    Canopy-scale biophysical controls of transpiration and evaporation in the Amazon Basin
    Kaniska Mallick, Ivonne Trebs, Eva Boegh, Laura Giustarini, Martin Schlerf, Darren Drewry, Lucien Hoffmann, Celso von Randow, Bart Kruijt, Alessandro Araùjo, Scott Saleska, James R. Ehleringer, Tomas F. Domingues, Jean Pierre H. B. Ometto, Antonio D. Nobre, Osvaldo Luiz Leal de Moraes, Matthew Hayek, J. William Munger, and Steve Wofsy
    Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2015-552,2016
    Manuscript under review for HESS (discussion: open, 0 comments)
    While quantifying vegetation water use over multiple plant functions types in the Amazon Basin, we found substantial biological control during drought as well as water stress period and dominant climatic control during water surplus period. This work has a direct implication in understanding the resilience of the Amazon forest in the spectre of frequent drought menace as well as the role of drought induced plant biological functioning in modulating the water-carbon coupling in this ecosystem.
13. Yuanyuan Huang, Stefan Gerber: Nitrogen restrictions buffer modeled interactions of water with the carbon cycle 2016-01-23T13:58:26.084883-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Terrestrial carbon and water cycles are coupled at multiple spatiotemporal scales and are crucial to carbon sequestration. Water related climate extremes, such as drought and intense precipitation, can substantially affect the carbon cycle. Meanwhile, nitrogen is a limiting resource to plant and has therefore the potential to alter the coupling of water and carbon cycles on land. Here we assess the effect of nitrogen limitation on the response of the terrestrial carbon cycle to moisture anomalies using Geophysical Fluid Dynamics Laboratory's land surface model LM3V-N. We analyzed the response of three central carbon fluxes: net primary productivity (NPP), heterotrophic respiration (Rh), and net ecosystem productivity (NEP, the difference between NPP and Rh) and how these fluxes were altered under anomalies of the standardized precipitation and evapotranspiration index (SPEI). We found that globally, the correlations between each of the carbon flux and SPEI depended on the timescale and a strong legacy effect of SPEI anomalies on Rh. Consideration of nitrogen constraints reduced anomalies in carbon fluxes in response to extreme dry/wet events. This nitrogen-induced buffer constrained the growth of plants under wet extremes and allowed for enhanced growth during droughts. Extra gain of soil moisture from the downregulation of canopy transpiration by nitrogen limitation and shifts in the relative importance of water and nitrogen limitation during dry/wet extreme events are possible mechanisms contributing to the buffering of modeled NPP and NEP. Responses of Rh to moisture anomalies were much weaker compared to NPP, and N buffering effects were less evident.
14. Russell L. Scott, Joel A. Biederman, Erik P. Hamerlynck, Greg A. Barron-Gafford: The carbon balance pivot point of southwestern U.S. semiarid ecosystems: Insights from the 21st century drought 2015-12-22T14:43:14.182369-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Global-scale studies indicate that semiarid regions strongly regulate the terrestrial carbon sink. However, we lack understanding of how climatic shifts, such as decadal drought, impact carbon sequestration across the wide range of structural diversity in semiarid ecosystems. Therefore, we used eddy covariance measurements to quantify how net ecosystem production of carbon dioxide (NEP) differed with relative grass and woody plant abundance over the last decade of drought in four Southwest U.S. ecosystems. We identified a precipitation “pivot point” in the carbon balance for each ecosystem where annual NEP switched from negative to positive. Ecosystems with grass had pivot points closer to the drought period precipitation than the predrought average, making them more likely to be carbon sinks (and a grass-free shrubland, a carbon source) during the current drought. One reason for this is that the grassland located closest to the shrubland supported higher leaf area and photosynthesis at the same water availability. Higher leaf area was associated with a greater proportion of evapotranspiration being transpiration (T/ET), and therefore with higher ecosystem water use efficiency (gross ecosystem photosynthesis/ET). Our findings strongly show that water availability is a primary driver of both gross and net semiarid productivity and illustrate that structural differences may contribute to the speed at which ecosystem carbon cycling adjusts to climatic shifts.
15. Jia Yang, Hanqin Tian, Bo Tao, Wei Ren, Shufen Pan, Yongqiang Liu, Yuhang Wang: A growing importance of large fires in conterminous United States during 1984–2012 2015-12-30T09:27:05.097465-05:00 Journal of Geophysical Research: Biogeosciences+
    
    Fire frequency, extent, and size exhibit a strong linkage with climate conditions and play a vital role in the climate system. Previous studies have shown that the frequency of large fires in the western United States increased significantly since the mid-1980s due to climate warming and frequent droughts. However, less work has been conducted to examine burned area and fire emissions of large fires at a national scale, and the underlying mechanisms accounting for the increases in the frequency of large fires are far from clear. In this study, we integrated remote-sensed fire perimeter and burn severity data sets into the Dynamic Land Ecosystem Model to estimate carbon emissions from large fires (i.e., fires with size larger than 1000 acres or 4.05 km2) in conterminous United States from 1984 to 2012. The results show that average area burned by large fires was 1.44 × 104 km2 yr−1 and carbon emissions from large fires were 17.65 Tg C yr−1 during the study period. According to the Mann-Kendall trend test, annual burned area and pyrogenic carbon emissions presented significant upward trends at the rates of 810 km2 yr−1 and 0.87 Tg C yr−1, respectively. Characteristic fire size (fire size with the largest contribution to the total burned area) in the period of 2004–2012 increased by 176.1% compared to the period of 1984–1993. We further found that the larger fires were associated with higher burn severity and occurred more frequently in the warmer and drier conditions. This finding implies that the continued warming and drying trends in the 21st century would enhance the total burned area and fire emissions due to the contributions of larger and more severe wildfires.
16. L. Rowland: Death from drought in tropical forests is triggered by hydraulics not carbon starvation 2015-11-23 Nature+
    
    Drought threatens tropical rainforests over seasonal to decadal timescales, but the drivers of tree mortality following drought remain poorly understood. It has been suggested that reduced availability of non-structural carbohydrates (NSC) critically increases mortality risk through insufficient carbon supply to metabolism (‘carbon starvation’). However, little is known about how NSC stores are affected by drought, especially over the long term, and whether they are more important than hydraulic processes in determining drought-induced mortality. Using data from the world’s longest-running experimental drought study in tropical rainforest (in the Brazilian Amazon), we test whether carbon starvation or deterioration of the water-conducting pathways from soil to leaf trigger tree mortality. Biomass loss from mortality in the experimentally droughted forest increased substantially after >10 years of reduced soil moisture availability. The mortality signal was dominated by the death of large trees, which were at a much greater risk of hydraulic deterioration than smaller trees. However, we find no evidence that the droughted trees suffered carbon starvation, as their NSC concentrations were similar to those of non-droughted trees, and growth rates did not decline in either living or dying trees. Our results indicate that hydraulics, rather than carbon starvation, triggers tree death from drought in tropical rainforest.

Soil Respiration

1. Paulo M. Brando, Claudinei Oliveria-Santos, Wanderley Rocha, Roberta Cury, Michael T. Coe: Effects of experimental fuel additions on fire intensity and severity: unexpected carbon resilience of a neotropical forest 2016-01-10T23:26:59.646469-05:00 Global Change Biology+
   
   Global changes and associated droughts, heat waves, logging activities, and forest fragmentation may intensify fires in Amazonia by altering forest microclimate and fuel dynamics. To isolate the effects of fuel loads on fire behavior and fire-induced changes in forest carbon cycling, we manipulated fine fuel loads in a fire experiment located in southeast Amazonia. We predicted that a 50% increase in fine fuel loads would disproportionally increase fire intensity and severity (i.e., tree mortality and losses in carbon stocks) due to multiplicative effects of fine fuel loads on the rate of fire spread, fuel consumption, and burned area. The experiment followed a fully replicated randomized block design (N = 6) comprised of unburned control plots and burned plots that were treated with and without fine fuel additions. The fuel addition treatment significantly increased burned area (+22%) and consequently canopy openness (+10%), fine fuel combustion (+5%), and mortality of individuals ≥5 cm in diameter at breast height (dbh; +37%). Surprisingly, we observed nonsignificant effects of the fuel addition treatment on fireline intensity, and no significant differences among the three treatments for (i) mortality of large trees (≥30 cm dbh), (ii) aboveground forest carbon stocks, and (iii) soil respiration. It was also surprising that postfire tree growth and wood increment were higher in the burned plots treated with fuels than in the unburned control. These results suggest that (i) fine fuel load accumulation increases the likelihood of larger understory fires and (ii) single, low-intensity fires weakly influence carbon cycling of this primary neotropical forest, although delayed postfire mortality of large trees may lower carbon stocks over the long term. Overall, our findings indicate that increased fine fuel loads alone are unlikely to create threshold conditions for high-intensity, catastrophic fires during nondrought years.
2. Craig A. Emmerton, Vincent L. St. Louis, Elyn R. Humphreys, John A. Gamon, Joel D. Barker, Gilberto Z. Pastorello: Net ecosystem exchange of CO2 with rapidly changing high Arctic landscapes 2015-12-26T08:55:04.517149-05:00 Global Change Biology+
   
   High Arctic landscapes are expansive and changing rapidly. However, our understanding of their functional responses and potential to mitigate or enhance anthropogenic climate change is limited by few measurements. We collected eddy covariance measurements to quantify the net ecosystem exchange (NEE) of CO2 with polar semidesert and meadow wetland landscapes at the highest latitude location measured to date (82°N). We coupled these rare data with ground and satellite vegetation production measurements (Normalized Difference Vegetation Index; NDVI) to evaluate the effectiveness of upscaling local to regional NEE. During the growing season, the dry polar semidesert landscape was a near-zero sink of atmospheric CO2 (NEE: −0.3 ± 13.5 g C m−2). A nearby meadow wetland accumulated over 300 times more carbon (NEE: −79.3 ± 20.0 g C m−2) than the polar semidesert landscape, and was similar to meadow wetland NEE at much more southerly latitudes. Polar semidesert NEE was most influenced by moisture, with wetter surface soils resulting in greater soil respiration and CO2 emissions. At the meadow wetland, soil heating enhanced plant growth, which in turn increased CO2 uptake. Our upscaling assessment found that polar semidesert NDVI measured on-site was low (mean: 0.120–0.157) and similar to satellite measurements (mean: 0.155–0.163). However, weak plant growth resulted in poor satellite NDVI–NEE relationships and created challenges for remotely detecting changes in the cycling of carbon on the polar semidesert landscape. The meadow wetland appeared more suitable to assess plant production and NEE via remote sensing; however, high Arctic wetland extent is constrained by topography to small areas that may be difficult to resolve with large satellite pixels. We predict that until summer precipitation and humidity increases enough to offset poor soil moisture retention, climate-related changes to productivity on polar semideserts may be restricted.
3. Ana López-Ballesteros, Penélope Serrano-Ortiz, Enrique P. Sánchez-Cañete, Cecilio Oyonarte, Andrew S. Kowalski, Óscar Pérez-Priego, Francisco Domingo: Enhancement of the net CO2 release of a semiarid grassland in SE Spain by rain pulses 2016-01-09T04:05:01.261648-05:00 Journal of Geophysical Research: Biogeosciences+
   
   Occasional rain events occur over the dry season in semiarid ecosystems and cause immediate, large increases in the net CO2 efflux which gradually decrease over a few days following the rain event. In a semiarid grassland located in SE Spain, these precipitation pulses represent only 7% of dry season length but provoked approximately 40% of the carbon emitted during the dry seasons over 2009–2013. We performed a manipulation experiment to decompose the net ecosystem pulse response into its biological processes in order to quantify how much of a role photosynthesis and aboveground respiration play compared to soil respiration. Experimental results showed that while soil respiration was the dominant component of the net CO2 flux (net ecosystem CO2 exchange, NEE) over the irrigation day and the day after (80% of NEE), plant photosynthesis remained inactive until 2 days after the pulse, when it appeared to become as prevalent as soil respiration (approximately 40% of NEE). Additionally, aboveground respiration was generally secondary to soil respiration over the whole experiment. However, statistical results showed that aboveground carbon exchange was not significantly affected by the rain pulse, with soil respiration being the only component significantly affected by the rain pulse.

Throughfall Exclusion

1. : Total Atmospheric Mercury Deposition in Forest Areas in Korea Wed, 27 Jan 2016 04:05:27 +0100 Atmospheric Chemistry and Physics+
   
   Total Atmospheric Mercury Deposition in Forest Areas in Korea
   Jin-Su Han, Yong-Seok Seo, Moon-Kyung Kim, Thomas M. Holsen, and Seung-Muk Yi
   Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-7,2016
   Manuscript under review for ACP (discussion: open, 0 comments)
   Atmospheric mercury dry and wet deposition, mercury in throughfall and litterfall, and mercury volatilization from soil were measured during August 2008 to February 2010 in a temperate deciduous forest in Korea. The yearly estimated mercury budget was calculated using two input approaches. For this location the annual mercury accumulation was estimated to be 6.8 μg m^-2 yr^-1 or 3.9 μg m^-2 yr^-1 depending on the approach used.
2. Karen L. Vandecar, Christiane W. Runyan, Paolo D'Odorico, Deborah Lawrence, Birgit Schmook, Rishiraj Das: Phosphorus input through fog deposition in a dry tropical forest 2015-12-02T17:32:46.942272-05:00 Journal of Geophysical Research: Biogeosciences+
   
   In many tropical forests, where phosphorus (P) is considered a limiting nutrient, atmospheric deposition can contribute significantly to available P. Previous studies have shown that P inputs from atmospheric deposition are enhanced by plant canopies. This effect is explained as the result of increased deposition of P-rich aerosol particles (dry deposition) and fog droplets (fog or “occult” deposition) onto leaf surfaces. Here we studied the importance of fog as a source of P to a P-limited dry tropical forest. Throughout an 80 day period during the dry season when fog is most common, we sampled fog water and bulk precipitation in a clearing and measured leaf wetness and throughfall in an adjacent secondary and mature forest stand. During the study period, total P (PT) concentrations in fog water ranged from 0.15 to 6.40 mg/L, on average fourteenfold greater than PT concentrations in bulk precipitation (0.011 to 0.451 mg/L), and sixfold and sevenfold greater than throughfall PT concentrations in the secondary and mature forest stands, respectively (0.007 to 1.319 mg/L; 0.009 to 0.443 mg/L). Based on leaf area index, the frequency of fog deposition, and amount of water deposited per fog event, we estimate that fog delivers a maximum of 1.01 kg/ha/yr to secondary forest stands and 1.75 kg/ha/yr to mature forest stands, compared to 0.88 kg/ha/yr to secondary forest stands and 1.98 kg/ha/yr to mature forest stands via throughfall (wet + dry deposition) and stemflow. Thus, fog deposition may contribute substantially to available P in tropical dry forests.

Heterotrophic Respiration

1. Yuanyuan Huang, Stefan Gerber: Nitrogen restrictions buffer modeled interactions of water with the carbon cycle 2016-01-23T13:58:26.084883-05:00 Journal of Geophysical Research: Biogeosciences+
   
   Terrestrial carbon and water cycles are coupled at multiple spatiotemporal scales and are crucial to carbon sequestration. Water related climate extremes, such as drought and intense precipitation, can substantially affect the carbon cycle. Meanwhile, nitrogen is a limiting resource to plant and has therefore the potential to alter the coupling of water and carbon cycles on land. Here we assess the effect of nitrogen limitation on the response of the terrestrial carbon cycle to moisture anomalies using Geophysical Fluid Dynamics Laboratory's land surface model LM3V-N. We analyzed the response of three central carbon fluxes: net primary productivity (NPP), heterotrophic respiration (Rh), and net ecosystem productivity (NEP, the difference between NPP and Rh) and how these fluxes were altered under anomalies of the standardized precipitation and evapotranspiration index (SPEI). We found that globally, the correlations between each of the carbon flux and SPEI depended on the timescale and a strong legacy effect of SPEI anomalies on Rh. Consideration of nitrogen constraints reduced anomalies in carbon fluxes in response to extreme dry/wet events. This nitrogen-induced buffer constrained the growth of plants under wet extremes and allowed for enhanced growth during droughts. Extra gain of soil moisture from the downregulation of canopy transpiration by nitrogen limitation and shifts in the relative importance of water and nitrogen limitation during dry/wet extreme events are possible mechanisms contributing to the buffering of modeled NPP and NEP. Responses of Rh to moisture anomalies were much weaker compared to NPP, and N buffering effects were less evident.
2. J. F. Tjiputra, A. Grini, H. Lee: Impact of idealized future stratospheric aerosol injection on the large-scale ocean and land carbon cycles 2016-01-06T15:17:28.890712-05:00 Journal of Geophysical Research: Biogeosciences+
   
   Using an Earth system model, we simulate stratospheric aerosol injection (SAI) on top of the Representative Concentration Pathways 8.5 future scenario. Our idealized method prescribes aerosol concentration, linearly increasing from 2020 to 2100, and thereafter remaining constant until 2200. In the aggressive scenario, the model projects a cooling trend toward 2100 despite warming that persists in the high latitudes. Following SAI termination in 2100, a rapid global warming of 0.35 K yr−1 is simulated in the subsequent 10 years, and the global mean temperature returns to levels close to the reference state, though roughly 0.5 K cooler. In contrast to earlier findings, we show a weak response in the terrestrial carbon sink during SAI implementation in the 21st century, which we attribute to nitrogen limitation. The SAI increases the land carbon uptake in the temperate forest-, grassland-, and shrub-dominated regions. The resultant lower temperatures lead to a reduction in the heterotrophic respiration rate and increase soil carbon retention. Changes in precipitation patterns are key drivers for variability in vegetation carbon. Upon SAI termination, the level of vegetation carbon storage returns to the reference case, whereas the soil carbon remains high. The ocean absorbs nearly 10% more carbon in the geoengineered simulation than in the reference simulation, leading to a ∼15 ppm lower atmospheric CO2 concentration in 2100. The largest enhancement in uptake occurs in the North Atlantic. In both hemispheres' polar regions, SAI delays the sea ice melting and, consequently, export production remains low. In the deep water of North Atlantic, SAI-induced circulation changes accelerate the ocean acidification rate and broaden the affected area.
3. Yujie He, Jinyan Yang, Qianlai Zhuang, Jennifer W. Harden, Anthony D. McGuire, Yaling Liu, Gangsheng Wang, Lianhong Gu: Incorporating microbial dormancy dynamics into soil decomposition models to improve quantification of soil carbon dynamics of northern temperate forests 2015-12-22T14:43:34.594535-05:00 Journal of Geophysical Research: Biogeosciences+
   
   Soil carbon dynamics of terrestrial ecosystems play a significant role in the global carbon cycle. Microbial-based decomposition models have seen much growth recently for quantifying this role, yet dormancy as a common strategy used by microorganisms has not usually been represented and tested in these models against field observations. Here we developed an explicit microbial-enzyme decomposition model and examined model performance with and without representation of microbial dormancy at six temperate forest sites of different forest types. We then extrapolated the model to global temperate forest ecosystems to investigate biogeochemical controls on soil heterotrophic respiration and microbial dormancy dynamics at different temporal-spatial scales. The dormancy model consistently produced better match with field-observed heterotrophic soil CO2 efflux (RH) than the no dormancy model. Our regional modeling results further indicated that models with dormancy were able to produce more realistic magnitude of microbial biomass (<2% of soil organic carbon) and soil RH (7.5 ± 2.4 Pg C yr−1). Spatial correlation analysis showed that soil organic carbon content was the dominating factor (correlation coefficient = 0.4–0.6) in the simulated spatial pattern of soil RH with both models. In contrast to strong temporal and local controls of soil temperature and moisture on microbial dormancy, our modeling results showed that soil carbon-to-nitrogen ratio (C:N) was a major regulating factor at regional scales (correlation coefficient = −0.43 to −0.58), indicating scale-dependent biogeochemical controls on microbial dynamics. Our findings suggest that incorporating microbial dormancy could improve the realism of microbial-based decomposition models and enhance the integration of soil experiments and mechanistically based modeling.

Fertilization

1. Andrea D. Almeida Castanho, David Galbraith, Ke Zhang, Michael T. Coe, Marcos H. Costa, Paul Moorcroft: Changing Amazon biomass and the role of atmospheric CO2 concentration, climate, and land use 2016-01-19T18:44:00.211132-05:00 Global Biogeochemical Cycles+
   
   The Amazon tropical evergreen forest is an important component of the global carbon budget. Its forest floristic composition, structure, and function are sensitive to changes in climate, atmospheric composition, and land use. In this study biomass and productivity simulated by three dynamic global vegetation models (Integrated Biosphere Simulator, Ecosystem Demography Biosphere Model, and Joint UK Land Environment Simulator) for the period 1970–2008 are compared with observations from forest plots (Rede Amazónica de Inventarios Forestales). The spatial variability in biomass and productivity simulated by the DGVMs is low in comparison to the field observations in part because of poor representation of the heterogeneity of vegetation traits within the models. We find that over the last four decades the CO2 fertilization effect dominates a long-term increase in simulated biomass in undisturbed Amazonian forests, while land use change in the south and southeastern Amazonia dominates a reduction in Amazon aboveground biomass, of similar magnitude to the CO2 biomass gain. Climate extremes exert a strong effect on the observed biomass on short time scales, but the models are incapable of reproducing the observed impacts of extreme drought on forest biomass. We find that future improvements in the accuracy of DGVM predictions will require improved representation of four key elements: (1) spatially variable plant traits, (2) soil and nutrients mediated processes, (3) extreme event mortality, and (4) sensitivity to climatic variability. Finally, continued long-term observations and ecosystem-scale experiments (e.g. Free-Air CO2 Enrichment experiments) are essential for a better understanding of the changing dynamics of tropical forests.
2. Jinyan Yang, Yujie He, Doug P. Aubrey, Qianlai Zhuang, Robert O. Teskey: Global patterns and predictors of stem CO2 efflux in forest ecosystems 2015-12-14T23:01:54.618357-05:00 Global Change Biology+
   
   Stem CO2 efflux (ES) plays an important role in the carbon balance of forest ecosystems. However, its primary controls at the global scale are poorly understood and observation-based global estimates are lacking. We synthesized data from 121 published studies across global forest ecosystems and examined the relationships between annual ES and biotic and abiotic factors at individual, biome, and global scales, and developed a global gridded estimate of annual ES. We tested the following hypotheses: (1) Leaf area index (LAI) will be highly correlated with annual ES at biome and global scales; (2) There will be parallel patterns in stem and root CO2 effluxes (RA) in all forests; (3) Annual ES will decline with forest age; and (4) LAI coupled with mean annual temperature (MAT) and mean annual precipitation (MAP) will be sufficient to predict annual ES across forests in different regions. Positive linear relationships were found between ES and LAI, as well as gross primary production (GPP), net primary production (NPP), wood NPP, soil CO2 efflux (RS) and RA. Annual ES was correlated with RA in temperate forests after controlling for GPP and MAT, suggesting other additional factors contributed to the relationship. Annual ES tended to decrease with stand age. Leaf area index, MAT and MAP predicted 74% of variation in ES at global scales. Our statistical model estimated a global annual ES of 6.7 ± 1.1 Pg C yr−1 over the period of 2000-2012 with little interannual variability. Modelled mean annual ES was 89 ± 53, 248 ± 127 and 506 ± 262 g C m−2 yr−1 for boreal, temperate, and tropical forests, respectively. We recommend that future studies report ES at a standardized constant temperature, incorporate more manipulative treatments, such as fertilization and drought, and whenever possible, simultaneously measure both aboveground and belowground CO2 fluxes. This article is protected by copyright. All rights reserved.
3. Martin P. Girardin, Edward H. Hogg, Pierre Y. Bernier, Werner A. Kurz, Xiao Jing Guo, Guillaume Cyr: Negative impacts of high temperatures on growth of black spruce forests intensify with the anticipated climate warming 2015-10-28T01:16:17.4325-05:00 Global Change Biology+
   
   An increasing number of studies conclude that water limitations and heat stress may hinder the capacity of black spruce (Picea mariana (Mill.) B.S.P.) trees, a dominant species of Canada's boreal forests, to grow and assimilate atmospheric carbon. However, there is currently no scientific consensus on the future of these forests over the next century in the context of widespread climate warming. The large spatial extent of black spruce forests across the Canadian boreal forest and associated variability in climate, demography, and site conditions pose challenges for projecting future climate change responses. Here we provide an evaluation of the impacts of climate warming and drying, as well as increasing [CO2], on the aboveground productivity of black spruce forests across Canada south of 60°N for the period 1971 to 2100. We use a new extensive network of tree-ring data obtained from Canada's National Forest Inventory, spatially explicit simulations of net primary productivity (NPP) and its drivers, and multivariate statistical modeling. We found that soil water availability is a significant driver of black spruce interannual variability in productivity across broad areas of the western to eastern Canadian boreal forest. Interannual variability in productivity was also found to be driven by autotrophic respiration in the warmest regions. In most regions, the impacts of soil water availability and respiration on interannual variability in productivity occurred during the phase of carbohydrate accumulation the year preceding tree-ring formation. Results from projections suggest an increase in the importance of soil water availability and respiration as limiting factors on NPP over the next century due to warming, but this response may vary to the extent that other factors such as carbon dioxide fertilization, and respiration acclimation to high temperature, contribute to dampening these limitations.

Total Belowground Allocation

1. Gregory P. Asner, Roberta E. Martin: Convergent Elevation Trends in Canopy Chemical Traits of Tropical Forests 2015-11-19T01:38:25.930482-05:00 Global Change Biology+
   
   The functional biogeography of tropical forests is expressed in foliar chemicals that are key physiologically-based predictors of plant adaptation to changing environmental conditions including climate. However, understanding the degree to which environmental filters sort the canopy chemical characteristics of forest canopies remains a challenge. Here we report on the elevation and soil-type dependence of forest canopy chemistry among 75 compositionally and environmentally distinct forests in nine regions, with a total of 7819 individual trees representing 3246 species collected, identified and assayed for foliar traits. We assessed whether there are consistent relationships between canopy chemical traits and both elevation and soil type, and evaluated the general role of phylogeny in mediating of patterns of canopy traits within and across communities. Chemical trait variation and partitioning suggested a general model based on four inter-connected findings. First, geographic variation at the soil Order level, expressing broad changes in fertility, underpins major shifts in foliar phosphorus (P) and calcium (Ca). Second, elevation-dependent shifts in average community leaf dry mass per area (LMA), chlorophyll, and carbon allocation (including non-structural carbohydrates) are most strongly correlated with changes in foliar Ca. Third, chemical diversity within communities is driven by differences between species rather than by plasticity within species. Finally, elevation- and soil-dependent changes in N, LMA and leaf carbon allocation are mediated by canopy compositional turnover, whereas foliar P and Ca are driven more by changes in site conditions than by phylogeny. Our findings have broad implications for understanding the global ecology of humid tropical forests, and their functional responses to changing climate. This article is protected by copyright. All rights reserved.

Stem CO2 efflux

1. Kristina J. Anderson-Teixeira, Maria M. H. Wang, Jennifer C. McGarvey, David S. LeBauer: Carbon dynamics of mature and regrowth tropical forests derived from a pantropical database (TropForC-db) 2016-01-21T03:19:04.279301-05:00 Global Change Biology+
   
   Tropical forests play a critical role in the global carbon (C) cycle, storing ~45% of terrestrial C and constituting the largest component of the terrestrial C sink. Despite their central importance to the global C cycle, their ecosystem-level C cycles are not as well characterized as those of extra-tropical forests, and knowledge gaps hamper efforts to quantify C budgets across the tropics and to model tropical forest- climate interactions. To advance understanding of C dynamics of pantropical forests, we compiled a new database, the Tropical Forest C database (TropForC-db), which contains data on ground-based measurements of ecosystem-level C stocks and annual fluxes along with disturbance history. This database currently contains 3,568 records from 845 plots in 178 geographically distinct areas, making it the largest and most comprehensive database of its type. Using TropForC-db, we characterized C stocks and fluxes for young, intermediate-aged, and mature forests. Relative to existing C budgets of extra-tropical forests, mature tropical broadleaf evergreen forests had substantially higher gross primary productivity (GPP) and ecosystem respiration (Reco), their autotropic respiration (Ra) consumed a larger proportion (~67%) of GPP, and their woody stem growth (ANPPstem) represented a smaller proportion of net primary productivity (NPP, ~32%) or GPP (~9%). In regrowth stands, aboveground biomass increased rapidly during the first 20 years following stand-clearing disturbance, with slower accumulation following agriculture and in deciduous forests, and continued to accumulate at a slower pace in forests aged 20-100 years. Most other C stocks likewise increased with stand age, while potential to describe age trends in C fluxes was generally data-limited. We expect that TropForC-db will prove useful for model evaluation and for quantifying the contribution of forests to the global C cycle. The database version associated with this publication is archived in Dryad (DOI:10.5061/dryad.t516f) and a dynamic version is maintained at https://github. com/forc-db. This article is protected by copyright. All rights reserved.
2. Keryn I. Paul, Stephen H. Roxburgh, Jerome Chave, Jacqueline R. England, Ayalsew Zerihun, Alison Specht, Tom Lewis, Lauren T. Bennett, Thomas G. Baker, Mark A. Adams, Dan Huxtable, Kelvin D. Montagu, Daniel S. Falster, Mike Feller, Stan Sochacki, Peter Ritson, Gary Bastin, John Bartle, Dan Wildy, Trevor Hobbs, John Larmour, Rob Waterworth, Hugh T.L. Stewart, Justin Jonson, David I. Forrester, Grahame Applegate, Daniel Mendham, Matt Bradford, Anthony O'Grady, Daryl Green, Rob Sudmeyer, Stan J. Rance, John Turner, Craig Barton, Elizabeth H. Wenk, Tim Grove, Peter M. Attiwill, Elizabeth Pinkard, Don Butler, Kim Brooksbank, Beren Spencer, Peter Snowdon, Nick O'Brien, Michael Battaglia, David M Cameron, Steve Hamilton, Geoff McAuthur, Jenny Sinclair: Testing the generality of above-ground biomass allometry across plant functional types at the continent scale 2015-12-18T10:39:37.539111-05:00 Global Change Biology+
   
   Accurate ground-based estimation of the carbon stored in terrestrial ecosystems is critical to quantifying the global carbon budget. Allometric models provide cost-effective methods for biomass prediction. But do such models vary with ecoregion or plant functional type? We compiled 15,054 measurements of individual tree or shrub biomass from across Australia to examine the generality of allometric models for prediction above-ground biomass. This provided a robust case study because Australia includes ecoregions ranging from arid shrublands to tropical rainforests, and has a rich history of biomass research, particularly in planted forests. Regardless of ecoregion, for five broad categories of plant functional type (shrubs; multi-stemmed trees; trees of the genus Eucalyptus and closely related genera; other trees of high wood density; and other trees of low wood density), relationships between biomass and stem diameter were generic. Simple power-law models explained 84-95% of the variation in biomass, with little improvement in model performance when other plant variables (height, bole wood density), or site characteristics (climate, age, management) were included. Predictions of stand-based biomass from allometric models of varying levels of generalisation (species-specific, plant functional type) were validated using whole-plot harvest data from 17 contrasting stands (range: 9 to 356 Mg ha−1). Losses in efficiency of prediction were < 1% if generalised models were used in place of species-specific models. Furthermore, application of generalised multi-species models did not introduce significant bias in biomass prediction in 92% of the 53 species tested. Further, overall efficiency of stand-level biomass prediction was 99%, with a mean absolute prediction error of only 13%. Hence, for cost-effective prediction of biomass across a wide range of stands, we recommend use of generic allometric models based on plant functional types. Development of new species-specific models is only warranted when gains in accuracy of stand-based predictions are relatively high (e.g. high-value monocultures). This article is protected by copyright. All rights reserved.
3. Jinyan Yang, Yujie He, Doug P. Aubrey, Qianlai Zhuang, Robert O. Teskey: Global patterns and predictors of stem CO2 efflux in forest ecosystems 2015-12-14T23:01:54.618357-05:00 Global Change Biology+
   
   Stem CO2 efflux (ES) plays an important role in the carbon balance of forest ecosystems. However, its primary controls at the global scale are poorly understood and observation-based global estimates are lacking. We synthesized data from 121 published studies across global forest ecosystems and examined the relationships between annual ES and biotic and abiotic factors at individual, biome, and global scales, and developed a global gridded estimate of annual ES. We tested the following hypotheses: (1) Leaf area index (LAI) will be highly correlated with annual ES at biome and global scales; (2) There will be parallel patterns in stem and root CO2 effluxes (RA) in all forests; (3) Annual ES will decline with forest age; and (4) LAI coupled with mean annual temperature (MAT) and mean annual precipitation (MAP) will be sufficient to predict annual ES across forests in different regions. Positive linear relationships were found between ES and LAI, as well as gross primary production (GPP), net primary production (NPP), wood NPP, soil CO2 efflux (RS) and RA. Annual ES was correlated with RA in temperate forests after controlling for GPP and MAT, suggesting other additional factors contributed to the relationship. Annual ES tended to decrease with stand age. Leaf area index, MAT and MAP predicted 74% of variation in ES at global scales. Our statistical model estimated a global annual ES of 6.7 ± 1.1 Pg C yr−1 over the period of 2000-2012 with little interannual variability. Modelled mean annual ES was 89 ± 53, 248 ± 127 and 506 ± 262 g C m−2 yr−1 for boreal, temperate, and tropical forests, respectively. We recommend that future studies report ES at a standardized constant temperature, incorporate more manipulative treatments, such as fertilization and drought, and whenever possible, simultaneously measure both aboveground and belowground CO2 fluxes. This article is protected by copyright. All rights reserved.
4. Richard K. F. Nair, Micheal P. Perks, Andrew Weatherall, Elizabeth M. Baggs, Maurizio Mencuccini: Does canopy nitrogen uptake enhance carbon sequestration by trees? 2015-12-14T06:43:34.471036-05:00 Global Change Biology+
   
   Temperate forest 15N isotope trace experiments find nitrogen (N) addition-driven carbon (C) uptake is modest as little additional N is acquired by trees; however, several correlations of ambient N deposition against forest productivity imply a greater effect of atmospheric nitrogen deposition than these studies. We asked whether N deposition experiments adequately represent all processes found in ambient conditions. In particular, experiments typically apply 15N to directly to forest floors, assuming uptake of nitrogen intercepted by canopies (CNU) is minimal. Additionally, conventional 15N additions typically trace mineral 15N additions rather than litter N recycling and may increase total N inputs above ambient levels. To test the importance of CNU and recycled N to tree nutrition, we conducted a mesocosm experiment, applying 54 g N/15N ha−1 yr−1 to Sitka spruce saplings. We compared tree and soil 15N recovery among treatments where enrichment was due to either (1) a 15N-enriched litter layer, or mineral 15N additions to (2) the soil or (3) the canopy. We found that 60% of 15N applied to the canopy was recovered above ground (in needles, stem and branches) while only 21% of 15N applied to the soil was found in these pools. 15N recovery from litter was low and highly variable. 15N partitioning among biomass pools and age classes also differed among treatments, with twice as much 15N found in woody biomass when deposited on the canopy than soil. Stoichiometrically calculated N effect on C uptake from 15N applied to the soil, scaled to real-world conditions, was 43 kg C kg N−1, similar to manipulation studies. The effect from the canopy treatment was 114 kg C kg N−1. Canopy treatments may be critical to accurately represent N deposition in the field and may address the discrepancy between manipulative and correlative studies.

Nature and Science

1. Jeanine L. Olsen: The genome of the seagrass Zostera marina reveals angiosperm adaptation to the sea 2016-01-27 Nature+
   
   Seagrasses colonized the sea on at least three independent occasions to form the basis of one of the most productive and widespread coastal ecosystems on the planet. Here we report the genome of Zostera marina (L.), the first, to our knowledge, marine angiosperm to be fully sequenced. This reveals unique insights into the genomic losses and gains involved in achieving the structural and physiological adaptations required for its marine lifestyle, arguably the most severe habitat shift ever accomplished by flowering plants. Key angiosperm innovations that were lost include the entire repertoire of stomatal genes, genes involved in the synthesis of terpenoids and ethylene signalling, and genes for ultraviolet protection and phytochromes for far-red sensing. Seagrasses have also regained functions enabling them to adjust to full salinity. Their cell walls contain all of the polysaccharides typical of land plants, but also contain polyanionic, low-methylated pectins and sulfated galactans, a feature shared with the cell walls of all macroalgae and that is important for ion homoeostasis, nutrient uptake and O2/CO2 exchange through leaf epidermal cells. The Z. marina genome resource will markedly advance a wide range of functional ecological studies from adaptation of marine ecosystems under climate warming, to unravelling the mechanisms of osmoregulation under high salinities that may further inform our understanding of the evolution of salt tolerance in crop plants.
2. Johannes Lehmann: The contentious nature of soil organic matter 2015-11-23 Nature+
   
   Instead of containing stable and chemically unique ‘humic substances’, as has been widely accepted, soil organic matter is a mixture of progressively decomposing organic compounds; this has broad implications for soil science and its applications.
3. Diana H. Wall: Soil biodiversity and human health 2015-11-23 Nature+
   
   Soil biodiversity sustains human health and its loss can be mitigated by sustainable management.
4. L. Rowland: Death from drought in tropical forests is triggered by hydraulics not carbon starvation 2015-11-23 Nature+
   
   Drought threatens tropical rainforests over seasonal to decadal timescales, but the drivers of tree mortality following drought remain poorly understood. It has been suggested that reduced availability of non-structural carbohydrates (NSC) critically increases mortality risk through insufficient carbon supply to metabolism (‘carbon starvation’). However, little is known about how NSC stores are affected by drought, especially over the long term, and whether they are more important than hydraulic processes in determining drought-induced mortality. Using data from the world’s longest-running experimental drought study in tropical rainforest (in the Brazilian Amazon), we test whether carbon starvation or deterioration of the water-conducting pathways from soil to leaf trigger tree mortality. Biomass loss from mortality in the experimentally droughted forest increased substantially after >10 years of reduced soil moisture availability. The mortality signal was dominated by the death of large trees, which were at a much greater risk of hydraulic deterioration than smaller trees. However, we find no evidence that the droughted trees suffered carbon starvation, as their NSC concentrations were similar to those of non-droughted trees, and growth rates did not decline in either living or dying trees. Our results indicate that hydraulics, rather than carbon starvation, triggers tree death from drought in tropical rainforest.
5. Catherine Ritz: Potential sea-level rise from Antarctic ice-sheet instability constrained by observations 2015-11-18 Nature+
   
   Large parts of the Antarctic ice sheet lying on bedrock below sea level may be vulnerable to marine-ice-sheet instability (MISI), a self-sustaining retreat of the grounding line triggered by oceanic or atmospheric changes. There is growing evidence that MISI may be underway throughout the Amundsen Sea embayment (ASE), which contains ice equivalent to more than a metre of global sea-level rise. If triggered in other regions, the centennial to millennial contribution could be several metres. Physically plausible projections are challenging: numerical models with sufficient spatial resolution to simulate grounding-line processes have been too computationally expensive to generate large ensembles for uncertainty assessment, and lower-resolution model projections rely on parameterizations that are only loosely constrained by present day changes. Here we project that the Antarctic ice sheet will contribute up to 30 cm sea-level equivalent by 2100 and 72 cm by 2200 (95% quantiles) where the ASE dominates. Our process-based, statistical approach gives skewed and complex probability distributions (single mode, 10 cm, at 2100; two modes, 49 cm and 6 cm, at 2200). The dependence of sliding on basal friction is a key unknown: nonlinear relationships favour higher contributions. Results are conditional on assessments of MISI risk on the basis of projected triggers under the climate scenario A1B (ref. 9), although sensitivity to these is limited by theoretical and topographical constraints on the rate and extent of ice loss. We find that contributions are restricted by a combination of these constraints, calibration with success in simulating observed ASE losses, and low assessed risk in some basins. Our assessment suggests that upper-bound estimates from low-resolution models and physical arguments (up to a metre by 2100 and around one and a half by 2200) are implausible under current understanding of physical mechanisms and potential triggers.
6. Rick D. Stuart-Smith: Thermal biases and vulnerability to warming in the world’s marine fauna 2015-11-11 Nature+
   
   How marine communities will respond to climate change depends on the thermal sensitivities of existing communities; existing reef communities do not show a perfect fit between current temperatures and the thermal niches of the species within them and this thermal bias is a major contributor to projected local species loss.
7. Michael Ghidiu: Conductive two-dimensional titanium carbide ‘clay’ with high volumetric capacitance 2014-11-26 Nature+
   
   Safe and powerful energy storage devices are becoming increasingly important. Charging times of seconds to minutes, with power densities exceeding those of batteries, can in principle be provided by electrochemical capacitors—in particular, pseudocapacitors. Recent research has focused mainly on improving the gravimetric performance of the electrodes of such systems, but for portable electronics and vehicles volume is at a premium. The best volumetric capacitances of carbon-based electrodes are around 300 farads per cubic centimetre; hydrated ruthenium oxide can reach capacitances of 1,000 to 1,500 farads per cubic centimetre with great cyclability, but only in thin films. Recently, electrodes made of two-dimensional titanium carbide (Ti3C2, a member of the ‘MXene’ family), produced by etching aluminium from titanium aluminium carbide (Ti3AlC2, a ‘MAX’ phase) in concentrated hydrofluoric acid, have been shown to have volumetric capacitances of over 300 farads per cubic centimetre. Here we report a method of producing this material using a solution of lithium fluoride and hydrochloric acid. The resulting hydrophilic material swells in volume when hydrated, and can be shaped like clay and dried into a highly conductive solid or rolled into films tens of micrometres thick. Additive-free films of this titanium carbide ‘clay’ have volumetric capacitances of up to 900 farads per cubic centimetre, with excellent cyclability and rate performances. This capacitance is almost twice that of our previous report, and our synthetic method also offers a much faster route to film production as well as the avoidance of handling hazardous concentrated hydrofluoric acid.
8. Nuno Carvalhais: Global covariation of carbon turnover times with climate in terrestrial ecosystems 2014-09-24 Nature+
   
   The response of the terrestrial carbon cycle to climate change is among the largest uncertainties affecting future climate change projections. The feedback between the terrestrial carbon cycle and climate is partly determined by changes in the turnover time of carbon in land ecosystems, which in turn is an ecosystem property that emerges from the interplay between climate, soil and vegetation type. Here we present a global, spatially explicit and observation-based assessment of whole-ecosystem carbon turnover times that combines new estimates of vegetation and soil organic carbon stocks and fluxes. We find that the overall mean global carbon turnover time is  years (95 per cent confidence interval). On average, carbon resides in the vegetation and soil near the Equator for a shorter time than at latitudes north of 75° north (mean turnover times of 15 and 255 years, respectively). We identify a clear dependence of the turnover time on temperature, as expected from our present understanding of temperature controls on ecosystem dynamics. Surprisingly, our analysis also reveals a similarly strong association between turnover time and precipitation. Moreover, we find that the ecosystem carbon turnover times simulated by state-of-the-art coupled climate/carbon-cycle models vary widely and that numerical simulations, on average, tend to underestimate the global carbon turnover time by 36 per cent. The models show stronger spatial relationships with temperature than do observation-based estimates, but generally do not reproduce the strong relationships with precipitation and predict faster carbon turnover in many semi-arid regions. Our findings suggest that future climate/carbon-cycle feedbacks may depend more strongly on changes in the hydrological cycle than is expected at present and is considered in Earth system models.
9. Ru-Jin Huang: High secondary aerosol contribution to particulate pollution during haze events in China 2014-09-17 Nature+
   
   Rapid industrialization and urbanization in developing countries has led to an increase in air pollution, along a similar trajectory to that previously experienced by the developed nations. In China, particulate pollution is a serious environmental problem that is influencing air quality, regional and global climates, and human health. In response to the extremely severe and persistent haze pollution experienced by about 800 million people during the first quarter of 2013 (refs 4, 5), the Chinese State Council announced its aim to reduce concentrations of PM2.5 (particulate matter with an aerodynamic diameter less than 2.5 micrometres) by up to 25 per cent relative to 2012 levels by 2017 (ref. 6). Such efforts however require elucidation of the factors governing the abundance and composition of PM2.5, which remain poorly constrained in China. Here we combine a comprehensive set of novel and state-of-the-art offline analytical approaches and statistical techniques to investigate the chemical nature and sources of particulate matter at urban locations in Beijing, Shanghai, Guangzhou and Xi’an during January 2013. We find that the severe haze pollution event was driven to a large extent by secondary aerosol formation, which contributed 30–77 per cent and 44–71 per cent (average for all four cities) of PM2.5 and of organic aerosol, respectively. On average, the contribution of secondary organic aerosol (SOA) and secondary inorganic aerosol (SIA) are found to be of similar importance (SOA/SIA ratios range from 0.6 to 1.4). Our results suggest that, in addition to mitigating primary particulate emissions, reducing the emissions of secondary aerosol precursors from, for example, fossil fuel combustion and biomass burning is likely to be important for controlling China’s PM2.5 levels and for reducing the environmental, economic and health impacts resulting from particulate pollution.
10. Sean T. Michaletz: Convergence of terrestrial plant production across global climate gradients 2014-07-20 Nature+
    
    Net primary production is affected by temperature and precipitation, but whether this is a direct kinetic effect on plant metabolism or an indirect ecological effect mediated by changes in plant age, plant biomass or growing season length is unclear — this study develops metabolic scaling theory to be able to answer this question and applies it to a global data set of plant productivity, concluding that it is indirect effects that explain the influence of climate on productivity, which is characterized by a common scaling relationship across climate gradients.
11. Eita Sasaki: Co-opting sulphur-carrier proteins from primary metabolic pathways for 2-thiosugar biosynthesis 2014-05-11 Nature+
    
    Sulphur is an essential element for life and is ubiquitous in living systems. Yet how the sulphur atom is incorporated into many sulphur-containing secondary metabolites is poorly understood. For bond formation between carbon and sulphur in primary metabolites, the major ionic sulphur sources are the persulphide and thiocarboxylate groups on sulphur-carrier (donor) proteins. Each group is post-translationally generated through the action of a specific activating enzyme. In all reported bacterial cases, the gene encoding the enzyme that catalyses the carbon–sulphur bond formation reaction and that encoding the cognate sulphur-carrier protein exist in the same gene cluster. To study the production of the 2-thiosugar moiety in BE-7585A, an antibiotic from Amycolatopsis orientalis, we identified a putative 2-thioglucose synthase, BexX, whose protein sequence and mode of action seem similar to those of ThiG, the enzyme that catalyses thiazole formation in thiamine biosynthesis. However, no gene encoding a sulphur-carrier protein could be located in the BE-7585A cluster. Subsequent genome sequencing uncovered a few genes encoding sulphur-carrier proteins that are probably involved in the biosynthesis of primary metabolites but only one activating enzyme gene in the A. orientalis genome. Further experiments showed that this activating enzyme can adenylate each of these sulphur-carrier proteins and probably also catalyses the subsequent thiolation, through its rhodanese domain. A proper combination of these sulphur-delivery systems is effective for BexX-catalysed 2-thioglucose production. The ability of BexX to selectively distinguish sulphur-carrier proteins is given a structural basis using X-ray crystallography. This study is, to our knowledge, the first complete characterization of thiosugar formation in nature and also demonstrates the receptor promiscuity of the A. orientalis sulphur-delivery system. Our results also show that co-opting the sulphur-delivery machinery of primary metabolism for the biosynthesis of sulphur-containing natural products is probably a general strategy found in nature.
12. Johan Rockström: [Editorial] Future Earth 2016-01-22 Science+
    
    The new year ushers in important international agendas secured at the end of 2015: the Paris climate agreement to limit global warming to a 1.5° to 2°C increase and adoption of the United Nations Sustainable Development Goals. Both actions reflect the world's recognition that development in all nations hinges on a stable and resilient Earth system. This is a political paradigm shift, fortified by three decades of remarkable advancements in Earth system science. The International Geosphere-Biosphere Programme (IGBP), which ended in December 2015, can take considerable credit for coordinating and catalyzing much of this fundamental research. The recently launched Future Earth research program builds on this legacy and is the right response to the new scientific challenges. Author: Johan Rockström
13. Donghui Guo: [Report] Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts 2016-01-22 Science+
    
    Nitrogen (N)–doped carbon materials exhibit high electrocatalytic activity for the oxygen reduction reaction (ORR), which is essential for several renewable energy systems. However, the ORR active site (or sites) is unclear, which retards further developments of high-performance catalysts. Here, we characterized the ORR active site by using newly designed graphite (highly oriented pyrolitic graphite) model catalysts with well-defined π conjugation and well-controlled doping of N species. The ORR active site is created by pyridinic N. Carbon dioxide adsorption experiments indicated that pyridinic N also creates Lewis basic sites. The specific activities per pyridinic N in the HOPG model catalysts are comparable with those of N-doped graphene powder catalysts. Thus, the ORR active sites in N-doped carbon materials are carbon atoms with Lewis basicity next to pyridinic N. Authors: Donghui Guo, Riku Shibuya, Chisato Akiba, Shunsuke Saji, Takahiro Kondo, Junji Nakamura
14. Wei Xuan: [Report] Cyclic programmed cell death stimulates hormone signaling and root development in Arabidopsis 2016-01-22 Science+
    
    The plant root cap, surrounding the very tip of the growing root, perceives and transmits environmental signals to the inner root tissues. In Arabidopsis thaliana, auxin released by the root cap contributes to the regular spacing of lateral organs along the primary root axis. Here, we show that the periodicity of lateral organ induction is driven by recurrent programmed cell death at the most distal edge of the root cap. We suggest that synchronous bursts of cell death in lateral root cap cells release pulses of auxin to surrounding root tissues, establishing the pattern for lateral root formation. The dynamics of root cap turnover may therefore coordinate primary root growth with root branching in order to optimize the uptake of water and nutrients from the soil. Authors: Wei Xuan, Leah R. Band, Robert P. Kumpf, Daniël Van Damme, Boris Parizot, Gieljan De Rop, Davy Opdenacker, Barbara K. Möller, Noemi Skorzinski, Maria F. Njo, Bert De Rybel, Dominique Audenaert, Moritz K. Nowack, Steffen Vanneste, Tom Beeckman

Authors of Interest

1. Yiqi Luo, Anders Ahlström, Steven D. Allison, Niels H. Batjes, Victor Brovkin, Nuno Carvalhais, Adrian Chappell, Philippe Ciais, Eric A. Davidson, Adien Finzi, Katerina Georgiou, Bertrand Guenet, Oleksandra Hararuk, Jennifer W. Harden, Yujie He, Francesca Hopkins, Lifen Jiang, Charlie Koven, Robert B. Jackson, Chris D. Jones, Mark J. Lara, Junyi Liang, A. David McGuire, William Parton, Changhui Peng, James T. Randerson, Alejandro Salazar, Carlos A. Sierra, Matthew J. Smith, Hanqin Tian, Katherine E. O. Todd-Brown, Margaret Torn, Kees Jan Groenigen, Ying Ping Wang, Tristram O. West, Yaxing Wei, William R. Wieder, Jianyang Xia, Xia Xu, Xiaofeng Xu, Tao Zhou: Toward more realistic projections of soil carbon dynamics by Earth system models 2016-01-21T17:40:19.692208-05:00 Global Biogeochemical Cycles+
   
   Soil carbon (C) is a critical component of Earth system models (ESMs), and its diverse representations are a major source of the large spread across models in the terrestrial C sink from the third to fifth assessment reports of the Intergovernmental Panel on Climate Change (IPCC). Improving soil C projections is of a high priority for Earth system modeling in the future IPCC and other assessments. To achieve this goal, we suggest that (1) model structures should reflect real-world processes, (2) parameters should be calibrated to match model outputs with observations, and (3) external forcing variables should accurately prescribe the environmental conditions that soils experience. First, most soil C cycle models simulate C input from litter production and C release through decomposition. The latter process has traditionally been represented by first-order decay functions, regulated primarily by temperature, moisture, litter quality, and soil texture. While this formulation well captures macroscopic soil organic C (SOC) dynamics, better understanding is needed of their underlying mechanisms as related to microbial processes, depth-dependent environmental controls, and other processes that strongly affect soil C dynamics. Second, incomplete use of observations in model parameterization is a major cause of bias in soil C projections from ESMs. Optimal parameter calibration with both pool- and flux-based data sets through data assimilation is among the highest priorities for near-term research to reduce biases among ESMs. Third, external variables are represented inconsistently among ESMs, leading to differences in modeled soil C dynamics. We recommend the implementation of traceability analyses to identify how external variables and model parameterizations influence SOC dynamics in different ESMs. Overall, projections of the terrestrial C sink can be substantially improved when reliable data sets are available to select the most representative model structure, constrain parameters, and prescribe forcing fields.
2. Xia Xu, Zheng Shi, Xuecheng Chen, Yang Lin, Shuli Niu, Lifen Jiang, Ruiseng Luo, Yiqi Luo: Unchanged carbon balance driven by equivalent responses of production and respiration to climate change in a mixed grass prairie 2015-12-14T23:28:59.045972-05:00 Global Change Biology+
   
   Responses of grassland carbon (C) cycling to climate change and land use remain a major uncertainty in model prediction of future climate. To explore the impacts of global change on ecosystem C fluxes and the consequent changes in C storage, we have conducted a field experiment with warming (+ 3 °C), altered precipitation (doubled and halved), and annual clipping at the end of growing seasons in a mixed grass prairie in Oklahoma, USA from 2009 to 2013. Results showed that although ecosystem respiration (ER) and gross primary production (GPP) negatively responded to warming, net ecosystem exchange of CO2 (NEE) did not significantly change under warming. Doubled precipitation stimulated and halved precipitation suppressed ER and GPP equivalently, with the net outcome being unchanged in NEE. These results indicate that warming and altered precipitation do not necessarily have profound impacts on ecosystem C storage. In addition, we found that clipping enhanced NEE due to a stronger positive response of GPP compared to ER, indicating that clipping could potentially be an effective land practice that could increase C storage. No significant interactions between warming, altered precipitation, and clipping were observed. Meanwhile, we found that belowground net primary production (BNPP) in general was sensitive to climate change and land use though no significant changes were found in NPP across treatments. Moreover, negative correlations of the ER/GPP ratio with soil temperature and moisture did not differ across treatments, highlighting the roles of abiotic factors in mediating ecosystem C fluxes in this grassland. Importantly, our results suggest that belowground C cycling (e.g., BNPP) could respond to climate change with no alterations in ecosystem C storage in the same period. This article is protected by copyright. All rights reserved.
3. Jinyan Yang, Yujie He, Doug P. Aubrey, Qianlai Zhuang, Robert O. Teskey: Global patterns and predictors of stem CO2 efflux in forest ecosystems 2015-12-14T23:01:54.618357-05:00 Global Change Biology+
   
   Stem CO2 efflux (ES) plays an important role in the carbon balance of forest ecosystems. However, its primary controls at the global scale are poorly understood and observation-based global estimates are lacking. We synthesized data from 121 published studies across global forest ecosystems and examined the relationships between annual ES and biotic and abiotic factors at individual, biome, and global scales, and developed a global gridded estimate of annual ES. We tested the following hypotheses: (1) Leaf area index (LAI) will be highly correlated with annual ES at biome and global scales; (2) There will be parallel patterns in stem and root CO2 effluxes (RA) in all forests; (3) Annual ES will decline with forest age; and (4) LAI coupled with mean annual temperature (MAT) and mean annual precipitation (MAP) will be sufficient to predict annual ES across forests in different regions. Positive linear relationships were found between ES and LAI, as well as gross primary production (GPP), net primary production (NPP), wood NPP, soil CO2 efflux (RS) and RA. Annual ES was correlated with RA in temperate forests after controlling for GPP and MAT, suggesting other additional factors contributed to the relationship. Annual ES tended to decrease with stand age. Leaf area index, MAT and MAP predicted 74% of variation in ES at global scales. Our statistical model estimated a global annual ES of 6.7 ± 1.1 Pg C yr−1 over the period of 2000-2012 with little interannual variability. Modelled mean annual ES was 89 ± 53, 248 ± 127 and 506 ± 262 g C m−2 yr−1 for boreal, temperate, and tropical forests, respectively. We recommend that future studies report ES at a standardized constant temperature, incorporate more manipulative treatments, such as fertilization and drought, and whenever possible, simultaneously measure both aboveground and belowground CO2 fluxes. This article is protected by copyright. All rights reserved.
4. C. C. Treat, M. C. Jones, P. Camill, A. Gallego-Sala, M. Garneau, J. W. Harden, G. Hugelius, E. S. Klein, U. Kokfelt, P. Kuhry, J. Loisel, P. J. H. Mathijssen, J. A. O'Donnell, P. O. Oksanen, T. M. Ronkainen, A. B. K. Sannel, J. Talbot, C. Tarnocai, M. Väliranta: Effects of permafrost aggradation on peat properties as determined from a pan-Arctic synthesis of plant macrofossils 2016-01-14T16:06:45.912977-05:00 Journal of Geophysical Research: Biogeosciences+
   
   Permafrost dynamics play an important role in high-latitude peatland carbon balance and are key to understanding the future response of soil carbon stocks. Permafrost aggradation can control the magnitude of the carbon feedback in peatlands through effects on peat properties. We compiled peatland plant macrofossil records for the northern permafrost zone (515 cores from 280 sites) and classified samples by vegetation type and environmental class (fen, bog, tundra and boreal permafrost, and thawed permafrost). We examined differences in peat properties (bulk density, carbon (C), nitrogen (N) and organic matter content, and C/N ratio) and C accumulation rates among vegetation types and environmental classes. Consequences of permafrost aggradation differed between boreal and tundra biomes, including differences in vegetation composition, C/N ratios, and N content. The vegetation composition of tundra permafrost peatlands was similar to permafrost-free fens, while boreal permafrost peatlands more closely resembled permafrost-free bogs. Nitrogen content in boreal permafrost and thawed permafrost peatlands was significantly lower than in permafrost-free bogs despite similar vegetation types (0.9% versus 1.5% N). Median long-term C accumulation rates were higher in fens (23 g C m−2 yr−1) than in permafrost-free bogs (18 g C m−2 yr−1) and were lowest in boreal permafrost peatlands (14 g C m−2 yr−1). The plant macrofossil record demonstrated transitions from fens to bogs to permafrost peatlands, bogs to fens, permafrost aggradation within fens, and permafrost thaw and reaggradation. Using data synthesis, we have identified predominant peatland successional pathways, changes in vegetation type, peat properties, and C accumulation rates associated with permafrost aggradation.
5. Tao Zhou, Peijun Shi, Gensuo Jia, Yongjiu Dai, Xiang Zhao, Wei Shangguan, Ling Du, Hao Wu, Yiqi Luo: Age-dependent forest carbon sink: Estimation via inverse modeling 2015-12-02T17:15:34.414507-05:00 Journal of Geophysical Research: Biogeosciences+
   
   Forests have been recognized to sequester a substantial amount of carbon (C) from the atmosphere. However, considerable uncertainty remains regarding the magnitude and time course of the C sink. Revealing the intrinsic relationship between forest age and C sink is crucial for reducing uncertainties in prediction of forest C sink potential. In this study, we developed a stepwise data assimilation approach to combine a process-based Terrestrial ECOsystem Regional model, observations from multiple sources, and stochastic sampling to inversely estimate carbon cycle parameters including carbon sink at different forest ages for evergreen needle-leaved forests in China. The new approach is effective to estimate age-dependent parameter of maximal light-use efficiency (R2 = 0.99) and, accordingly, can quantify a relationship between forest age and the vegetation and soil C sinks. The estimated ecosystem C sink increases rapidly with age, peaks at 0.451 kg C m−2 yr−1 at age 22 years (ranging from 0.421 to 0.465 kg C m−2 yr−1), and gradually decreases thereafter. The dynamic patterns of C sinks in vegetation and soil are significantly different. C sink in vegetation first increases rapidly with age and then decreases. C sink in soil, however, increases continuously with age; it acts as a C source when the age is less than 20 years, after which it acts as a sink. For the evergreen needle-leaved forest, the highest C sink efficiency (i.e., C sink per unit net primary productivity) is approximately 60%, with age between 11 and 43 years. Overall, the inverse estimation of carbon cycle parameters can make reasonable estimates of age-dependent C sequestration in forests.
6. Yujie He, Jinyan Yang, Qianlai Zhuang, Jennifer W. Harden, Anthony D. McGuire, Yaling Liu, Gangsheng Wang, Lianhong Gu: Incorporating microbial dormancy dynamics into soil decomposition models to improve quantification of soil carbon dynamics of northern temperate forests 2015-12-22T14:43:34.594535-05:00 Journal of Geophysical Research: Biogeosciences+
   
   Soil carbon dynamics of terrestrial ecosystems play a significant role in the global carbon cycle. Microbial-based decomposition models have seen much growth recently for quantifying this role, yet dormancy as a common strategy used by microorganisms has not usually been represented and tested in these models against field observations. Here we developed an explicit microbial-enzyme decomposition model and examined model performance with and without representation of microbial dormancy at six temperate forest sites of different forest types. We then extrapolated the model to global temperate forest ecosystems to investigate biogeochemical controls on soil heterotrophic respiration and microbial dormancy dynamics at different temporal-spatial scales. The dormancy model consistently produced better match with field-observed heterotrophic soil CO2 efflux (RH) than the no dormancy model. Our regional modeling results further indicated that models with dormancy were able to produce more realistic magnitude of microbial biomass (<2% of soil organic carbon) and soil RH (7.5 ± 2.4 Pg C yr−1). Spatial correlation analysis showed that soil organic carbon content was the dominating factor (correlation coefficient = 0.4–0.6) in the simulated spatial pattern of soil RH with both models. In contrast to strong temporal and local controls of soil temperature and moisture on microbial dormancy, our modeling results showed that soil carbon-to-nitrogen ratio (C:N) was a major regulating factor at regional scales (correlation coefficient = −0.43 to −0.58), indicating scale-dependent biogeochemical controls on microbial dynamics. Our findings suggest that incorporating microbial dormancy could improve the realism of microbial-based decomposition models and enhance the integration of soil experiments and mechanistically based modeling.