Climatic change in the Russian Altai, southern Siberia, based on palynological and geomorphological results, with implications for climatic teleconnections and human history since the middle Holocene

Frank Schlütz

Two pollen diagrams from near the Chuya Basin, Russian Altai, are presented together with results from geomorphological archives. The Kuray Range profile (2330 m a.s.l.) is situated just above the forest line and starts with the weak cooling of the middle Atlantic period (ca. 6500–5900 b.p.) that bisects the Holocene optimum, as in other parts of Siberia. Taiga (boreal coniferous forest) with Picea obovata established afterwards but was displaced at ca. 5300 b.p. by the pronounced cooling of the early Sub-boreal. A gradual recovery of taiga stopped and it disappeared around 3400 b.p. at the end of the middle Sub-boreal. Since then the climatic and vegetational conditions much resemble those of the Sub-atlantic. In the last two millennia, three climatic declines are documented in the second diagram from the Tarkhata Valley (2210 m a.s.l.) from the dry limit of Larix sibirica: a cold and wet one around the 5th century a.d., a cold and dry one around a.d. 1200 and the Little Ice Age which started around a.d. 1600. In addition, several of the climatic periods and especially local human influence can be proved by the geomorphological findings. Most of the climatic changes seem to be connected with socio-ecological changes in Asia and even Europe, with movements of the Huns and Mongols, which show the possible dimensions of global climatic change. Hints of teleconnections between Siberia, the Mediterranean and the Himalayas via the North Atlantic and the Arctic Oscillation are discussed.

Quaternary Science Reviews 28 (2009) 1449–1471 Contents lists available at ScienceDirect Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev Holocene climatic change and the nomadic Anthropocene in Eastern Tibet: palynological and geomorphological results from the Nianbaoyeze Mountains Frank Schlütz a, *, Frank Lehmkuhl b a b Department of Palynology and Climate Dynamics, Albrecht-von-Haller Institute for Plant Sciences, University of Göttingen, Untere Karspüle 2, 37073 Göttingen, Germany Department of Geography, RWTH Aachen University, 52056 Aachen, Germany a r t i c l e i n f o a b s t r a c t Article history: Received 28 September 2008 Received in revised form 21 January 2009 Accepted 23 January 2009 Our study provides detailed information on the Lateglacial landscape and vegetation development of Tibet. Based on a suite of geomorphological and palynological proxy data from the Nianbaoyeze Shan on the eastern margin of the Tibetan Plateau (33 N/101 E, 3300–4500 m asl.), we reconstruct the current state as a function of climate history and the longevity of human inﬂuence. Study results constrain several major phases of aeolian sedimentation between 50–15 ka and various glacier advances during the Late Pleistocene, the Holocene and the Little Ice Age. Increased aeolian deposition was primarily associated with periods of more extensive glacial ice extent. Fluvial and alluvial sediment pulses document an increase of erosion starting at 3926 79 cal yr B.P., coinciding with cooling (Neoglacial) and a growing anthropo-zoogenic inﬂuence. Evidence for periglacial mass movements indicate that the late Holocene cooling started at around 2000 cal yr B.P., demonstrating increased surface activity under the combined effects of human inﬂuence and climate deterioration. The onset of peat growth generally depended on local conditions that include relief, meso-climate and in more recent times also on soil compaction due to animal trampling. We distinguish three initiation periods of peat growth: 12,700– 10,400 cal yr B.P. for ﬂat basins inside last glacial terminal moraines; 7000–5000 cal yr B.P. for the main valley ﬂoors; and 3000–1000 cal yr B.P. for the higher terrace surfaces. The Holocene vegetation history started with an open landscape dominated by pioneer shrubs along braided rivers (<10,600–9800 cal yr B.P.), followed by the spreading of conifers (Picea, Juniperus, Abies) and Betula-trees accompanied by a successive closing of the vegetation cover by Poaceae, Cyperaceae and herbs (9800–8300 cal yr B.P.). First signs of nomadic presence appear as early as 7200 cal yr B.P., when temperatures were up to 2 C warmer than today. Forest remained very patchy with strong local contrasts. During the following cooling phase (5900–2750 cal yr B.P.) the natural vegetation was transformed by nomadic grazing to Bistorta-rich Kobresia pygmaea-pastures. Modern nomadic migration routes were established at least 2200 years ago. Overgrazing and trampling led to the shrinking of Bistorta and the spreading of annual weeds. Short-lived cold events (8000, 6200, 3500 cal yr B.P.) impacted on the vegetation only temporarily. As the transformation of the natural Poaceae-rich vegetation into Kobresia-pastures modiﬁed the inﬂuence of the Tibetan Plateau (‘‘hot plate’’) on the monsoon system, our data even point to an early start of a nomadic (!) Anthropocene nearly 6000 years ago. Against the background of a very long grazing history, modern Tibet must be seen as a cultural landscape. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction The Tibetan Plateau and bordering mountains occupy an area of ca 2.2 106 km2. With an average elevation of more than 4500 m asl and the largest glaciated area outside the Polar Region it * Corresponding author. Tel.: þ495513910318; fax: þ49551398449. E-mail address: fschlue@gwdg.de (F. Schlütz). 0277-3791/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2009.01.009 plays an important role in regional and global atmospheric circulation. The Plateau and the bordering mountain ranges are inﬂuenced by ﬁve major climatic systems: the mid-latitude westerlies, the South and East Asian monsoons, the Siberian high-pressure system and the El Niño Southern Oscillation (ENSO). The relative importance of each system as a moisture and heat source varies throughout the region, with the eastern fringe of Tibet and the southern slopes of the Himalaya being the wettest (e.g. Böhner, 1996). The interplay of atmospheric forces over the plateau and their relationship to the broad climate shifts of the Pleistocene and 1450 F. Schlütz, F. Lehmkuhl / Quaternary Science Reviews 28 (2009) 1449–1471 Holocene periods are important for understanding the dynamics of global change (Ruddiman and Kutzbach, 1989; Prell and Kutzbach, 1992). One of the most direct effects of the high altitude and aridity of the Tibetan Plateau is the atmospheric warming in spring and summer enhancing monsoon dynamics. For a long time scientists have considered vegetation distribution and dynamics in the remote and sparsely populated parts of the Tibetan Plateau as a result of climatic inﬂuences only, thereby neglecting the possible role of human inﬂuence (Guo, 1993; Fang et al., 2002). Even pollen spectra of surface samples have been analyzed in terms of the recent climate only (Shen, 2003; Shen et al., 2006) and fossil pollen spectra are interpreted primarily as indicative of past climate change (Gasse et al., 1991, 1996; van Campo and Gasse, 1993; van Campo et al., 1996; Liu et al., 1998; Lingyu et al., 1999; Shen et al., 2006; Herzschuh et al., 2006b). No doubt, the vegetation history of Tibet reﬂects climatic changes with correlations to global patterns (Gupta et al., 2003; Hong et al., 2003; Feng et al., 2006; Herzschuh, 2006; Yu et al., 2006), but development and species composition of the vegetation is not less sensitive to herbivores. The impact of grazing, trampling and fences on the vegetation is shown by many ecological studies in several parts of High and Central Asia (Li et al., 1984; Tsuyuzaki et al., 1990; Holzner and Kriechbaum, 2000; Fernandez-Gimenez and Allen-Diaz, 2001; Du et al., 2004; Shang and Long, 2005; Wei et al., 2005; Miehe et al., 2006, 2008). Human utilization as a driving factor for landscape development is a global phenomenon resulting in largely domesticated ecosystems (Kareiva et al., 2007). Even naturally appearing vegetation is often not pristine (Willis et al., 2004). With the development of the Tibetan nomadic culture the natural grazing system with only wild herbivores was transformed over thousands of years into a more and more anthropozoogenic system, leading to the dominance of domesticated herds. This was already highlighted through the work of Thelaus (1992) and Frenzel (1994) who used palynological data from the Zoige Basin 100 km NW of the Nianbaoyeze Shan to date the onset of nomadic inﬂuences to at least 4500 cal yr B.P. (Thelaus, 1992) and 5800 cal yr B.P. (Frenzel, 1994). Based on a variety of ecological and palaeoecological sources the essential role of the Tibetan nomads in the history of vegetation and landscape has been found in several studies (Miehe et al., 1998, 2006, 2007, 2008; Kaiser et al., 2006, 2008). The present ecosystems are a snap shot of thousands of years of co-evolution of plants, animals and humans under the speciﬁc climatic conditions of the Tibetan Plateau (Miller, 1999a,b; Tolvanen, 2001). During the last half of the 20th century the ecological equilibrium has undergone vast changes through livestock grazing (Miller, 2005). It is worth to stressing that anthropo-zoogenic vegetation changes have feedbacks with the climate system as well (Li et al., 2002; Du et al., 2004). To understand the roles of climate and human impact, we undertook a systematic study using palynological and geomorphological methods on the eastern margin of the Tibetan Plateau, in the Nianbaoyeze Shan region (¼Nianbaoyeze Mountains, Fig. 1). Resulting palynological reconstructions allow us to examine the relationship between vegetation, human impact, and climate change. The area of the Nianbaoyeze Shan was selected as this mountain system is situated on the absolute western most limits of modern forests, which stretch from the Chinese lowlands into our study area (Fig. 2). Because of the proximity to the modern forest margin the vegetation in this region is highly sensitive to changing conditions of climate and human inﬂuence. Our investigation focuses on geomorphic mapping and the detailed analyses of selected peat cores. The area and study locations of all sections are shown in Fig. 1. The high spatial resolution employed allows us to address the interplay of climate change and human impact during the Holocene in detail. Geomorphological and palynological records were obtained during several joint Chinese–German expeditions starting in 1991. In addition to geological and geomorphological ﬁeldwork, 16 peat cores were taken from the Nianbaoyeze Shan area ranging in altitude between 3870 and 4170 m asl. Here we present a selection of pollen data that focus on (1) the Holocene vegetation history in altitudes of around 4000 m asl, (2) the interplay of climate and early human inﬂuence, (3) the local differentiation of vegetation development in this speciﬁc ecotone with forest relicts in alpine sedge mats under nomadic use. We investigated various geoarchives, those that are mostly independent from nomadic land use – such as glacier activity – and those probably heavily inﬂuenced by climate change and/or overgrazing, like buried soils. 2. Regional setting The Nianbaoyeze Shan is the easternmost part of the NW–SW trending Bayan Har Shan. The granite dome of Nianbaoyeze (about 820 km2 in area, see Fig. 1) is situated at the main water divide of the Huang He and Yangtze River systems. With the highest peak at 5369 m, it rises about 500–800 m above the surrounding peneplain (‘‘main surface’’ cf. Lehmkuhl and Spönemann, 1994). Pleistocene glacial landforms such as moraines, cirques, and U-shaped valleys are well-developed. Moraines that chieﬂy consist of granites and widespread granitic erratics from the Nianbaoyeze batholith clearly mark the extent of multiple Pleistocene glaciations. Loess deposits of several decimeters thickness accumulated especially in basins below 3400 m, e.g. the Basin of Aba. The higher areas up to the boundary of alpine meadows between 3500 and 4300 m asl are covered by about 50–60 cm of sandy silt of aeolian origin (Lehmkuhl and Liu, 1994; Lehmkuhl, 1995). A small modern glacier around the highest summit covers an area of about 5.1 km2. The present snowline (ELA ¼ equilibrium line altitude) is calculated to be at an average elevation of 5100 m. The climate is controlled by the East Asian and South Asian monsoon from May to October delivering 80% of the total annual precipitation. The very dry winter with a monthly precipitation of less than 10 mm is due to the winter monsoon which in turn is controlled through the Siberian High. For precipitation and temperature distribution in the area see Fig. 3. In general, there is a decrease of annual precipitation from more than 1000 mm at the south-eastern margin of the Tibetan Plateau in the Sichuan Basin towards the west to about 300 mm in Madoi (4272 m asl) on the NW Tibetan Plateau. In the Nianbaoyeze area climate data are limited but local authorities have reported an annual precipitation of 975 mm in the S part and 582 mm in the NW part (Lehmkuhl and Liu, 1994). The average vertical gradient of temperature is about 0.55 C/100 m (Domrös and Peng, 1988; Böhner, 1996, 2006). On higher areas of the plateau the mean annual temperature (MAAT) is 0.1 C in Jiuzhi (3629 m asl, NE margin of Nianbaoyeze). Aba (3275 m asl, SE of Nianbaoyeze) is a warmer region (MAAT 3.2 C) within the coniferous forest zone (Figs. 2 and 3) which was severely decimated by logging after World War II. Agriculture, mainly barley, is practiced to altitudes of up to 3300 m in the Aba Basin. In Jiuzhi there are 10 months of possible snowfall and no frost-free month. The ground surface is frozen for nearly half the year. The vegetation in Western Sichuan, from the Sichuan Basin towards the Tibetan Plateau, is one the most diverse in the holarctic region (Mutke and Barthlott, 2005). Several centuries of intensive land use have changed the natural vegetation. At lower elevations forests have been converted into agricultural land and in the upper regions into pastures. The number of species decreases with higher altitudes and in a NW direction due to precipitation and temperature reductions. There is a clear differentiation between N- and S-facing slopes due to solar radiation. F. Schlütz, F. Lehmkuhl / Quaternary Science Reviews 28 (2009) 1449–1471 1451 Fig. 1. Study area of the Nianbaoyeze Shan on the eastern margin of the Tibetan Plateau. The Pollen diagrams are shown from north to south: (3) Lerzha River, (9) Ximenco Valley, (14) Jiea Basin, (15) Kekehe West, (16) Kekehe East. Fig. 2 shows the simpliﬁed vertical belts of vegetation from the Sichuan Basin in the east to the Nianbaoyeze and beyond to the Anyêmaqên Mountains in the west. At higher elevations, especially in the western part (Fig. 2), the vegetation structure is less complex and biodiversity is lower than in the deeply incised river valleys of the Minjiang and Daduhe (Fig. 2; von Wissmann, 1960/61; Hou, 1982; Schweinfurth, 1986; Zhou et al., 1986; Chen, 1987; Li et al., 1990). 1452 F. Schlütz, F. Lehmkuhl / Quaternary Science Reviews 28 (2009) 1449–1471 Fig. 2. Vegetation zones at the eastern fringe of the Tibetan Plateau (modiﬁed from Lehmkuhl, 1995). Tafel (1914) reported on the deliberate deforestation by ﬁre by Tibetans following his journey in eastern Tibet (1905–1908). Chinese deforestation and farming started at the end of the last century in the valleys (Tafel, 1914) and was intensiﬁed during the Cultural Revolution (1966–1976). Based on historical documents Tang et al. (1994) reconstructed forest cover for ﬁve counties in the upstream of the Minjiang River. According to their results, the forest coverage declined from about 50% in the Yuan Dynasty (1271–1368 AD), to 30% the 1950s, 18.8% at the end of the 1970s, and 12% in 1981. However, deforestation by the Tibetan nomads in this area began millennia beforehand (Thelaus, 1992; Frenzel, 1994). Tree remnants, possibly indicating former forest cover, are shown in Fig. 4. 3. Material and methods 3.1. Geomorphology The geomorphological ﬁeldwork included mapping and the analyses of structures and landforms, aided by the use of barometric altimetry and leveling, maps, Landsat-TM images, and aerial Fig. 3. Climatic diagrams of Jiuzhi (33 250 N/101 290 E) and Aba (32 540 N/101420 E). For location of Jiuzhi and Aba see Fig. 2. photographs. Selected exposures were studied to examine sediment and landform associations. In addition, at 16 sites cores from peat bogs were taken (Figs. 1 and 5). Detailed sedimentological and geomorphological analyses were conducted on alluvial fans and terraces, as well as glacial, periglacial mass movement, aeolian, and lacustrine deposits in order to provide a framework of the late Quaternary evolution of landforms and sediments in this area. This included the sampling of suitable material for radiocarbon and luminescence dating from key locations and horizons. As luminescence dates are not subject to a calibration process like radiocarbon dates, they are reported as ka. Radiocarbon dates were determined by conventional 14C-decay counting in Hannover, Germany, and calibrated using CalPal (for details see next section on Palynology). All dating results including location and other details are shown in Table 1, Figs. 1 and 5. The radiocarbon data from fossil soils are based on the analysis of humic acids. The geomorphological investigations focused on ﬁve research themes: (1) timing and climatic signiﬁcance of glacier advances and (2) aeolian sedimentation, (3) dynamics of periglacial mass movements such as increased soliﬂuction in mountain areas, (4) ﬂuvial sedimentation including slope wash and (5) the beginning of peat growth at different sites in relation to their geomorphic context. Because they are independent from human activities pre-Little Ice Age glacier fluctuations are an excellent climatic proxy. Lehmkuhl (1995) presented the ﬁrst results from luminescence data from aeolian sediments overlying glacial and glacioﬂuvial deposits from this area. Owen et al. (2003) used cosmogenic surface exposure dating (SED) to constrain the timing of terminal moraines and to provide a framework for Late Quaternary glacier advances. Holocene glacier advances have rarely been investigated and age control is largely missing due to the lack of organic matter (Lehmkuhl, 1997). During our ﬁeldwork glacier ﬂuctuations of the Nianbaoyeze peak and especially the Ximenco Valley were investigated (Figs. 1 and 6). In southern Tibet Holocene glacier advances are dated for the Little Ice Age (LIA) and the so-called Neoglacial period from about 3000 years ago (Wang and Fan, 1987). Late glacial aeolian material (loess-like sediments and sandy silts) covers most terraces and slopes, as shown for other regions of Central Asia and Mongolia (Lehmkuhl, 1997; Lehmkuhl et al., 2007). In the Nianbaoyeze a widespread layer of approximately 50 cm of F. Schlütz, F. Lehmkuhl / Quaternary Science Reviews 28 (2009) 1449–1471 1453 Fig. 4. A) U-shaped Deka River valley (w4000 m asl) in the centre of the Nianbaoyeze Shan, about 4 km east of peat core 13 (location shown in Fig. 1). Some scrubs are visible on the southern slope (left side) and Picea trees on the northern slope (right side). Photo: F. Lehmkuhl, 27.08.1991. B) Kekehe River in the southern part of the Nianbaoyeze about 3600 m asl. Remnants of Juniper trees can be seen on the northern slope located about 6 km east of peat core 16 (pollen diagram Kekehe East, Fig. 1). Photo: F. Lehmkuhl, 07.09.1991. silt and ﬁne sand covers most slope debris, Pleistocene till and terraces. Largely de-carbonated brown and black soils (Luvisoils and mountain Tschernosem) are developed in cover-beds of aeolian origin. Samples for luminescence dating were taken from basal and higher layers of the loess-like sediments at numerous sites (Lehmkuhl and Liu, 1994). Fossil soils overridden by soliﬂuction debris provide ages of solifluction phases indicating cooling periods (Gamper, 1985). Buried fossil A-horizons in alpine meadows indicate periods of relative weak geomorphic processes followed by phases of increased soliﬂuction activity. Such sections were investigated on the northern slope of the Nianbaoyeze and in two 5 m long sections close to the settlement of Manzhang in about 4300 m asl, 50 km west of the Nianbaoyeze (Fig. 7). Fluvial sediments were studied in a section north of the Nianbaoyeze on the eastern bank of the Ha’a River. In-between these different sediments, fossil soil horizons frequently occur, as can also be shown for other regions on the Tibetan Plateau (e.g. Lehmkuhl et al., 2000, 2007; Klinge and Lehmkuhl, 2005; Kaiser et al., 2006). In addition, in some of the peat cores ﬂuvial sediments occur. Peats and fossil soils were used for dating. 3.2. Palynology In the Nianbaoyeze Shan and its surroundings 16 peat cores were drilled (Table 1). The sites were chosen based on geomorphic context and are shown in Fig. 1. All proﬁles were taken with a Dachnowski corer and a Russian chamber corer. Sites were chosen for three Fig. 5. Summary of stratigraphy and age control of all 16 peat cores. For location see Fig. 1. 1454 F. Schlütz, F. Lehmkuhl / Quaternary Science Reviews 28 (2009) 1449–1471 Table 1 Information on geo-archives used for this study and geochronological data from the Nianbaoyeze Shan and Manzhang. Location Environment Sample No. Lab No. Coordinates Fossil soil and peat basis Ma’erang Lake Ximenco Valley, lateral moraine Nigequ Valley Jiukehe River peat basis fossil soil peat basis fossil soil 1 2 4 5 (08) (10) (18) (20) Hv Hv Hv Hv 18028 18029 18030 18031 N N N N Peat and humic layers in alluvial fan deposits Ha’a River peat Ha’a River peat Ha’a River peat Ha’a River peat Jiea Basin peat Jiea Basin peat Jiea Basin peat 6 6 6 6 7 7 7 (21) (21) (21) (21) (24) (24) (24) Hv Hv Hv Hv Hv Hv Hv 18034 18033 18032 18035 18036 18038 18037 Fossil soil in soliﬂuction lobes Nigequ River catchment Manzhang Manzhang Manzhang Manzhang Manzhang Manzhang Manzhang Manzhang fossil fossil fossil fossil fossil fossil fossil fossil fossil 3 (17) 113 114 115 116 117 118 119 120 Hv Hv Hv Hv Hv Hv Hv Hv Hv Peat coarings basal data Jiukehe River Ekuo River Lerzha Rivera Jiukehe (Jiuke River) Jiukehe Jiukehe Ximenco Valley Ximenco Valley Ximenco Valleya Ximenco Valley Nigequ Valley Ha’acuo Valley Magenacuo Jiea Basina Kekehe Westa Kekehe Easta peat peat peat peat peat peat peat peat peat peat peat peat peat peat peat peat Core No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 a b c soil soil soil soil soil soil soil soil soil m asl Depth (cm) Age [B.P.] cal yr B.P. Process 33 160 E 101040 33 190 E 101070 33 260 E 101110 33 260 E 101060 4300 4380 4010 4000 40 70–80 87 65 840 55 1385 170 2170 125 10750 85 786 72 1298 181 2162 146 12733 77 glacier activity glacier activity ﬂuvial activity glacier activity N N N N N N N 33 310 33 310 33 310 33 310 33 320 33 320 33 320 E 101170 E 101170 E 101170 E 101170 E 101050 E 101050 E 101050 3760 3760 3760 3760 3870 3870 3870 310 325 480 605 76 78 120 4410 235 3130 70 7035 145 3605 60 2820 75 2370 80 2405 70 5021 316 3346 79 7862 133 3926 79 2955 99 2491 143 2519 135 ﬂuvial ﬂuvial ﬂuvial ﬂuvial ﬂuvial ﬂuvial ﬂuvial 18418 19031 19032 19033 19947 19034 19035 19036 19037 N N N N N N N N N 33 240 33 210 33 210 33 210 33 210 33 210 33 210 33 210 33 210 E 101110 E 100 240 E 100 240 E 100 240 E 100 240 E 100 240 E 100 240 E 100 240 E 100 240 4300 4350-4370 4350-4370 4350-4370 4350-4370 4350-4370 4350-4370 4350-4370 4350-4370 40 see see see see see see see see 290 55 525 85 1850 65 1775 70 2320 80 1715 75 1200 70 2015 80 1180 65 372 67 564 64 1789 72 1703 91 2009 97 1635 84 1131 92 1993 97 1108 86 cryogene cryogene cryogene cryogene cryogene cryogene cryogene cryogene cryogene Hv 18410 Hv 21316 Hv 18411 Hv 18412 Hv 18413 no data Hv 18414 Hv 18415 Hv 21319 Hv 18417 Hv 20057 Hv 21321 Hv 20058 Hv 21322 Hv 20059 Hv 20060 N N N N N N N N N N N N N N N N 33 220 E 101020 33 130 E 100 590 33 210 E 101020 33 210 E 101020 33 220 E 101020 33 220 E 101020 33 190 E 101070 33 210 E 101060 33 240 E 101060 33 250 E 101060 33 250 E 101120 33 180 E 101140 33 120 E 101120 33 160 E 101 240 33 090 E 101000 33 080 E 101010 4050 4100 4170 4185 4170 4170 4070 4065 4050 4020 4140 4010 3990 3870 4130 3960 90–100 110–120 310–320 74–79 91–96 78 35–40 80–89 92–94 45–50 221–234 90–100 85–90 75–86 52–59 52–67 6030 190 3635 115 9185 145 2440 65 5690 125 no data 1020 175 3055 190 4900 180 2465 70 4295 75 1630 70 1380 95 4320 150 4655 75 5400 95 6903 230 3978 216 10397 160 2535 129 6501 134 no data 963 175 3231 230 5636 216 2548 127 4861 116 1528 89 1288 91 4936 248 5410 95 6164 114 peat accumulation peat accumulation peat accumulation peat accumulation peat accumulation peat accumulation peat accumulation base of ﬂuvial layerb peat accumulation peat accumulation peat accumulation peat accumulation base of ﬂuvial layerc peat accumulation peat accumulation peat accumulation Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. 7 7 7 7 7 7 7 7 activity activity activity activity activity activity activity activity activity activity activity activity activity activity activity activity Pollen diagrams ( ¼ peat coring No. 3, 9, 14–16). Peat accumulation on top of ﬂuvial layer. Fluvial layer embedded in peat. reasons: 1) to obtain detailed information on the vegetation history, 2) to collect samples from different parts of the Nianbaoyeze, 3) to develop an understanding of peat development in different relief/ geomorphic positions. Most proﬁles are from the northern and north-western parts of the Nianbaoyeze, in the catchment of the Huang He (Fig.1: 1, 3–12). In addition, one proﬁle is from the western part (Fig. 1: 2), two from the eastern (Fig. 1: 13, 14), and two from the southern part of the mountains (Fig. 1: 15, 16). Most of them are situated in valley ﬂoors; only two are from higher terraces. As there are no peat bogs in valley ﬂoors of the steeper southern part of the Nianbaoyeze (catchment of the Yangtze River), those two sites (15, 16) are located on ﬂat slopes in-between two smaller tributary rivers. In two of the valleys several proﬁles were drilled tracing the maximum extent of Pleistocene glaciers: Lerzha River: No. 1, 3, 4, 5, 6 (Fig. 8) and the valley of Ximenco Lake: No. 7, 8, 9, 10 (Figs. 6 and 9). While nine proﬁles were cored inside the extent of the Late Quaternary glaciations of the Nianbaoyeze, seven further proﬁles are in front of the LGM glacier extent: No. 1, 5, 6, 10, 11, 14, 16 (Fig. 1, Table 1). Fig.10 shows drilling site 14 in the Jiea Basin east and Fig.11 site 16 Kekehe East in the south of the Nianbaoyeze Mountains. After preliminary examinations ﬁve peat cores were chosen for detailed palynological investigations (Fig. 1: 3, 9, 14, 15, 16). In addition 6 surface samples were analyzed (Fig. 12). The pollen diagrams Lerzha River (Fig. 13) and Ximenco Valley (Fig. 14) from the northern part of the Nianbaoyeze (Fig. 1) cover most of the Holocene and examine local differences over a 10 km distance. Two neighboring proﬁles from the southern part of the mountains Kekehe West (Fig. 15) and Kekehe East (Fig. 16) underline the strengthening of local differences. The proﬁle Jiea Basin (Fig. 17) shows the vegetation development over the last 6000 years in the eastern foreland of the mountain range. All pollen samples were prepared by standard methods using KOH, HF and acetolysis (Erdtman, 1960; Moore et al., 1999). Standard pollen and spore identiﬁcations were carried out with a 500fold magniﬁcation, but for ambiguous pollen grains a magniﬁcation of 1250 was used in addition to phase contrast and oil immersion. Around 140 palynological types were distinguished, a selection of which are presented in the pollen diagrams. Identiﬁcation is based on type slides and literature (Zhang et al., 1990; Ying et al., 1993; Wang et al., 1995; Zhang and Zhou, 1998; Beug and Miehe, 1999; Schlütz, 1999; Beug, 2004) with type names adapted to the Tibetan ﬂora. The calculation of pollen percentages is based on the sum of arboreal pollen (AP) and non-arboreal pollen (NAP) types; pollen grains of Cyperaceae, water plants and spores are not included. In case of the surface sample No. 6, a Caryophyllaceae cushion, the Caryophyllaceae pollen was also excluded. The taxa are arranged F. Schlütz, F. Lehmkuhl / Quaternary Science Reviews 28 (2009) 1449–1471 1455 Fig. 6. Upper part of the Ximenco Valley, 4620 m asl. The U-shaped valley slopes consist of granite. The dashed lines indicate lateral and two terminal moraines of the Little Ice Age (LIA) which are covered by a fossil soil (2) that was dated to 1298 181 cal yr B.P. (Hv 18029, Table 1). The dots marc the positions of Rhododendron scrubs on the terminal moraines and date to 1884–1888 and 1911. At the front: location of peat core 7 where peat accumulation started at about 963 175 cal yr B.P. (Hv 18414). Photo: F. Lehmkuhl, 09.08.1991. according to their ecology, grow forms and chronological emergence. Radiocarbon dates were determined in Hannover (Germany) by conventional 14C-dating of bulk samples (Table 2). Calibrated ages of our data and data from the literature were (re-)calculated with the online program CalPal (calibration set CalPal2007_HULU). Resulting age-depth models are summarized in Fig. 18 and given in calibrated years B.P. in the text (B.P. ¼ 1950 AD). Due to technical reasons time axes in the pollen diagrams refer to calibrated years before the year of drilling (1991 AD). The pollen diagrams are plotted with the software C2 (Juggins, 2007) and visually divided into individual local pollen zones. In addition to the selected pollen diagrams, summaries of signiﬁcant pollen curves are given to illustrate and discuss characteristics of local, regional and super-regional developments (Figs. 19 and 20). 3.3. Local vegetation and palynological indicators Kobresia-mats, also called alpine meadows, are dominated by Kobresia pygmaea. They are the predominant vegetation type in the Nianbaoyeze Shan area as is the case for most other areas in eastern Tibet (Miehe et al., 2008). Because of its small size (usually no bigger than 2 cm) K. pygmaea evades grazing by Yaks and other herbivores. In addition to species of Poaceae, Bistorta (Bistorta viviparum, B. macrophyllum), Gentianaceae, Ranunculaceae (Trollius, Anemone), Asteraceae (Leontopodium, Saussurea) and Saxifraga occur. North of the Nianbaoyeze patches of Kobresia tibetica-mats with Triglochin maritima are found. Alpine shrubs are dominated by Salix oritrepha accompanied by other Salix species, Caragana jubata, Rosaceae (Cotoneaster, Dasiphora (Potentilla) fruticosa, Sibiraea, Spiraea) and in the north also by Rhododendron. Their herbaceous layer consists of Poaceae, Anemone, Asteraceae (Anaphalis, Aster, Leontopodium, Ligularia, Saussurea, and Taraxacum), Gentianaceae, Potentilla, Astragalus, Euphorbia, Pedicularis and Saxifraga (Zhou et al., 1986; Zhang et al., 1988; CAS, 1992). Pasturing deeply affects species composition and structure of the vegetation. To a large degree the Kobresia-mats are an anthropo-zoogenic replacement community and may be better called Kobresia-pastures. Most of the taxa react not linearly but unimodal to grazing, as Trollius beneﬁts already from a relative low (Li et al., 1984), Anemone, Bistorta and Poaceae from a moderate grazing pressure (Li et al., 1984; Holzner and Kriechbaum, 2000). They disappear with increasing grazing pressure while species of Leontopodium (Senecio-type), Saussurea (Saussurea-type) and Ligularia (Cichorioideae) prevail. Pastures rich in Leontopodium are of lowest grazing quality (Li et al., 1984; Holzner and Kriechbaum, 2000). Based on the presence or absence of Bistorta (B. macrophylla) Zhang et al. (1988) divide the Kobresia-pastures of eastern Tibet into strongly and weakly grazed types (the latter with Bistorta). Beside Bistorta, other Polygonaceae offer several easily distinguishable pollen types with good indicator values (Hedberg, 1946; Zhang and Zhou, 1998; Zhou et al., 2004). Rheum is mostly found on slopes with regularly open ground. Species of the Rumex-type (Oxyria digyna, R. acetosa, R. patientia) occur on mountain slopes, in moist valleys, at water sides and along ditches. Koenigia islandica is the only species of the Koenigia-type at high altitudes (Zhang and Zhou, 1998; Wu and Raven, 2003; Zhou et al., 2004), and is an annual plant on frequently destroyed surfaces which also applies to the species of the Cephalophilon-type (Zhang and Zhou, 1998; Wu and Raven, 2003; Zhou et al., 2004). Thus, the occurrence of the Cephalophilon-type is often accompanied by several anthropozoogenic indicators (Schlütz et al., 2007). The Boraginaceae are almost annual plants growing on open (sandy) ground (Wu and Raven, 1995). 4. Results and discussion 4.1. Geomorphology The geomorphological and sedimentological investigation focuses on ﬁve (1–5) key research themes as introduced above. 1456 F. Schlütz, F. Lehmkuhl / Quaternary Science Reviews 28 (2009) 1449–1471 Fig. 7. A) Soliﬂuction lobe in 4280 m altitude on the northern slope in the northern part of the Nianbaoyeze Shan (section 3, Nigequ River). A fossil soil in about 40 cm depth was dated to 372 67 cal yr B.P. (Hv 18418). Photo: F. Lehmkuhl, 09.08.1991. B) Two sections with fossil soliﬂuction layers (humic horizons) close to the settlement Manzhang in 4350 and 4370 m asl. (Sample No. refer to Table 1). (1, 2) Results on extent and timing of Late Quaternary glacier advances and aeolian sedimentation from this region were ﬁrst published by Lehmkuhl and Liu (1994) and Lehmkuhl (1995). Most samples from loess, loess-like sediments and sandy loess on different terrace sequences within the Nianbaoyeze Mountains provide ages for a main aeolian deposition period between 50 and 15 ka (Lehmkuhl, 1995). Based on these data from aeolian mantle deposits overlying terminal moraines it is probable that the documented glacier advances are associated with maximum cooling periods during the Last glacial cycle. This is in accordance with the Chinese literature, where the last glaciation is commonly divided into two main stages. These are thought to represent glaciations that occurred during marine oxygen isotope stages MIS2 and MIS-4 and are separated by an interstadial that lasted from about 55 to 32 ka (e.g. Li et al., 1985; Li and Pan, 1989; Thompson et al., 1989, 1997; Zhang et al., 1991). New results derived from cosmogenic surface exposure dating (Owen et al., 2003; Lehmkuhl and Owen, 2005) suggest that glaciers in the more monsooninﬂuenced regions of Tibet (such as the Nianbaoyeze and the Anyêmaqên Shan further to the West) advanced during times of increased insolation, such as MIS-3 and the early Holocene. This may be explained by an increased moisture ﬂux during these times generating higher precipitation totals, which in turn led to positive glacial mass balances and consequent glacial advances. Precipitation would have been reduced during insolation minima of MIS-2 (global Last Glacial Maximum: LGM), however, temperatures were low enough to lead to positive glacier mass balances, allowing glaciers to advance, albeit not as far as during MIS-3. Nevertheless, the sedimentation of aeolian sandy silt occurred during the Early Holocene. We mapped several terminal moraines upstream of the maximum ice margins that demonstrate later ice advances during Lateglacial times in the Nianbaoyeze (Lehmkuhl, 1995). However, as of yet there are no absolute age data on these moraines. The beginning peat growth indicates that the main valleys were ice free at the beginning of the Holocene, and some parts during the Lateglacial period (e.g. the fossil soil in section No. 5: 12,733 77 cal yr B.P.; peat coring No. 3: 10,397 160 cal yr B.P.). F. Schlütz, F. Lehmkuhl / Quaternary Science Reviews 28 (2009) 1449–1471 1457 Fig. 8. Lerzha valley, north-western part of Nianbaoyeze Shan showing the locations of ﬁve peat cores taken in 1991. For better orientation the LGM terminal moraine is marked (xx). The shown core numbers and dating results refer to Table 1. A) View towards the headwater section of the valley. B) Coring site 4 in the upper part of the valley. C) Coring sites 5 and 6 on terrace. D) Coring site 3 (pollen diagram ‘‘Lerzha River’’) within a glacially carved basin close to the LGM terminal moraine. Photos: F. Lehmkuhl, August 1991. Fig. 9. Lake Ximenco in the northern part of Nianbaoyeze Shan showing the location of peat core No. 9 (¼pollen diagram ‘‘Ximenco Valley’’) in 4050 m asl. The LGM moraine in the background is marked (xx). Photo: F. Lehmkuhl, August 1991. 1458 F. Schlütz, F. Lehmkuhl / Quaternary Science Reviews 28 (2009) 1449–1471 Fig. 10. Jiea Basin in the eastern part of Nianbaoyeze Shan showing the location of peat core No. 14 (¼pollen diagram ‘‘Jiea Basin’’). Several gullies seen on the slopes in the distance were caused by Holocene erosion. Fluvial sediments from the bank of a small nearby creek indicating a phase of higher ﬂuvial activity were dated to 2519 135 cal yr B.P. (Hv 18037) in section 7. Photo: F. Lehmkuhl, August 1991. Indications of Holocene glacier re-advances were found in front of the north facing modern glacier of Nianbaoyeze and in the Ximenco Valley (Fig. 6). Lateral and two terminal moraines enclose an area without or only sparse vegetation. Rhododendron scrubs on these two lateral moraines date to 1884–1888 (outer wall) and 1911 (inner terminal moraine; Lehmkuhl, 1995). On the lateral moraine a buried fossil soil was dated to 1298 181 cal yr B.P. (No. 2, Fig. 6) with the moraine pre-dating soil formation. We suggest that these terminal and lateral moraines correlate to the LIA. (3) Periglacial mass movements occur in areas above 4300 m asl. Fossil soils overridden by soliﬂuction lobes and debris provide ages of increased soliﬂuction activity and indicate cooling periods (Gamper, 1985; Lehmkuhl, 1995; Grunert et al., 2000). Buried fossil A-horizons in alpine meadows mark periods of relative geomorphologic stability followed by phases of increased soliﬂuction activity. Such sections were investigated on the northern slope of the Nianbaoyeze and in two 5 m long sections close to the settlement of Manzhang in about 4300 m asl, 50 km west of the Fig. 11. Flat surfaces with widespread peat bogs above the Kekehe River in the southern part of Nianbaoyeze Shan showing the location of peat core No. 16 (¼pollen diagram ‘‘Kekehe East’’), 3960 m asl. The Kekehe valley trends west-east and is part of Yangtze River catchment. Photo: F. Lehmkuhl, September 1991. 1459 F. Schlütz, F. Lehmkuhl / Quaternary Science Reviews 28 (2009) 1449–1471 pollen sum Androsace Cyperaceae Gentianaceae Rumex-type Cichorioideae Koenigia-type Trollius Caltha-type Caryophyllaceae Bistorta Anemone-type Thalictrum Chenopodiaceae Artemisia Alnus grasses & herbaceous Poaceae Quercus Pinus Spiraea lo. di. Sibiraea Rhododendron Salix oritrepha-type Salix Potentilla-type Hippophae trees & shrubs Betula Juniperus Picea forest 1 2 North 3 4 West 5 6 South 7 5 10 10 5 10 5 5 5 5 5 5 5 5 10 20 30 10 20 30 40 50 5 10 20 15 10 5 5 1 1 5 5 5 100 500 Fig. 12. Surface samples 1–7 from pastures under high grazing pressure in the north, from pastures with short summer grazing in the west and more extensive summer pasturing south of the Nianbaoyeze Shan, lo. di. ¼ long distance. Nianbaoyeze (Fig. 7). These sections show increased soliﬂuction activity at around 2000 cal yr B.P. and during the Little Ace Age (372 67, 504 64 cal yr B.P., Table 1). (4) Fluvial and alluvial sedimentation patterns were investigated in a section of alluvial fan deposits north of the Nianbaoyeze on the eastern bank of the Ha’a River (section 6). A second example from the Jiea Basin (section 7) also shows peat growth and slope wash deposits, while in some of the peat cores ﬂuvial sediments occur. The radiocarbon data are derived from peat and fossil soils. These sequences document ﬂuvial erosion and slope wash of small pebbles and silt in small catchments or on slopes. The ﬁrst peat growth on the alluvial fan of the Ha’a River began at 3926 79 cal yr B.P., 6.05 m below the modern surface (see Table 1). However, older radiocarbon data in the fan deposits at depths of 4.80 m (7862 133 cal yr B.P.) and 3.10 m (5021 316 cal yr B.P.), suggest erosion and re-sedimentation of older organic material from the slopes. The peat growth in the Jiea Basin (section 7, see Fig. 10) starts before 4320 150 cal yr B.P. Younger organic material from the slopes indicate erosion on the gentle slopes with the onset of the Neoglacial in the Jiea Basin (2955 99 cal yr B.P.). At the N and E slopes of the Nianbaoyeze alluvial fans started to accumulate at 3900 cal yr B.P. and 2500 cal yr B.P., respectively (Lehmkuhl, 1995). The higher sedimentation rate in the alluvial fans during the Late Holocene was driven by enhanced erosion that was caused by reduced vegetation cover due to climate cooling and/or by grazing. In addition, in some of the peat cores small ﬂuvial pebbles indicate ﬂuvial activity: e.g. No. 4: 2535 139 cal yr B.P., No. 13: 1288 91 cal yr B.P. (5) The peat growth in the different sections depends on the relief and geomorphic situation. An example of the different geomorphic situations is given in Fig. 8 for ﬁve locations in the Lerzha Valley on the north-western part of Nianbaoyeze Shan. The onset of peat growth can be categorized into three time periods: (I) >10,000 cal yr B.P. (12,700–10,400 cal yr B.P.): peat accumulates in ﬂat basins, especially in basins inside the terminal moraines of the last glacial cycle. (II) 7000–5000 cal yr B.P. (6900, 6500, 6100, 5400, 4900 cal yr B.P.): peat growth mainly in the valley ﬂoor. (III) 3000–1000 cal yr B.P. and LIA: peat growth also on higher terraces and on gravel terraces surfaces (3200, 2500, 2500, 1500, 1300, 1000 cal yr B.P.). This is in accordance with the ﬂuvial and glacial activity discussed above. The peat growth in the Ximenco Valley in front of the LIA moraines (coring No. 7, Fig. 6) started beyond 963 175 cal yr B.P. 4.2. Palynology 4.2.1. Surface samples The modern pollen rain was studied by analyzing six moss cushions and one cushion of Arenaria kansuensis (Caryophyllaceae) taken at locations remote from the drilling sites (Fig. 12). In most samples Picea percentages were below 1.5% (max. 3.5%) suggesting scarcity of trees in the area. Values of Juniperus and Betula are between 1.5–9% and 1.5–8%, respectively. The higher values highlight the tendency over the last few centuries of these taxa to spread out, which for Juniperus is recorded in all four and for Betula in three pollen diagrams (Fig. 19). Accordingly, lower values of Cyperaceae mark the latest decrease of pollen production from Kobresia-pastures caused by increased grazing pressure in the recent past (Fig. 20). Pollen values obtained from the surface samples differ from those collected from the top samples of the peat cores because the latter represent averages of a longer time including a sub-recent period of relative low grazing pressure. The moss cushions probably reﬂect pollen rain of only a few and possibly as little as 1–2 years (Vermoere et al., 2000; Räsänen et al., 2004). Samples 1 (4300 m), 2 (4100 m) and 3 (4100 m) were taken from the northern fringe of Nianbaoyeze Shan some distance above Ximenco Lake. Here Rhododendron shrubs grow and pollen of Rhododendron were found in all three samples (0.3–3%). The north of the Nianbaoyeze Shan is at present under relatively high grazing pressure, which is reﬂected by high values of Artemisia and the Anemone-type, but a low appearance of Bistorta. The west of the Nianbaoyeze Shan is only grazed for a short period during summer and surface samples 4 (4380 m), 5 (4300 m) and 6 (4200 m) are rich in pollen of Bistorta but poor in Anemonetype and Artemisia. High values of Caryophyllaceae and Androsace indicate a cushion rich and open vegetation cover. The Juniperus value of 7% in sample 7 from near the Kekehe River on the south is only slightly higher than those from the top of the neighboring pollen diagrams Kekehe West and East (6.5% and 5.5%). It should be noted that the Juniperus value of sample 7 would be signiﬁcantly higher without the extremely high Artemisia value of 45%. Due to the proportional calculation the latest spreading of Juniper shrubs due to strong grazing is not well reﬂected. 4.2.2. Lerzha River, 4170 m, north-western Nianbaoyeze Shan As far as we know the proﬁle Lerzha River is the highest altitude pollen diagram from eastern Tibet. For the nomads from the north the area offers good pastures in wintertime (Fig. 8 D). The 3.3 m long core consists of a Cyperaceae-peat with organic mud and silt at 1460 LR 1 150 300 450 600 5 5 5 5 5 5 Myriophyllum spicatum-type Hippuris Sparganium-type Riccia-type Pteridium-type Selaginella pollen sum others 300 LR 2 9000 10000 LR 3 LR 4 LR 5 F. Schlütz, F. Lehmkuhl / Quaternary Science Reviews 28 (2009) 1449–1471 15 5 Primula veris-group Cyperaceae 10 5 5 5 5 5 5 5 5 Tribulus Pedicularis palustris-type Cichorioideae Koenigia-type Cephalophilon-type Stellera Valerianaceae Rumex-type Rheum 15 Saussurea-type 5 15 10 Bistorta 15 Triglochin-type 10 Caltha-type 10 Matricaria-type 15 Gentianaceae p.p. grasses & herbaceous Aconogonon-type Anemone-type 15 Trollius 10 5 Chenopodiaceae Senecio-type 20 Thalictrum 10 20 30 40 50 Artemisia 25 50 5 5 5 5 5 5 5 5 5 5 5 5 5 5 10 15 Salix p.p. trees & shrubs Potentilla-type Salix oritrepha-type Rosaceae p.p. Ephedra fragilis-type Ephedra distachya-type Sibiraea-type Caragana-type Pinus Quercus Tsuga Juglans/Pterocarya Ulmus-type Alnus Pistacia Sum AP long distance Poaceae 15 Hippophae 10 5 5 5 Abies Juniperus Arceuthobium Betula 8000 7000 6000 5000 4000 3000 2000 0 1000 Picea cal yr before 1991 forest the base. The pollen diagram (Fig. 13) is divided into 5 local pollen zones named LR 1–5, representing typical stages of Holocene vegetation development in eastern Tibet. LR 1: pioneer shrubs and tree groves in the beginning of the early Holocene (10,600–9800 cal yr B.P.) As is the case for the whole Holocene period, the pollen sum is dominated by the NAP with AP values of only 25–45% in the LR 1, while the values of Cyperaceae are over 100%. The percentages of Picea (up to 4.8%), Abies (up to 3.5%) and Juniperus (ca 1.7%) are somewhat higher than recorded for the most recent past, indicating the existence of patches of forests. Those groves were in number and/or expanse larger than today, especially during the midHolocene optimum. But it is highly unlikely that they formed a closed forest cover. From the recent ecology, we can expect that Picea and Abies occupied north facing slopes, while Juniperus formed stands on the south facing slopes. Typical for eastern Tibet are high values of Hippophaë between 5 and 15% in the early Holocene (Schlütz et al., 2007). Pioneer shrubs of for instance Hippophaë thibetana colonized the ﬂoodplains of the Lerzha River. Species of Salix, the Potentilla-type (D. fruticosa), and of Rosaceae were part of a suite pioneer shrubs. Increased river activity during the early Holocene was driven by high rainfall probably due to the re-establishment of the monsoonal regime, as well as high surface runoff due to sparse vegetation cover on surrounding slopes. Long distance transport of AP from southeast China by moisture bearing air masses is documented by pollen of Pinus, Quercus and Tsuga. The increase of Betula points to a successive establishment of deciduous forests on gradually consolidating soils. Most probably the Lerzha River was a braided river at this time with branches of slowly moving or standing water as is indicated by some pollen types of water plants (Myriophyllum spicatum-type, Hippuris) and fruits of Potamogeton (not shown in the diagram). During the whole of the Holocene, pollen of Cyperaceae dominate the spectra (100–695%) reaching 190–390% in LR 1. This reﬂects the local occurrence of Cyperaceae (Cyperaceae-peat) and possibly the establishment of the ﬁrst patches of K. pygmaea. Poaceae (20–35%), Artemisia (8–18%) and Thalictrum (max. 8%) indicate more steppe-like vegetation, however, species of these taxa also occur within Kobresia-mats. Species of the Senecio-type (up to 7%) and ﬂowering plants of other insect pollinated groups (Gentianaceae, Matricaria-type, Bistorta, Anemone-type) contributed to a colorful appearance of the grassland. A sharp decrease of Hippophaë and Salix is probably linked to an increase of Cyperaceae marking a signiﬁcant slowdown of the ﬂuvial dynamics. This is reﬂected by the development of soils, more stabilized land surfaces and a moderated runoff regime due to a closed vegetation cover since 9800 cal yr B.P. LR 2: stabilization in the later early Holocene (9800–8300 cal yr B.P.) Low values of Hippophaë, Salix and the Potentilla-type mark the whole LR 2 as a time of low river dynamics. In places the pioneer shrubs were followed by Betula forests, but mostly by Kobresia-mats and Poaceae, forming a binding surface cover. In summer the swamp at the Lerzha site must have been (reddish)yellow by the ﬂowers of Trollius, indicating local grazing pressure, while high values of Poaceae demonstrate a generally low inﬂuence of megaherbivores. The low ﬂuvial activity led to the loss of water ﬁlled but disconnected meander channels and their associated plant communities (M. spicatum-type, Hippuris, Sparganium-type). Fig. 13. Simpliﬁed pollen diagram ‘‘Lerzha River’’, 4170 m asl (percentages based on pollen sum without Cyperaceae, exaggeration multiplier 10, for dating results see Table 2). 300 XV 1 XV 2 XV 3 F. Schlütz, F. Lehmkuhl / Quaternary Science Reviews 28 (2009) 1449–1471 10 pollen sum 10 5 150 300 Myriophyllum spicatum-type others Hippuris Riccia-type 5 Cyperaceae 10 Boraginaceae 5 5 Fabaceae p.p. Koenigia-type 5 5 Cichorioideae Euphorbia 15 5 Brassicaceae Bupleurum-type 20 Trollius 15 5 10 Senecio-type 15 Gentianaceae p.p. 15 Bistorta grasses & herbaceous Saussurea-type Anemone-type 10 Thalictrum 15 Chenopodiaceae 10 20 30 40 50 60 Artemisia 25 50 Poaceae 5 5 Alnus Juglans/Pterocarya 5 5 Pinus Quercus lo. dist. 5 Sum AP 15 5 5 5 Rosaceae p.p. Sibiraea-type Spiraea-type Hippophae 20 Potentilla-type trees and shrubs 5 5 Ephedra fragilis-type Ephedra distachya-type 10 Salix 5 5 Juniperus Betula 10 Abies LR 3: the climatic optimum of the middle Holocene (8300–5900 cal yr B.P.) The strong decrease of Poaceae and the increase of AP point to climatic conditions more favorable for trees (Picea, Abies, Juniperus, Betula). Situated at the drought and cold limit of forests, the climate of the Nianbaoyeze Shan must have become warmer and more humid, beneﬁting trees and causing a decrease of steppe elements (Poaceae, Artemisia). This is also reﬂected in the decrease of alpine shrubs such as S. oritrepha (Zhou et al., 1986). The climatic optimum reached its maximum around 7600– 7000 cal yr B.P. with values of Picea being 8–9% and of Juniperus up to 4.5%. A spreading of Hippophaë, Salix and D. fruticosa (Potentillatype) indicate slope erosion which was likely triggered by enhanced monsoonal rainfall. However, compared to the early Holocene, ﬂuvial erosion was probably less signiﬁcant partly because of a well-developed vegetation cover including an at least 1 m thick peat layer at the coring site. With the higher amount of precipitation, more Pinus pollen delivered over long distance was washed out of the monsoonal air masses. Spores of Selaginella emphasize the seasonality of rainfalls. Before the climate optimum a short cold event is recorded for eastern Tibet at 8000 cal yr B.P. (Yu et al., 2006) leading to a temporary decrease of tree taxa and a lower production of biomass. The latter enhanced the grazing pressure of herbivores on the remaining vegetation as seen by the spreading of grazing indicators (Senecio-type, Gentianaceae p.p., Matricaria-type, not shown in the diagram: Swertia-type 25%, Liliaceae 15%). Low values of Poaceae indicate the replacement of grassland by expanding forests, while the decrease of Cyperaceae may point to the destruction of Cyperaceae-swamps by the river. On the other hand the climatic optimum is also the time of the ﬁrst appearance of the well known grazing indicators Caragana-type, Tribulus and Stellera. They are accompanied by other indicators of grazing like Pedicularis palustris-type and Cichorioideae (i.e. Taraxacum). At the same time the growth of annual plants (Koenigia, Polygonum glaciale-type) points to an opening of the turf surfaces and spores of the Pteridium-type point to ﬁre. This seems to indicate that the Nianbaoyeze became more attractive for animal herds due to higher biomass production and a milder climate, causing a decrease of Poaceae and Cyperaceae pollen production by browsing before ﬂowering. Possibly the grazing can be associated with a very early presence of nomads, who used ﬁre as a tool for preparing proliﬁc pastures (Miehe et al., 2007). As burnt plant fragments occurred throughout the whole proﬁle, ﬁre can be seen to a certain degree as a natural factor. After 7000 cal yr B.P. Picea was decreased by a colder climate leading to vegetation resembling more and more that of later periods. Poaceae and Cyperaceae recovered and Caltha scaposa (only known Caltha species in the region) being typical for damp grazed meadows (Wu and Raven, 2001), became abundant for some time. The increase of the Triglochin-type may point to local salinisation due to evaporation or animal impacts. LR 4: Kobresia–Bistorta-pastures in the later middle Holocene (5900–2750 cal yr B.P.) Decreases of Picea and Betula together with an increase of Bistorta around 5900 cal yr B.P. indicate a change to cold-moist conditions after the climatic optimum. The high values of Bistorta stand for K. pygmaea-pastures tinted pinkish-white by the rich occurrence of B. viviparum and/or B. macrophylla, typical for moderate grazing (Li et al., 1984; Zhou et al., 1986; Zhang et al., 1988). Recent experiments show that without grazing K. pygmaea is 10000 9000 8000 7000 6000 5000 4000 3000 2000 0 1000 Picea cal yr before 1991 forest 1461 Fig. 14. Simpliﬁed pollen diagram ‘‘Ximenco Valley’’, 4050 m asl (percentages based on pollen sum without Cyperaceae, exaggeration multiplier 10, for dating results see Table 2). 1462 300 KW 1 KW 2 F. Schlütz, F. Lehmkuhl / Quaternary Science Reviews 28 (2009) 1449–1471 5 pollen sum 5 Botrychium 150 300 450 5 Riccia-type others Pteridium-type 15 Cyperaceae 5 Apiaceae 10 Trollius 10 Aconogonon-type 15 Gentianaceae p.p. 5 Anemone-type 5 Onagraceae 5 Lamiaceae 5 ValeDips 5 Cichorioideae 5 Liliaceae 15 Koenigia-type 15 Saussurea-type 15 Papaver-type 10 Bistorta grasses & herbaceous 5 Thalictrum 10 Chenopodiaceae 30 40 50 60 70 Artemisia 10 20 overgrown by tall Poaceae (Miehe et al., 2008). Consequentially, the Kobresia-mats that dominate the modern vegetation over 450,000 km2 in eastern Tibet are the result of grazing under cold, damp conditions over 6000 years. Even though the relative importance of wild herds and nomadic livestock on grazing pressure cannot yet be accurately assessed, the Nianbaoyeze Shan can thus be seen as an old cultural landscape. A short climatic change around 3500 cal yr B.P., marked by a decrease of Betula and increases of pioneer shrubs (Hippophaë, Potentilla-type), is probably correlated to one of the cold events reported by Yu et al. (2006). In addition, the lasting increase of the Senecio-type and the Gentianaceae p.p. and the lower values of Artemisia hint at a general change to colder conditions after 3500 cal yr B.P. This may have been driven by a stronger Siberian High reinforcing the cold winter monsoon since the middle of the 4th millennium BC leading to severe winters and a short growing season in summer (Chotinskij, 1982; Schlütz and Lehmkuhl, 2007). With the spreading of Cyperaceae, pioneer shrubs diminished (S. oritrepha-, Sibiraea-, Caragana-type). LR 5: climatic and human influence in the late Holocene (from 2750 cal yr B.P. to present) Around 2750 cal yr B.P. the values of Betula fall below 3% while Juniperus exceeds over 3% reaching up to 4.5%. These opposing trends may reﬂect a tendency to colder and dryer conditions, with a lower regional pollen production which led to a relative increase of long distance pollen (Pinus, Quercus). Local salinisation is reﬂected by the Triglochin-type. During the last 500 years the AP falls below 5%, indicating diminishing forests to the recent, with only a few small groves. This was caused by climate change – especially the cooling during the LIA – and growing grazing pressure. The rise of grazing occurred in two steps reﬂected (1) by the increase of pollen taxa indicating moderate grazing like Bistorta and the Anemone-type and (2) by the Saussurea-type that represent heavy grazing (Li et al., 1984). This is also suggested by the strong increase (38%) and following decline (17%) of Poaceae, as Poaceae beneﬁts from grazing until the pressure becomes too strong (Holzner and Kriechbaum, 1998; Miehe et al., 2006). Likewise, Cyperaceae decreased under strong grazing. Rheum and the Rumex-type, two taxa comprising several grazing weeds, have reached the highest values over the last 11,000 years in recent times. 25 50 Poaceae 5 Sum AP 5 5 5 Alnus 5 Quercus long dist. Larix Tsuga 5 Pinus 5 Sibiraea-type 5 5 Rosaceae p.p. 5 Potentilla-type shrubs 5 Ephedra fragilis-type Salix 5 Caragana-type 5 Hippophae 10 Betula 5 Juniperus 9000 8000 7000 6000 5000 4000 3000 2000 0 1000 Picea cal yr before 1991 tress 4.2.3. Ximenco Valley, 4050 m, northern Nianbaoyeze Shan The proﬁle was taken at 4050 m asl in a swamp near the Ximenco Lake (Fig. 9). The area is used today as summer pasture with a summer settlement nearby. The 1.45 m long proﬁle consists of a Cyperaceae-peat with an organic mud with silt at the base and a silt layer at 100 cm depth (6250 cal yr B.P.). For three samples the pollen sum is below 60, the average of the other samples is about 260. The pollen diagram Ximenco Valley (Fig. 14) covers most of the Holocene and is divided into 3 local pollen zones (XV 1–3) representing regional and local stages of the vegetation history. Because of a low number of samples and relatively poor pollen preservation detailed conclusions for XV 1 (10,000–5800 cal yr B.P.) are difﬁcult to draw. Compared to the Lerzha sequence the number of AP is relatively small (Fig. 19) and hardly exceeds 14.5%. The maximum of Betula (10.5%) at the base is followed by high values of Pinus which can be correlated with LR 2 and the pre climatic optimum period, respectively. The second highest value of Betula (9%) and the highest of Picea (4%) appear in the climatic optimum at 7200 cal yr B.P. The increase in Betula correlates with pollen and Fig. 15. Simpliﬁed pollen diagram ‘‘Kekehe West’’, 4130 m asl (percentages based on pollen sum without Cyperaceae, exaggeration multiplier 10, for dating results see Table 2). 300 KE 1 KE 2 F. Schlütz, F. Lehmkuhl / Quaternary Science Reviews 28 (2009) 1449–1471 150 300 pollen sum 5 Cyperaceae 5 Primula 5 Pedicularis oederi-type 5 Koenigia-type 5 Caltha-type 10 Cichorioideae 5 Saxifraga stellaris-type 10 Saussurea-type 15 Apiaceae 15 10 Bistorta 10 Senecio-type 20 Gentianaceae p.p. grasses & herbaceous Trollius 5 Anemone-type 5 Chenopodiaceae 20 Thalictrum 10 20 30 40 Artemisia 25 50 Poaceae 5 Sum AP 5 5 Ulmus-type 5 Quercus long dist. 5 Alnus Juglans/Pterocarya 5 Pinus 5 Ephedra distachya-type 5 Ephedra fragilis-type 5 Rosaceae p.p. 5 Sibiraea-type 10 Caragana-type 10 Potentilla-type shrubs Salix oritrepha-type 15 1463 spores of plants associated with open ground (Koenigia-type, Riccia-type), indicating Betula sprouting as a pioneer tree on disturbed soils. This may highlight the role of ﬂuvial activity in the vegetation development. The scarcity of Picea around the Ximenco Valley suggests that the high Picea values in the Lerzha diagram represent localized forest patches in the valley, with no support for extensive forest stands for the whole of the Nianbaoyeze. The high values of the water plant Myriophyllum reﬂect proximity to a lake. Thus the Ximenco area was sparse in trees during both the early Holocene and the climatic optimum. The XV 2 (5800–2750 cal yr B.P.) was a time of expansion for trees (Picea 5.5%, Betula 6.5%) and shrubs. For the ﬁrst time pioneer shrubs of Salix (up to 7%), D. fruticosa (Potentilla-type over 11%) and Hippophaë are documented together with indicators of open ground (Brassicaceae, Riccia-type). With the cooling following the climatic optimum, the biodiversity of the Kobresia-mats changed due to the spread of grazing weeds (Trollius, Bupleurum-type). However, the area was grazed since the early Holocene, as is indicated by the presence of Bistorta, the Anemone-type, Cichorioideae and Euphorbia. The decrease of Picea and Betula marks a cooling trend. Because of this cooling XV 3 (base at 2750 cal yr B.P.) starts with pioneer shrubs (Salix, Dasiphora, Hippophaë). Bistorta-rich pastures occurred around 900 cal yr B.P. Increasing grazing pressure led to the dominance of Ranunculaceae (Anemone-type), the diminution of Bistorta and Cyperaceae and the colonization of open ground by annual plants (Koenigia-type, Boraginaceae) and Hepaticae (Riccia-type). The opening of the Kobresia turf gave way to a successful germination of Juniperus, of which the shrubs are protected against grazing by resins. Thus this time constitutes a period of growing nomadic inﬂuence on the vegetation. 4.2.4. Kekehe West, 3960 m, southern Nianbaoyeze Shan The core Kekehe West was drilled on the south side of the Nianbaoyeze Shan on a ﬂat rock terrace at 3960 m altitude. Shrubs and some trees are situated near the coring site. Based on our agedepth model (Fig. 18) the 80 cm Cyperaceae-peat represent about 9500 years. The relatively uniform vegetation history is shown in Fig. 15 and was divided into 2 sections (KW 1 and 2). During KW 1 (9600–3700 cal yr B.P.) AP was lower than in any other pollen diagram, indicating a lack of trees and only very few shrubs. The Kobresia-mats were rich in Poaceae and spores of the poorly competitive fern Botrychium were found. With pronounced grazing (Artemisia, Caltha-type, Saussurea-type) Poaceae declined and the pastures became Cyperaceae-dominated. Onagraceae and Pteridium indicate that ﬁres also played a role in this process and burnt plant fragments were found in every sample of the proﬁle. In the KW 2 (since 3700 cal yr B.P. ago) Juniperus and Hippophaë are recorded for the ﬁrst time, indicating that shrubs (H. thibetana) and shrubs or trees (Juniperus) developed following a cool phase. This cooling entailed a decrease in vegetation cover and stronger erosion similar to the record from the Lerzha Valley. Strong grazing since 1200 cal yr B.P. (Saussurea-type, Caltha-type, Gentianaceae) became in recent times even stronger (Anemone-type, Juniperus) causing the decrease of Cyperaceae and Bistorta. 5 Salix 10 Hippophae 10 Betula 15 Juniperus 6000 5000 4000 3000 2000 0 1000 Picea cal yr before 1991 tress 4.2.5. Kekehe East, 4130 m, southern Nianbaoyeze Shan The Kekehe West site at 4130 m asl is located near a tributary of the Kekehe and is surrounded by shrub vegetation. In the KE 1 (6100–2900 cal yr B.P.) values of AP are around 40% (Fig. 16). Following a phase of pioneer shrubs (Salix, S. oritrepha-type, Potentilla-type), trees of Picea (14%) and Juniperus (8%) became Fig. 16. Simpliﬁed pollen diagram ‘‘Kekehe East’’, 3960 m asl (percentages based on pollen sum without Cyperaceae, exaggeration multiplier 10, for dating results see Table 2). 1464 300 JB 1 JB 2 JB 3 F. Schlütz, F. Lehmkuhl / Quaternary Science Reviews 28 (2009) 1449–1471 5 pollen sum 5 Botrychium 150 300 5 Riccia-type others Pteridium-type 5 Cyperaceae 5 Rheum 5 Pedicularis oederi-type 5 Rumex-type 5 Bupleurum-type 5 Saxifraga hirculus-type 10 Primula 5 Apiaceae p.p. 10 Saussurea-type 40 Caltha-type 30 Sanguisorba filiformis-type 5 Anemone-type grasses & herbaceous more frequent at 3300 cal yr B.P., while Betula values stayed below 5%. Kekehe West was the most forested place in the Nianbaoyeze during the whole of the Holocene period. However, the localized character of this phenomenon is clearly demonstrated by the absence of forest at the nearby Kekehe East site (1.5 km distance). Due to the cooling tree abundance decreased and shrubs stayed at low values (KE 2, since 2900 cal yr B.P.). The Cyperaceae were favored by the colder climate and used as pastures (Artemisia, Anemone-type, Gentianaceae p.p.). With increased grazing indicators such as Bistorta, Trollius, the Saxifraga stellaris-type and Cichorioideae became even more common since 2250 cal yr B.P. This trend and the decrease of Betula-trees due to anthropogenic use (wood and bark used for several purposes), mark the growing inﬂuence of nomads. Higher values of the Anemone-type, Trollius and Apiaceae represent an increasing anthropo-zoogenic inﬂuence that caused the retreat of Bistorta and Cyperaceae. 5 Bistorta 10 Matricaria-type 10 Aconogonon-type 5 Thalictrum 15 Chenopodiaceae 20 30 40 Artemisia 10 4.2.6. Jiea Basin, 3870 m, east of the Nianbaoyeze Shan The site is located in 3870 m asl about 15 km east of the Nianbaoyeze Shan in a swampy basin at the upper reaches of the Jiea River (Fig. 10). The 91 cm of Cyperaceae-peat cover about 6000 years and are rich in silt until 1600 cal yr B.P. At present, the area is used as pasture for several weeks in spring and autumn by the nomads on their way to and from their summer pastures in the central Nianbaoyeze Shan. From the beginning of the JB 1 (5900–3000 cal yr B.P.) AP values were low (5–8%) until recent times (Fig. 17). Trees must have been very sparse or absent. Kobresia-pastures (Cyperaceae 150%) rich in Poaceae (40%) were dominant and strongly grazed (Anemone-type 16%). Decreases of Picea and Poaceae and an increase of Sanguisorba filiformis and Cyperaceae mark a change to colder and wetter conditions (JB 2, since about 3000 cal yr B.P.). From this time onwards ﬁres played an important role, as is indicated by the ﬁrst appearance of burnt plant remains which occur in all samples of JB 2. The basin became a wetland that was white and yellow spotted by the abundant ﬂowers of the grazing weeds S. filiformis and C. scaposa (Song and Dong, 2002). With Primula and Saxifraga, also typical consorts of S. filiformis occurred. Species of the Anemonetype and some Poaceae were edged out. The steep increase of S. filiformis to 28% and pure Cyperaceae-peat since 1600 cal yr B.P. mark a decrease of ﬂuvial inﬂux. The recent increase of Juniperus probably indicates the spreading of Juniper shrubs due to strong grazing. 25 Poaceae 5 Sum AP 5 5 Tsuga 5 Quercus lo. dist. Ulmus-type 5 Salix 5 5 Salix oritrepha-type 5 Rosaceae p.p. 5 Lonicera 5 Ephedra fragilis-type 5 Potentilla-type trees & shrubs Pinus 5 Hippophae 5 Betula 5 Juniperus 5 Abies 4.3.1. Late Pleistocene A fossil soil dated to 12,733 77 cal yr B.P. marks an early climatic amelioration in the Nianbaoyeze Shan before the climate cold reversal of the Younger Dryas period (Tschudi et al., 2003). The palynological record starts somewhat after the Pleistocene/Holocene boundary (11,500 cal yr B.P.; Herzschuh, 2006). 4.3.2. Early Holocene: pioneer shrubs (<10,600–9800 cal yr B.P.) and stabilization phase (9800–8300 cal yr B.P.) With the onset of the Holocene period deciduous trees (Betula: B. utilis) and conifers (Picea, Juniperus) began developing small forest stands (Fig. 21). Most likely they were restricted to favorable places with exposure patterns as today, with Picea on north (P. purpurea, P. asperata) and Juniper (J. tibetica) on south facing slopes. With slight delay Abies (A. faxoniana) started to occur 5000 4000 3000 2000 1000 0 Picea cal yr before 1991 forest 4.3. Synthesis Fig. 17. Simpliﬁed pollen diagram ‘‘Jiea Basin’’, 3870 m asl (percentages based on pollen sum without Cyperaceae, exaggeration multiplier 10, for dating results see Table 2). 1465 F. Schlütz, F. Lehmkuhl / Quaternary Science Reviews 28 (2009) 1449–1471 Table 2 Sample details and dating results from pollen proﬁles used for the age-depth model presented in Fig. 18. Pollen diagram Coordinates m asl Lab No. Depth (cm) Age [B.P.] cal yr B.P. Lerzha River 3 N 33 210 E 101020 4170 Ximenco Valley 9 N 33 240 E 101060 4050 14 N 33 160 E 101 240 3870 Hv Hv Hv Hv Hv Hv Hv Hv Hv Hv Hv 62–69 187–195 310–320 52–54 92–94 45–50 75–86 38–44 52–59 32–37 52–67 2680 100 6550 140 9185 145 1990 110 4900 180 1285 65 4320 150 2660 65 4655 75 1890 95 5400 95 2798 114 7443 118 10397 160 1964 134 5636 216 1202 71 4936 248 2796 48 5410 95 1832 108 6164 114 Jiea Basin Core No. 0 0 4130 3960 Kekehe West 15 N 33 09 E 101 00 Kekehe East 16 N 33 080 E 101010 together with Picea (Lerzha River). The re-establishment of deciduous and conifer forests in the early Holocene has also been reported by Thelaus (1992) and Frenzel (1994). Extended pioneer shrubs of Hippophaë (H. thibetana), Salix and D. fruticosa (Potentilla-type) grew primarily on river banks in a generally open landscape. This pioneering phase combined with simultaneous local loess sedimentation (fed by dust from riverbeds) seems to be characteristic for the early Holocene in eastern Tibet (Klinge and Lehmkuhl, 2005; Schlütz et al., 2007). The pioneer species beneﬁted from seasonally high river discharges that were caused by strong rainfall associated with the rapidly strengthening summer monsoon (Fan et al., 1996; Gasse et al., 1996; Hong et al., 2003). The advance of grassland coincides with a slowdown of the ﬂuvial dynamics at about 9800 cal yr B.P. which also diminished the role of riverbeds as local dust sources (Lehmkuhl et al., 2000; Klinge and Lehmkuhl, 2005). The consequent reduction of loess aerosol Fig. 18. Age-depth model for the pollen proﬁles (Figs. 13–17) based on calibrated ages as shown in Table 2. Basal ages are extrapolated. The shorter proﬁles show similar accumulation rates that accelerated about 3000 years ago to the accumulation speed of the proﬁle Lerzha River. 21318 21317 18411 21320 21319 21323 21322 21324 20059 21325 20060 particles, which had before stimulated rain drop nucleation, may have had some negative feedback on precipitation and the inﬂux of exotic pollen types. Probably more important however, was the decrease in summer monsoon precipitation as has been suggested from shrinking lake levels between 9600–8300 cal yr B.P. (Fan et al., 1996; Gasse et al., 1996) and a lower turf accumulation rate after 9200 cal yr B.P. (Thelaus, 1992). 4.3.3. Middle Holocene: the climatic optimum (8300–5900 cal yr B.P.) and the following cooling (5900–2750 cal yr B.P.) The increase of forests (Picea, Abies, Juniperus, Betula) at around 8300 cal yr B.P. (Lerzha River) mark the turn to the Holocene climatic optimum with increased precipitation and thermal amelioration. Chinese workers have described this phase as the Yali-Period or Qilongduo-Interval starting at about 8300 cal yr B.P. (Huang et al., 1981; Li et al., 1985; Lin and Wu, 1987; Wang and Fan, 1987; Li, 1988; Sun and Chen, 1991; Winkler and Wang, 1993). Pollen based studies by Kong et al. (1990) date this period to 8800/ 8300 cal yr B.P. for NE-Tibet (Qinghai Hu) and to 8000 cal yr B.P. for western Tibet (Bangong Co; van Campo et al., 1996). Wetter conditions are also indicated by several Tibetan lakes which reached their maximum extent after 8300 cal yr B.P. (Zhou et al., 1991; van Campo and Gasse, 1993; Avouac et al., 1996). The climatic amelioration was interrupted in eastern Tibet by a short cold event at 8000 cal yr B.P. (Yu et al., 2006) leading to a temporary decrease of trees at their outermost distribution limit (Lerzha River). The return of pioneer shrubs (Hippophaë, Salix, Dasiphora) and trees (Picea, Juniperus) indicates increasing precipitation and temperatures during the maximum of the summer monsoon between 7600 and 7000 cal yr B.P. Based on ostracods Lister et al. (1991) date the begin of the moisture optimum in NE-Tibet (Qinghai Hu) just prior to 7800 cal yr B.P. The onset of renewed climatic deterioration (precipitation and/or temperature) in the Nianbaoyeze Shan appears to be in sync across several other sites in Tibet: western Tibet/Bangong Co, 7200 cal yr B.P. (Fan et al., 1996; Gasse et al., 1996), northern Tibet/Kakitu Mountains, 7100 cal yr B.P. (Beug, 1987). Our results from Ximenco Valley and Kekehe West manifest a very restricted and patchy distribution of forests with strong local contrasts even during the climatic optimum. The data speak against the existence of a closed forest belt at any time during the Holocene. During the whole period the Nianbaoyeze Shan remained a sparsely wooded area at the western most border of forest occurrence. In concordance with Frenzel (1994), Herzschuh et al. (2006a) and Schlütz (1999) we see a weak temperature increase of about 1–2 C during the maximum of the Holocene climatic optimum (7000–7600 cal yr B.P.). The decrease of Picea forests at the end of the Atlantic period may have been caused by a shortlived cold event at about 6200 cal yr B.P. (Yu et al., 2006). The second period of peat growth in the main valleys started during this 1466 15 PB BO AT SB SA F. Schlütz, F. Lehmkuhl / Quaternary Science Reviews 28 (2009) 1449–1471 15 Jiea Basin 15 Kekehe West Artemisia 10 Kekehe East 20 Ximenco Valley 10 20 30 40 50 Lerzha River 10 20 30 40 Jiea Basin 10 20 30 40 50 60 70 10 20 30 40 50 60 Kekehe West Poaceae Kekehe East 10 20 30 40 50 Ximenco Valley 5 Lerzha River 10 Kekehe East 5 Kekehe West Juniperus 10 Jiea Basin 5 Ximenco Valley 5 Lerzha River 10 Jiea Basin 10 Kekehe West 15 Ximenco Valley Betula 5 Kekehe East 5 Lerzha River 15 Jiea Basin 5 Kekehe East 5 Kekehe West 10 Ximenco Valley 4.3.4. Early human influence First indicators of a nomadic inﬂuence were identiﬁed in the Lerzha Valley as early as 7200 cal yr B.P.: Caragana-type, Tribulus, Stellera, Cichorioideae, Koenigia, Polygonum glaciale-type, Pteridiumtype. The nomads may have used ﬁre (Pteridium) for preparing proliﬁc pastures (Miehe et al., 2007). It is obvious that the Nianbaoyeze Shan was attractive to nomads during the maximum of summer monsoon inﬂuence (7600–7000 cal yr B.P.). The ﬁrst occurrence of Stellera pollen in the Zoige basin seems to be somewhat younger (6700 cal yr B.P.; Thelaus, 1992). By grazing under a colder climate Kobresia-pastures rich in Bistorta were formed following 5900 cal yr B.P. The exact role and timing of nomadic inﬂuences on the regional vegetation development cannot yet be accurately estimated. The nearest known archaeological sites (100 km north at Zongri, 3500 m, Machang culture/period: 2100– 1900 BC; Chayet, 1994; Baumer and Weber, 2002) and the oldest written sources about Tibetan nomadism yield much younger dates (Shang-Dynasty: 18th–12th century BC; Hermanns, 1949; Beckwith, 1987; Aldenderfer and Zhang, 2004). However, even older palynological (SE-Tibet: 8500 cal yr B.P.; Schlütz et al., 2007), 10000 9000 8000 7000 6000 5000 4000 3000 2000 1000 0 Lerzha River cal yr before 1991 Picea period indicating an increase in biomass production and accumulation due to higher temperatures and moisture. With increasing grazing pressure and a colder climate from 5900 to 2750 cal yr B.P. Picea and Betula decreased (Lerzha River). Persistently low values of Cyperaceae may indicate more or less stable effective moisture, as precipitation decreased parallel with temperature. On the drier western part of the Tibetan Plateau lakes became hypersaline after 5800 cal yr B.P. (Wang et al., 2002). Due to cold temperatures and grazing, Poaceae decreased at most sites in the Nianbaoyeze Shan. Grazing weeds like Bistorta (Lerzha River) and Ranunculaceae (Ximenco Valley: Anemone-type, Trollius) spread especially on pastures of low biomass production north of the Nianbaoyeze Shan which were exposed to the cold winter monsoon while being located in the rain shadow during the summer monsoon. Next to climatic reasons, the tendency for further peat accumulation (Kekehe East, Jiea Basin) may have been enhanced by water retention above compacted soil surfaces through trampling. The increase of erosion and slope wash by ﬂuvial activity and sedimentation since about 4000 cal yr B.P. seems to reﬂect a stronger grazing pressure. At certain sites conifer forests recovered (Ximenco: Picea; Kekehe East: Picea, Juniperus), but not in the Lerzha Valley, where the grazing impact was probably too dominant. Thus, the patterns of climate and vegetation development resemble conditions at the southern border of boreal forests in Siberia, where after a cold phase (winter monsoon ampliﬁcation) dark taiga with Picea obovata spread again (Schlütz and Lehmkuhl, 2007). A temporary decrease of Betula after 3700 cal yr B.P. (Lerzha River) may reﬂect erosive processes during another short-lived cold event (Yu et al., 2006). Close to that time, a longer lasting cooling is recorded for southern Siberia probably associated with an enhanced winter monsoon (Schlütz and Lehmkuhl, 2007). It seems likely that at around 3700 cal yr B.P. the Siberian High had a strong inﬂuence on the climate of the Tibet Plateau leading to colder winters and shorter summers. Due to increasing human inﬂuence and cooling, forests then diminished appreciably as is demonstrated by Picea in the north (Ximenco Valley) and south (Kekehe East), Betula in the north (Lerzha River, Ximenco Valley) and Juniperus in the south (Kekehe East). Fig. 19. Summarized spatial-temporal vegetation history of the Nianbaoyeze as demonstrated by selected taxa (Picea – Artemisia) arranged from NW (black shaded) to SE (grey shaded) and the Jiea Basin (light grey). 1467 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 cal yr before 1991 Lerzha River 15 Ximenco Valley Bistorta 15 Kekehe West 15 Kekehe East 15 Jiea Basin 5 Lerzha River 20 20 Kekehe West 20 Kekehe East Ranunculus-type Ximenco Valley 20 Jiea Basin 20 Lerzha River 15 Ximenco Valley 5 Trollius 10 Kekehe West Kekehe East 15 Jiea Basin 5 Sang. filiformis-type Jiea Basin 20 5 Chenopodiaceae Kekehe East 5 Rheum Lerzha River miscellaneous 40 Chenopodiaceae Jiea Basin 15 Koenigia-type Ximenco Valley 5 Boraginaceae Ximenco Valley 5 Lerzha River 150 300 450 600 Ximenco Valley 150 300 450 Kekehe East 150 300 Jiea Basin Cyperaceae 150 300 Kekehe West SA SB AT BO PB 150 300 Fig. 20. Summarized spatial-temporal vegetation history of the Nianbaoyeze as demonstrated by selected taxa (Bistorta – Cyperaceae) arranged from NW (black shaded) to SE (grey shaded) and the Jiea Basin (light grey). F. Schlütz, F. Lehmkuhl / Quaternary Science Reviews 28 (2009) 1449–1471 1468 F. Schlütz, F. Lehmkuhl / Quaternary Science Reviews 28 (2009) 1449–1471 Fig. 21. Synopsis of Holocene vegetation changes and geomorphodynamics in the Nianbaoyeze Shan area based on palynological and geomorphological data. archaeological (Tibet Plateau: 8200 cal yr B.P.; Brantingham and Xing, 2006) and genetic data are suggesting an early yak domestication (Guo et al., 2006). Nevertheless, much more research with focus on the grazing history is needed to establish a coherent model. 4.3.5. Late Holocene (from 2750 cal yr B.P. to present) While Betula decreased Juniperus expanded in the Lerzha Valley, indicating drier conditions in the rain shadow north of the Nianbaoyeze Shan. A more recent decrease of Betula around 2250 cal yr B.P. on summer pastures of the southern Nianbaoyeze (Kekehe East) was caused by stronger nomadic inﬂuence (spreading of Bistorta and Trollius at several sites). At the same time, the Anemone-rich pastures of the Jiea Basin started to develop into their modern state by an increase of Trollius, S. filiformis and Chenopodiaceae. In our opinion, this synchronicity demonstrates the onset of the still existing annual nomadic migration patterns, with movements between summer pastures in the Nianbaoyeze Shan and the winter pastures around Aba or the Huang He area (Fig. 2) through the Jiea Basin. If correct, this represents the ﬁrst palynological evidence of annual nomadic migration. The increased soliﬂuction activity and glacier advances since about 3000 cal yr B.P. indicate cooling periods and/or increasing grazing pressure. In addition, the onset of peat growth on the higher terraces reﬂects climatically cooler and wetter conditions and/or higher water retention on compacted soil surfaces through trampling. During the second half of the 8th century AD Tibetan tribes waged several campaigns of conquest (Hermanns, 1949; Gernet, 1997). At the time of military expansions the nomadic inﬂuence on the vegetation expanded for the need of food supply. In the Jiea Basin S. filiformis and Chenopodiaceae suddenly spread out, Juniperus shrubs became more common while Betula and Picea (Kekehe East) trees were cut around 1200 cal yr B.P. In the 17th century the invasion of Golok tribes possibly interrupted the nomadic migrations (Tafel, 1914), and the Jiea Basin may have been grazed all year long (high Anemone-type, low S. filiformis), until the migration routes were re-established. Possibly a low biomass production during the LIA was the trigger for these socio-cultural changes. At all sites, a recent trend to pastures less dominated by Bistorta (B. macrophylla) was recorded. The corresponding spread of Ranunculaceae (Anemone-type, Trollius) and lower pollen production of Cyperaceae suggest a strengthening of grazing over the last few centuries. The general increase of Juniperus reﬂects the growth of shrubs. Other grazing weeds exceed their earlier values by far (Rheum, Koenigia) or occur for the ﬁrst time (Boraginaceae). Particular features of the pollen diagram Lerzha River such as the high values of Bistorta and low values of Ranunculaceae (Anemone-type, Trollius) seem to be typical for winter pastures. The poisonous ingredients of the Ranunculaceae decay with drying; thus offering a signiﬁcant selective advantage for members of this plant family on summer pastures (Roth et al., 2000; Jigjidsuren and Johnson, 2003). The question arises which natural vegetation type was replaced by the nomadically induced Kobresia-pastures? Palynological and ecological results point to a Poaceae-rich steppe-like vegetation as the natural vegetation of eastern Tibet (Miehe et al., 2006, 2008; Schlütz et al., 2007). The present and natural vegetation types (K. pygmaea 2–3 cm small, green all year round, Poaceae 1–2 m in height, fading in autumn etc.) clearly differ in their potential water and energy ﬂuxes (Albedo) with important consequences for the local and regional climate (Ganopolski et al., 1998; Li et al., 2002; Xue et al., 2004; Gu et al., 2005). Due to the large surface area (450,000 km2 K. pygmaeapastures) it is highly likely, that the anthropo-zoogenic vegetation changes have affected synoptic conditions. In particular, because the Tibetan Plateau as a critical summer heating source, is one of the most important drivers of the Asian monsoon system (Flohn, 1955; Duan and Wu, 2005; Wu et al., 2007; Wang et al., 2008). We conclude that the Tibetan nomads inﬂuence the monsoonal climate by livestock breeding since about 6000 years ago. It may therefore be adequate to introduce the term ‘‘nomadic Anthropocene’’ for this period (Crutzen and Stoermer, 2000; Crutzen and Steffen, 2003; Ruddiman, 2003, 2005; Miehe et al., submitted for publication). It is an open question, but which course would the summer monsoon strength have taken without nomadic inﬂuence? 5. Conclusions The present vegetation mosaic of Tibet with its pastures, tree groves and pioneer shrubs, is the result of a long lasting coevolution of landscape development, climate change and nomadic culture. Therefore, models using recent climate data and pollen spectra to reconstruct past climate changes are likely to fail or F. Schlütz, F. Lehmkuhl / Quaternary Science Reviews 28 (2009) 1449–1471 produce inaccurate results. Here we have intentionally used a ‘‘subjective’’ interpretation of pollen data to develop a new hypothesis. It would be a great challenge to include grazing as an essential factor into future statistical models. It is desirable to reﬁne the palynological and ecological knowledge with an emphasis on grazing indicators including spores of coprophilous fungi (van Geel et al., 2003; van Geel and Aptroot, 2006; Schlütz et al., 2007), carbonized plant fragments and pollen clumps (Schlütz and Lehmkuhl, 2007), instead of reducing the number of factored palynological taxa due to limited palynomorphological knowledge. To ﬁnd pollen of the insect pollinated grazing weeds (Boraginaceae, Bistorta, Caragana-type, Potentillatype, Rheum, S. filiformis, Tribulus, Trollius, Stellera etc.) in sufﬁcient amounts, cores must be taken inside the pastures. Lake sediments are less instructive in this regard. Nevertheless, it will remain difﬁcult to distinguish human and climatic effects, even when different proxy data are conﬂated. As demonstrated, analyzing geoarchives in high spatial resolution from areas sensitive to change seems to be an adequate way. We tried to demonstrate that short term cold events (Yu et al., 2006) had only transient inﬂuence on vegetation history when grazing pressure was low. Locally they may have induced the starting of peat accumulation (cold events at 6200, 5200, 3500, 1500 cal yr B.P.). Lasting vegetation changes stimulated by short climatic changes occurred only in times of a strong anthropozoogenic inﬂuence. Kobresia-pastures appear to be the right term for the K. pygmaea dominated vegetation of Tibet instead of Kobresia-meadows or -mats, as nomads have shaped them for more than 6000 years. Based on a high spatial resolution of our pollen data, the existence of annual nomadic migration patterns since about 2200 years can be demonstrated for the ﬁrst time. The increased soliﬂuction activity within and around the Nianbaoyeze Shan may have been driven not only by climate but also through nomadic land use. In spite of the remoteness and a seemingly weak human inﬂuence, large parts of the Tibetan Plateau are not a pristine or natural, but a cultural landscape (Kaiser et al., 2006; Miehe et al., 2006). In the words of Kingdon-Ward (1947): ‘‘But Tibet considered primarily as a grazing land seems to have been overlooked. Yet that is what it really is.’’ These insights are likely to be important for understanding the present climate evolution throughout eastern Asia and even parts of Africa, because the particular types of vegetation on the Tibetan Plateau may inﬂuence the global monsoon system. We propose the term ‘‘nomadic Anthropocene’’ for the last 6000 years, when nomads transformed the natural steppe-like vegetation into K. pygmaea-pastures (Miehe et al., submitted for publication). Acknowledgment We would like to thank H.-J. 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