4.1. Suitability of the Perceived Components Used to Survey Visual Aesthetics and Acceptance of Local Forest Types
The five components of perception visual diversity, comprehensibility, ecological aesthetics, restfulness and order/spatial arrangement were chosen for this survey of local forest types dominated by Norway spruce to explain the two levels of visual perception: visual aesthetics and acceptance. These two levels constitute descriptions of psychological/mental human effects engendered by appreciated physical forest structures [
7]. Visual diversity and comprehensibility are essentially more intuitive sensations, originally developed over the course of evolution [
74]. In this biologically influenced context, people asses the advantageousness of a forest type or view by its suitability as a habitat within a landscape and within the natural settings preferred by people, as was explained by [
57]. By comparison, the acceptance of a forest type is based on information and on an understanding of how forest ecosystems should be managed to fulfil the demands of recreation. Therefore, ecological aesthetics has high relevance for acceptance, but both aspects are not the same [
63,
75]. Differences in people’s imaginations with respect to forest management are related to differences in acceptance. For example, people can accept forest types and views they might not prefer on the basis of aesthetic reasons, but which are seen as acceptable in the context of the need for specific silvicultural techniques or measures to produce and use forest goods and services. This means that acceptance is rather a cognitive skill that can be influenced by knowledge. The valuation of dead wood, for instance, depends on knowledge of the functions of dead wood as a natural part of ecological processes and systems [
38]. Dead wood was perceived as a structural element within the three forest views by the respondents but not its ecological functions. This was evidenced by the fact that dead wood was not listed as an important or as an absent element.
The overall rankings of the three evaluated forest types led to the same order for both visual aesthetics and acceptance, starting with the most favourably assessed forest type, Norway spruce forests with the regeneration of deciduous tree species in the background (fv1), followed by the Norway spruce stand with European beech as a second layer in the foreground of the stand (fv3) and in third place the Norway spruce stand without regeneration (fv2). The results of the ordination analysis and the permutation tests proved that the differences in the visual effects were low between fv1 and fv3, but significant in comparison with fv2. However, Norway spruce forests enjoyed greater acceptance where the forest view included some deciduous tree species. The preference for mixed forests was demonstrated and primarily interpreted as a preference for visual diversity [
76,
77]. One basic problem of using a number of selected components to evaluate people’s perceptions in surveys is the high degree of subjectivity and complexity, which is typical for the field of perception psychology [
18,
74]. To ensure relevance in terms of practical forest management, this study was established as a preference-based study, which sought to determine causalities between forest structures, and the perception thereof, rather than using ‘black box’ systems [
78]. It is important to find out which components of psychological perception can be directly linked to specific structural categories of ecosystems as an important part of people’s environments [
44,
79]. Our attempt to test the functionality of negatively formulated control statements in the survey with respect to visual diversity (statement 3), restfulness (statement 7) and order/spatial arrangement (statement 8), in accordance with [
34], failed only for the negative control statement addressing visual diversity (statement 3). The reason for this was perhaps an unfavourable formulation of the statement with a double negative in the questionnaire. This led to irritation amongst respondents in terms of classification within the rating scale (
Table 3). A better formulation for this third statement would probably have been: ‘There is little to be discovered within this forest type’.
In this study, with its high degree of local colour, we used five components which were demonstrably known to be of particular relevance in evaluating the visual perception of forest structures [
33,
34,
74]. It could be shown by means of linear mixed models that the chosen components were mostly appropriate to obtain information about the visual perceptions and acceptance of forest types and structures by the people who participated. An extensive proportion of the explanation was provided by the component visual diversity (
Table 5), especially for the appraisal of the visual aesthetics. Visual diversity was substantially lower in terms of explaining acceptance (
Table 6). The results of previous studies documented considerable differences in the strength of relationships between aesthetic diversity and acceptance or acceptability [
34,
57,
75]. The visual diversity component of forest views involved not only the number of perceived structures or structural elements, but also the shapes, colours and the individual significance of structures or structural attributes for people [
45,
47]. Hence, from the freely associated statements provided by the respondents, tree species, species of ground vegetation and forest structures characterised by different colours could be assigned to the component visual diversity [
42,
43,
76,
80]. For example, the white bark of the birch tree in fv2 differed from the brown bark of the Norway spruce trees. This contrast of colours may have increased the total frequency of perception of visual diversity, even if the beholder’s knowledge (see ecological aesthetics) of tree species was low or the surrounding environment within the forest view was uniform [
26,
45,
81]. Palmer et al. [
82] and White et al. [
20] demonstrated that contrasts in form, colour, material and light conditions have an effect in terms of the perception of the visual diversity of landscapes and of forest ecosystems. The correlations between different visual components (
Table 3) can also produce certain overlay effects, which serve to increase or reduce the relevance of individual components and their meaning [
66]. The critical correlation coefficient value of 0.7 was reached only for ‘aesthetic effects’ (statement 1) and ‘ecological aesthetics’ (statement 5) in relation to ‘acceptance’ (statement 10) (
Table 3).
A relatively common feature of managed Norway spruce forests are the remains of tree crowns consisting of fresh green needles left on the forest floor after felling activities. Fresh needles and leaves are normally appraised positively in contrast to a brown needle-horizon on the floor, but in assessing the component ecological aesthetics, the observer reflects upon the fact that this element is not natural in origin, or the result of anthropogenic activities [
21,
36]. As a consequence, this element is valued negatively, with an additional negative feedback loop for visual diversity. In our survey, it could be shown that this reflection was not driven by the component comprehensibility, because neither visual aesthetics nor acceptance were significant explanatory variables within the models (
Table 5 and
Table 6). Although it could be assumed that the local people represented in the survey had some knowledge of the dominant tree species Norway spruce and the admixed species such as birch or European beech, this was not directly apparent from the results of the modelling process for visual aesthetics or acceptance. The meta-analyses presented by Stamps [
79], who also included studies concerned with forests, demonstrated that the relationship between comprehensibility and aesthetic preferences has a very broad range, with opposing statements made in different studies. In our study, the component ecological aesthetics was more relevant to explaining the acceptance of forest types than visual aesthetics (
Table 5 and
Table 6). This was in line with observations from Gundersen and Frivold [
83], who noticed that structures are only perceived when background knowledge of the forest ecosystem is available, or when the structure differs in form or colour from the rest [
61]. The aforementioned authors also pointed out, however, that ecological aesthetics can also be driven by emotional contexts. Examples of this might be a theoretical imagining or feeling about how a real natural forest structure should look, and which structural attributes are originally a part of natural forests. As a result, most respondents favour forests that look natural from their individual point of view in combination with some aspects of ecological knowledge; for example, deciduous tree species are rare in this region and should be an important component of these forests [
1,
21]. From the perspective of ecological aesthetics, fv3 obtained nearly the same positive evaluation scores as fv1, even though the beech looked relatively homogenous within fv3 and were established by means of significant anthropogenic intervention. The component restfulness obtained the second rank within the model (
Table 5), which explained the visual aesthetics, and was highly significant for the model explanation of acceptance (
Table 6). This could again be explained by the perception of naturalness. If the sense of closeness to nature increases during the visual reflection of a specific forest, the feeling of restfulness also increases [
84]. According to Kaplan’s [
12] ‘attention restoration theory’, restfulness can be described as an unconscious reaction which leads to specific attention directed towards objects within the environment or, as in this case, in the forest. Restfulness has also been described as a complex reaction summarising different components, which is why the relevance of this component for visual aesthetics should not be over-interpreted [
27]. The statements addressing order/spatial arrangement also obtained higher valuations, in particular for fv1 and fv3, but the notion of order/spatial arrangement in natural systems is possibly too abstract and may have been interpreted differently by the visitors (see also [
34]). This became particularly obvious in the case of fv2, which was assessed negatively in terms of order/spatial arrangement, even as the corresponding structural parameters and silvicultural valuation units suggested this was the most homogeneous structure for this forest type. The terms ‘order’ and ‘spatial arrangement of environmental structures’ were interpreted as examples to inspire a high level of individuality [
85,
86]. These authors summarised the considerations with the question: What is orderly and how much human impact/care is appropriate for the forest structure? In the case of our study, this was obvious; for example, in relation to fv1, where the trench and old stumps were the result of a direct human impact, but this impact was not deemed by the respondents to be negative with regard to visual aesthetics or acceptance. The last component in particular confirmed that silvicultural activities at local and regional scale are accepted by forest visitors whose appreciation of the visual aesthetics is underpinned by basic ecological knowledge and understanding. Over the last decade, the need for state and private forest enterprises to effectively present and communicate their forest management activities to the public has increased greatly for urban forests and in forest areas with a high appeal for tourists [
87,
88]. Public relations activities addressing the topic of forest conversion [
89], for example, the conversion of homogenous Norway spruce forests to European beech or mixed forests, have been undertaken by the state forestry administrations in Germany for many years now [
90]. As was explained in
Section 2.1, this was one reason for the choice of the three forest views selected as typical examples of frequent forest structures. The local population is well informed about the topic of forest conversion, the related ecological processes and silvicultural techniques. The preparation of additional information is required for tourists, and the further adaptation of forest structures along the main trails should be strengthened accordingly. The results showed that contrasting tree species mixtures with higher proportions of deciduous tree species should be supported by silvicultural measures. It was also shown that tourism concepts should integrate information for visitors, both local and non-local, about the ecological role and value of dead wood. These concepts might include guided forest tours, forest-related education events or educational forest trails [
38,
91].
4.2. Relationship between Forest Structures and Components of Visual Aesthetics
Overall, the participants in this study did not focus overly much on the structural details of the forest views [
28]. They described the coarse structures of the forest types and surrounding elements such as light conditions, canopy closure or the proportion of visible sky. This perception was driven by the human quest to identify the optimal proportion of light and shadow in the surrounding habitat at every time of the day [
81]. Tree regeneration was the forest structure most frequently referred to by the survey participants in response to the open questions (
Table 7). The specific identification of the perceived tree species combined two aspects of visual aesthetics: (i) the visual capacity for perception (cognitive), if the habitus and the colour of different tree species are comparable, and (ii) knowledge of tree species. The latter is part of ecological aesthetics and comprehensibility, but only in rare cases was more than one tree species named [
92]. On the other hand, it is conceivable that rarely mentioned structures have a specific and individual meaning for some respondents [
21]. Damage to individual trees was not described, which was consistent with [
93], who differentiated between the experiences of landscapes expressed by laypeople (emotional) and by experts (cognitive). Water was one desired aesthetic component of forests, as found previously by [
20]. More regeneration of deciduous tree species was listed as a desirable forest structure, serving to increase diversity and the proportion of ‘green’ within the forest types [
94]. Shades of the colour green have been demonstrated to have a strong psychological effect for humans and on the interpretation of what respondents have seen [
74,
95,
96]. At the same time, dense green regeneration in the foreground blocks views into the forest and reduces opportunities to see ground vegetation and wild animals, and hampers the picking of mushrooms [
97,
98], which are also desired activities. It is important, therefore, that causal analyses differentiate between visual aesthetics and recreational amenities, as was stressed by [
27].
Different qualities of relations between the components of visual aesthetics and acceptance could be proven for the three chosen forest types by the correlation analyses [
57,
79]. The first forest type with old Norway spruce trees in the overstorey and a mixture of mainly deciduous young trees in the background received the most positive overall assessments for visual aesthetics and acceptance, although this forest type was also dominated by conifers. This ranking could largely be attributed to the age and dimensions of the Norway spruces trees. It has frequently been shown that old trees with larger diameters (>40 cm in this forest stand) are perceived positively by forest visitors [
36,
99]. Humans show a strong mental connection to old and big trees in large part because of their worldwide cultural–historical significance as objects of self-reflection, religious and symbolic character [
100]. Another positive effect in terms of the visual aesthetics of older forests is the high visibility [
42,
99] or visible depth [
30,
83]. Old, mono-layered Norway spruce forests with low stocking densities and no regeneration layer in the foreground form a simple structure, which is relevant in the context of the theoretical background covered by the ‘information-process theory’ [
57]. The structural information encrypted in this forest type is relatively simply perceived by visual skills, while the visual value of fv1 was further improved by the clearly visible aggregate of a mixed deciduous tree species regenerating in the background [
86,
101]. Aggregated tree species admixtures are preferred in some forest regions of central Europe [
48,
81]. The positive visual effect of the visible regeneration is supported theoretically by the ‘prospect-and-refuge theory’ [
102], which emphasises that the possibility to easily spot interesting things exists within this forest type, and also to find options for a place of refuge. Both aspects have their origins in humankinds’ evolutionary development and behaviour within a natural environment. The admixed deciduous tree species in the background also contrast with the pure old Norway spruce trees in front in both form and colour [
61,
92]. This is in line with the valuation of restfulness, illustrating the fascination with the concentrated regeneration and highlighted by the visible depth [
12,
84]. The low proportion of dead wood had a positive effect in relation to the visual component order; it was not perceived as a disturbance [
42]. The third forest type differed significantly in structural composition from the perspective of silvicultural management but contained comparable key structural elements such as old Norway spruce trees combined with only one deciduous tree species. This led to an only slightly poorer valuation, with a small shift in the ordination results (compare
Figure 3) of fv3 compared to fv1. The dense European beech layer, established to achieve forest conversion, reduced the visibility and visible depth significantly, with a lower value in terms of aesthetic effects [
42,
83,
99]. Buhyoff et al. [
103] and Ribe [
104] found that for trees of dimensions less than 12 cm dbh the effect of visual aesthetics can be described as a bell-shaped curve, which increases with densities from 1000 to 4000 plants before decreasing again with higher tree densities. The Norway spruce trees in the overstorey of fv3 provided a low degree of canopy closure. This relativised the effect of density in the second layer and created a mystical effect with hidden areas in this forest type waiting to be discovered [
33]. The curiosity of visitors will be stimulated, but at the same time the accessibility of these forests is low. The second forest type comprising mono-layered Norway spruce trees with a high stocking density obtained the lowest rank in the assessment of visual aesthetics and acceptance. These middle-aged coniferous forests with low proportions of admixed tree species were obviously the forest types with the lowest visual attractiveness, even though Norway spruce has a high presence in this region and a particular cultural–historical significance [
77,
80]. The main reasons for this low appraisal were the lack of regeneration, the structural uniformity of the overstorey and the ground layer, and the limited visibility [
42,
53,
78]. On the basis of a structural description of this forest type using silvicultural standards, it could be assumed that there was little to discover in this forest type, yet this contradicted the number of valued structures listed by respondents. It appeared, therefore, that the visually perceived structures were linked to negative information with a low level of comprehensibility. Structures such as the dead wood found in the form of individual admixed birch trees and the high number of dead branches at eye level on the trunks of Norway spruce trees had a negative symbolic character. Some respondents used the term monoculture, which confirmed the negative valuation of the ecological aesthetic and the existing knowledge or intuitive perception that this forest was not naturally structured [
19,
105]. Moreover, the visual dominance of brown coloured forest structures led to an increasing desire for the colour green [
94,
106].