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Institut
Biodiversity loss could jeopardize ecosystem functioning. Yet, the evidences that support this demonstration have been mostly obtained in aquatic and grassland ecosystems. Howbiodiversity affects ecosystem functioning still remain largely unanswered in forests, particularly in subtropical broad-leaved evergreen forests (EBLF). Tree productivity, among a wealth of forest ecosystem functioning, is of particular interest because it reflects the carbon sink capacity and wood productivity. Biodiversity-productivity relationships have been usually investigated at community level. However, tree-tree interactions occur at small scale. Thus, local neighborhood approach may allow a better understanding of tree-tree interactions and their contributions to the effects of biodiversity on tree productivity / growth rates. This thesis aims to analyze the effects of biodiversity and the abiotic environmental factors on the tree growth rates using both local neighborhood and community-based approaches. Furthermore, tree growth rates vary among different tree species. Functional traits have been related to the species-specific growth rates to understand the effects of species identity. Therefore, I also evaluated the crown- and leaf traits to predict the interspecific difference in growth rates. For a better understanding of the mechanisms that underline the relationships of biodiversity and tree growth rates, data of high solution and along time series is required to scrutinize the tree-tree interactions. Thereupon, I evaluated the applicability of terrestrial laser scanning (TLS) in assessing the tree dendrometrics. This thesis was conducted in the Biodiversity Ecosystem Functioning (BEF)–China experiment, which is located in a mountainous subtropical region in southeast China. A total of 40 native broad-leaved tree species were planted. In the first study, I used the local neighborhood approach to analyze how local abiotic conditions (i.e. topographic and edaphic conditions) and local neighborhood (i.e. species diversity and competition by neighborhood) affect the annual growth rates of 6723 individual trees. The second study used the community approach to partition the effects of environmental factors (i.e. topographic and edaphic), functional diversity according to Rao’s quadratic entropy (FDQ) and community weight mean (CWM) of 41 functional traits on community tree growth rates. The main question of the third study was how the species-specific growth rates are related to five crown- and 12 leaf traits.
In the fourth study, I investigated 438 tree individuals for the congruence between the conventional direct field measurements and TLS measurements. It was found that tree growth rates were strongly influenced by the local topographic and edaphic conditions but not affected by the diversity of local neighborhood. In contrast, results obtained by using the community-based approach showed that FDQ and CWMs of various leaf traits rather than abiotic environmental factors had significant impact on the community means of growth rates. Tree-tree interactions already occur in early life stages of trees, which were evidenced by the significant effect of competition by local neighborhood. These findings imply that the effects of abiotic environmental factors may be more evident at local scale and biodiversity effects may vary at different spatial scales. The species-specific growth rates were found to be related to specific leaf traits but not to crown traits and were best explained by both types of traits in combination. This finding supports the niche theory and provides the evidence for using functional diversity to examine the BEF relationships. The TLS-retrieved total tree height, stem diameter at 5 cm above ground, and length and height of the longest branch were highly congruent with those obtained from direct measurements. It indicates that TLS is a promising tool for high resolution, non-destructive analyses of tree structures in young tree plantations. Being one of very few studies to incorporate the individual tree scale in examining the biodiversity-productivity relationships within the BEF researches, this thesis stresses the importance of using individual-tree based approach, functional diversity and TLS to find the evidences of explanatory mechanisms of the observed biodiversity and ecosystem functioning (e.g. tree growth rates) relationships. Biodiversity effects may evolve along the successional stages. Therefore, incorporating the interaction between biodiversity and time in analyzing BEF relationship is also encouraged.
The dissertation deals with the impact of nitrogen deposition on the functioning of heathland ecosystems. Special interests were the displacement of heather (Calluna vulgaris) by the purple moor-grass (Molinia caerulea) as well as the fate of nitrogen loads in dry heathland ecosystems. The results of the studies undertaken in the field and in the greenhouse are presented as five individual journal articles. The nature of nutrient limitation was studied by means of fertilisation experiments with nitrogen (N) and phosphorus for heather and purple moor-grass (Articles I and II). The impact of nitrogen deposition on the outcome of competition between these two species was analysed during a competition experiment in the greenhouse (Article III). The aim of a 15N tracer experiment was to determine the fate of nitrogen deposition as well as allocation patterns (Article IV). In addition, the response of purple moor-grass to the combined effects of nitrogen deposition and summer droughts was investigated in a second greenhouse experiment (Article V). The fertilisation experiments showed that the growth of heather as well as of purple moor-grass is predominantly limited by N (Articles I and II). However, the results of the competition experiment demonstrated that only purple moor-grass has the ability to benefit from additional N loads, which in turn gives the grass the opportunity to displace heather (Article III). Drought treatment resulted in strikingly reduced biomass production of purple moor-grass in N-fertilised pots, mainly as a result of dying aboveground biomass during dry periods (Article V). This striking susceptibility of purple moor-grass to the combination of nitrogen deposition and drought must be taken into account, when predicting future developments of dry heathlands. The results of the 15N tracer experiment showed that the investigated heath is still in an early stage of N saturation, as indicated by a high immobilisation capacity and negligible leaching losses of 15N (Article IV). The findings of the dissertation contribute to a better understanding of the processes underlying the encroachment of purple moor-grass in dry heathlands and can enhance heathland management. The results can also be used to to evaluate the current and future status of this ecosystem particularly with regard to the various stages of N saturation as well as in the determination of “Critical Loads”.
The role of tree diversity for individual tree growth, crown architecture and branch demography
(2012)
In the light of the concurrent loss of biodiversity, biodiversity and ecosystem functioning (BEF) research attracted a great deal of attention and emerged as one of the important fields of research in ecology. Since important ecological interactions such as competition occur between individuals, the understanding of individual tree growth was considered to be fundamental for forest related BEF research. Individual tree growth is determined by the above- and belowground interactions of a tree individual with its local neighbourhood. To obtain a deeper understanding of BEF relationships, I broadened the focus from individual tree growth (usually measured as diameter or biomass increment) to the arrangement and dynamics of the above-ground modules of trees in dependence of their local neighbourhood. More precisely, the main objective of the present thesis was to analyse the impact of tree diversity on individual tree growth, crown architectural and branch demographic variables. Thereby I considered crown architectural variables as important indicators of the competition for light. In addition, crown architectural variables impacted ecosystem services such as erosion control. Furthermore, the results of the present thesis contributed to the current discussion on species coexistence theories, which may be differentiated by two opposing views: one that relies on neutral processes and one that implicates a role for meaningful differences in the ecological strategy (niche) of co-occurring species. The studied forest ecosystems were the subtropical broad-leaved evergreen forests of southeast China, which have been under high human pressure due to a long history of intensive land-use. The area is of particular interest for BEF research due to the high species richness of woody plants, including many, yet poorly studied species, and due to the rough terrain with steep slopes, which cause severe soil erosion. The present thesis combines three observational with two experimental studies, applying the local neighbourhood approach along an age gradient from tree saplings to mature trees. In the Gutianshan National Nature Reserve (GNNR), I conducted two observational studies on permanent plots which were chosen according to a space-for-time substitution design. The aim of the first study was to reveal the effects of diversity (species richness, functional diversity) together with other biotic and abiotic variables on morphological growth parameters (crown area, crown displacement and stem inclination) of target trees of four tree species (Castanea henryi, Castanopsis eyrei, Quercus serrata and Schima superba). In the second study, the same target trees together with their neighbours were used to analyse the relation between stand related functional diversity and the horizontal and vertical structure of the canopy. The third study was conducted in a young secondary broad-leaved evergreen forest. Using two target species (Castanopsis fargesii and Quercus fabri), the role of diversity, intra- vs. inter-specific competition and the mode of competition (symmetric vs. asymmetric) on the target individuals was tested by analysing five-year radial growth increments. The two other studies were carried out in an experimentally established plantation, using saplings of four tree species (C. henryi, Elaeocarpus decipiens, Q. serrata and S. superba), which were planted in monoculture, twoand four-species combinations and in three densities. The fourth study focused on mechanisms of coexistence and the role of species richness, species composition, species identity and density on sapling growth. The fifth study tested the effect of sapling density and identity on the througfall kinetic energy, which represents a measure for the erosive power of rain. It was found that functional diversity does affect crown architectural and canopy related parameters of forests in the GNNR. However, no effects of species richness on radial-growth were detected in the younger forest. Since I also did not find strong effects of species richness on saplings in the experimental plantation, diversity effects may evolve at a later age stage. The importance of the diversity effect may be related reversely to that of species identity in an age gradient of forest stands. The findings suggest that different mechanisms of coexistence operate simultaneously but that their relative importance may shift through the life stages of trees. During the sapling stage, species-specific differences in growth and architectural traits support niche theory. In older forest stands, no species-specific differences in growth parameters could be detected. However, I did find effects of functional diversity on horizontal canopy structure. I conclude that mechanisms of coexistence may not only change with forest stand age, but may also differ for distinct traits. The present thesis, being the first to apply the local neighbourhood approach with regard to crown architecture and branch demography within the BEF field of research, stresses the importance of this individual based approach. Although the observed forest systems are very complex, crown architectural and canopy structural variables were found to be affected by diversity. The finding that the degree of erosive power of rain could be elucidated by crown architectural variables, encourages further studies to reveal possible relations between biodiversity and other ecosystem functions or services, which might be mediated by crown architectural and canopy structural variables.
Restoration and management of abandoned, dry Continental heathland and sandy grassland communities
(2018)
Whereas heathlands in the Atlantic biogeographical region have been in the centre of interest, dry heathland communities in the Continental biogeographical region have been widely neglected. Thereby, the conservation status of both habitat types is listed as unfavourable-bad across the Continental biogeographical region. Surprisingly, no detailed studies are available on cost-efficient and sustainable restoration and management schemes to successfully restore and maintain highly degraded, long-abandoned Continental heaths and sandy grasslands, and thus, to counteract the poor conservation status of the habitat types. This shows the great need for research for the Continental biogeographical region. Thus, the present thesis provides substantial knowledge about the population dynamics of the key plant species of dry heaths Calluna vulgaris (L.) HULL by investigating key processes in the biology of the species as well as about the restoration and management of long-abandoned, dry Continental heathland and sandy grassland communities. In order to better understand the process of successful Continental heathland restoration, I analysed the reproductive potential (seed production, soil seed bank, and germination ability of seeds) of degenerate Calluna stands as well as the effects of single and combined management options on the generative rejuvenation (i.e., recruitment and survival) of Calluna. The results are based on a comprehensive three-year field experiment including the management options year-round, low-intensity cattle and horse grazing, one-time mowing and one-time shallow soil disturbances combined with greenhouse investigations on the soil seed bank content and germination ability of Calluna seeds. The results showed that even after long-term abandonment, seed production of degenerate Calluna stands and the germination ability of seeds proved to be high, being similar to Atlantic heathlands, whereas the soil seed bank is considerably reduced probably due to the dry conditions in the Continental region. In addition, low-intensity grazing with free-ranging robust breeds and the combination with one-time mowing at the beginning of the restoration process is an effective means of supporting the generative rejuvenation of this key plant species in degraded Continental heaths. The second study of this thesis focussed on the first-year establishment of Calluna in managed and unmanaged dry heaths and heaths in mosaics with dry sandy grasslands. The germination ability of seeds of different life-history phases of Calluna was analysed to determine if the predominance of the late life-history phase restricts the rejuvenation process of this key plant species. In addition, beside effects of management measures (year-round, low-intensity grazing, one-time mowing, one-time shallow soil disturbances) the author analysed the most important safe site conditions that possibly influenced the germination and the first-year survival of Calluna. The results of the study combine field experiments with growth chamber investigations. The author found that life-history phase of Calluna did not significantly affect seed germination and thus, the predominance of the degenerate life-history phase does not restrict the rejuvenation process. In addition, the results of the study revealed that grazing and thus trampling intensity must be temporarily and locally enhanced at the beginning of the restoration process of highly degraded heaths to increase safe site availability for successful Calluna establishment. Thereby, shadowing is the most important safe site condition. In the third study, the author investigated the impacts of year-round low-intensity cattle and horse grazing on the development of the highly competitive grass Calamagrostis epigejos (henceforth referred to as Calamagrostis), as well as the vegetation structure and plant species richness of long-abandoned but nutrient-poor dry heathland and sandy grassland communities, their mosaics and Calamagrostis stands. Finally, I assessed the local conservation status of the habitat types after seven years of grazing in comparison to long-abandoned sites. The results are based on a comprehensive field study on two spatial scales (plot-level: 25 square meters, macroplot-level: 1 ha). The author found that grazing successfully reduced the coverage and prevented the further spread of Calamagrostis, while simultaneously maintained or improved characteristic species richness and vegetation structure across the different nutrient-poor vegetation types over time, and thus enhanced the local conservation status of habitat types of community interest. In conclusion, the results of the studies considerably improved the understanding of dry, Continental heathland and sandy grassland restoration and management. They provide evidence that even after long-term management abandonment, year-round low-intensity cattle and horse grazing is a suitable management tool for restoring, maintaining and even improving nutrient-poor heathland and sandy grassland communities. However, at the beginning of the restoration process, additional management measures are necessary to faster restore abandoned habitats, especially highly degraded heaths.