The role of tree species diversity for maintaining ecosystem services under multiple disturbances and climate change

Natural disturbances such as windthrow and bark beetle outbreaks are important drivers of long-term forest dynamics. While they are expected to promote tree species diversity, they can also negatively affect the provisioning of ecosystem goods and services (EGS). Especially in the face of climate change and altered disturbance regimes, the ability of forests to provide EGS is increasingly threatened. The maintenance or enhancement of tree species diversity is seen as a possibility to increase the resistance and resilience of forests and to secure the provisioning of EGS for the future. Thus, unravelling the relationship between tree species diversity and EGS under multiple disturbances and climate change has become increasingly important in ecology over the last decades. However, little is known on this relationship at large spatio-temporal scales due to the limitations of empirical studies. Dynamic vegetation models (DVM) can be used to assess the proposed benefits of tree species diversity in a holistic way. Forest landscape models, one important type of DVMs, are especially suitable to simulate forest dynamics and tree species diversity under changing climatic conditions and over large spatial and temporal scales. They also allow to study the complex relationship between tree species diversity and interacting disturbances while controlling for site history and environmental conditions. The forest landscape model LandClim was used to address the following objectives: (1) to assess how tree species diversity can influence the ability of a forest to provide single and multiple EGS, (2) to evaluate the impact of multiple disturbances on tree species diversity, and (3) to quantify the resistance and resilience of mixed-species forests to multiple disturbances and climate change. In Chapter I, I systematically evaluated the suitability of monocultures and mixed-species forests to provide single and multiple EGS at the landscape scale. To this end, I simulated all possible combinations of dominant tree species in two mesic landscapes in Central Europe (region around the Mt. Feldberg in the Black Forest in Germany, and the Dischma valley in the Canton of Grisons in Switzerland). To quantify their ability to provide EGS, several EGS metrics were examined: protection against natural hazards (avalanche and rockfall protection), aboveground biomass and habitat quality for birds. Simulation results indicated that mixed-species forests were generally better in providing multiple EGS whereas monocultures were often best for single EGS. Moreover, changing environmental conditions along an elevational gradient had a strong impact on the structure of the different species combinations and therefore on the provisioning of EGS. Thus, tree species diversity per se is not a good predictor of multifunctionality, but it is important to select species as part of the mixtures that occupy different ecological niches and that are best suited for the local environmental conditions. Forest disturbances can act as important drivers for biodiversity. In Chapter II, I evaluated the relative contribution of two distinctive disturbance agents (i.e., windthrow and ungulate browsing) for tree species diversity at the landscape scale and assessed the shape of the diversity-disturbance relationship. Simulations were run with varying browsing intensities and wind severities in four landscapes in Central Europe, differing in species pools and landscape complexity. My findings showed that tree species diversity in all four landscapes was influenced strongly by understorey (browsing) and overstorey (wind) disturbances, and their relative contribution depended highly on species composition, species-specific vulnerabilities to the disturbance, and forest structure, particularly tree size structure, mediated by growing conditions within the landscape. Tree species diversity was found to peak at intermediate browsing pressure, as dominant but browsing-intolerant species disappeared from the landscape. By contrast, more frequent and more severe wind regimes increased tree species diversity almost linearly by generating a mosaic of uneven-aged forest patches. Thus, the pattern and diversity of forest landscapes in Central Europe are shaped by understorey and overstorey disturbances. Tree species diversity may help to enhance forest resistance and resilience to disturbances and climate change. In Chapter III, I systematically explored the influence of multiple factors on the resistance and resilience to multiple disturbances and climate change at the landscape scale. To this end, I simulated a wide range of mixtures of three major Central European tree species (i.e., Norway spruce, European beech and Silver fir) under current and future climate, including wind and bark beetle disturbances. Simulation results indicated that even though spruce-dominated forests were least resistant to wind disturbance, increasing spruce proportion had a positive influence on resilience under current climate. Moreover, in the absence of climate change, forest resilience generally decreased with elevation and the co-occurrence of bark beetle outbreaks. Under climate change, elevation and spruce proportion reversed their influence on forest resilience, with elevation becoming an important positive factor and spruce proportion a negative factor. In interaction with bark beetle outbreaks, enhanced wind disturbances had a negative influence, particularly under water-limited conditions caused by climate change. As high resistance did not necessarily imply high resilience, the two properties should be considered separately in further studies. Additionally, especially the resistance and resilience of spruce-dominated forests to interacting disturbances and climate change could be enhanced if admixed with more drought- and disturbance tolerant species. Based on these findings, I recommend the following future research to further enhance our knowledge about the influence of tree species diversity on EGS provisioning and the diversity-disturbance relationship: (1) to assess the influence of subdominant species on EGS provisioning, (2) to evaluate EGS at multiple time points so as to increase the robustness of the findings, (3) to corroborate the wind module e.g. using a model comparison, and (4) to develop a more dynamic representation of browsing at the landscape scale in order to be able to assess more systematically interaction effects of windthrow and browsing on tree species diversity. Overall, this thesis demonstrated the usefulness of forest landscape models for biodiversity studies at spatial and temporal scales that are not accessible to either observation or experimentation. The findings highlighted the importance to focus on species with different functional traits in EGS studies, rather than on species diversity itself. Moreover, due to altered growth patterns of tree species under climate change, forest managers need to adapt forests continuously to maintain their resilience to disturbances under changing environmental conditions