Structure and functioning of soil animal communities in beech forests mixed with Douglas-fir, silver fir and spruce

Increasing temperatures and recurring drought events in the face of climate change are challenging European temperate forests. Notably, large-scale diebacks have been reported in monoculture Norway spruce stands. In this context, more drought-resistant alternative tree species are being promoted and implemented in silvicultural practices of tree species mixtures. Non-native Douglas fir as well as silver fir native to southern Europe are considered suitable options, especially when establishing mixed stands with European beech. However, consequences of implementing non-native Douglas-fir remain relatively unknown, particularly regarding the belowground food web. The soil fauna are a fundamental part of forest ecosystems, linking above-and belowground food webs and providing key ecosystem functions, such as nutrient cycling, nutrient availability and soil formation. Changes in their community composition and functionality are therefore important to understand to infer effects of the implementation of the non-native Douglas-fir into European forest ecosystem on the soil food web. Here we study soil fauna communities and their ecosystem functions in Douglas-fir - beech forests compared with beech and the native conifers silver fir and spruce, including different proportions of conifer-beech mixtures, across three regions in Germany: northern Bavaria, southern, and northern Lower Saxony. First we investigated macrofauna communities and trophic niches based on stable isotope analysis of 13C and 15N of predators, primary and secondary decomposers in Lower Saxony with two contrasting site conditions of nutrient-rich and nutrient-poor soils (Chapter 2). When comparing Douglas-fir forests with spruce and beech as well as the conifer - beech mixtures, Douglas-fir had a negative impact on soil macrofauna in nutrient-poor sites. However, Douglas-fir - beech mixtures mitigated the effects on density and diversity as well as the detrital shift, which was noticeable in the conifer forests compared to pure beech. The community composition of macrofauna in the conifer mixtures was in between the respective monocultures. In general, trophic groups differed more between regions than forest types, while trophic positions remained robust between regions. Forest type in loamy sites generally had little influence on soil macrofauna. Nevertheless, primary decomposers benefited from Douglas-fir and especially Douglas-fir - mixtures compared to spruce forests, profiting from lower acidification and higher quality of Douglas-fir leaf litter. To further investigate the influence of conifer-beech proportions, we studied the soil mesofauna (Oribatida, Collembola and Mesostigmata) in increasing proportions of Douglas-fir in beech dominated forests compared with silver fir - beech forests in the Spessart region of northern Bavaria (Chapter 3). Here, Oribatida communities shifted towards increasing proportions of families associated as primary decomposers in forests with higher Douglas-fir proportions, confirming a positive effect of Douglas-fir on primary decomposers for the soil fauna (Chapter 2 & 3). The community composition of Oribatida responded sensitively to the differences in forest types for Douglas-fir beech, silver fir - beech and pure beech stands on species and family - level, while the community composition of Collembola and Mesostigmata varied little. With only Collembola diversity increasing in Douglas-fir - beech forests compared to silver fir –beech mixtures, there were few differences in density and diversity between the forest types. Therefore the soil mesofauna further highlights the general robustness of the soil food web between different forest types and with increasing conifer proportions. Finally we looked into the energy fluxes of the soil food web of decomposers and predators comprising macrofauna and mesofauna with increasing proportions of conifers in beech stands, including Douglas-fir and silver fir (Chapter 4). Mesofauna dominated in biomass and energy fluxes, highlighting their importance towards ecosystem functions in acidic forest types as occurring in the Spessart. Energy fluxes of decomposers profited from moderate Douglas-fir proportions compared to silver - fir, while energy fluxes of predators increased in higher conifer proportions, with responses being conifer tree species specific. The diversity of macrofauna was driven by soil pH, with total energy fluxes positively driven by mesofauna diversity. Group specific energy fluxes increased with greater diversity of decomposers and predators, indicating a positive biodiversity ecosystem functioning relationship and the importance of maintaining a diverse soil food web. To conclude, the results of this thesis show the robustness of soil animal communities and food webs under the introduction of Douglas-fir into European temperate forest systems. However, soil conditions should be considered when introducing Douglas-fir, but the soil food web and the ecosystem functions are resilient to changes in forest types, especially by mixing Douglas-fir with beech. Douglas-fir - beech mixtures can positively influence primary decomposers and decomposition of soil fauna communities compared to native conifer tree species, especially in forests with lower Douglas-fir proportions (Chapter 5). Therefore, Douglas-fir can be considered an alternative to the climate vulnerable Norway spruce, regarding the influence on soil fauna. In general, by conserving biodiversity and promoting soil fauna communities favored by soil conditions, planting Douglas-fir in temperate beech forests can maintain key ecosystem functions and can contribute to the silviculture of the future. Further research needs to focus on further environmental conditions and basal resource distribution in Douglas-fir forests, to understand changes in abiotic and biotic factors such as resource shifts under Douglas-fir.