Direct and indirect effects of forest management on tree-hole inhabiting aquatic organisms and their functional traits

Ecological communities in forests have been shown to be strongly affected by forest management but a detailed understanding of how different components of management affect insect communities directly and indirectly via environmental variables, how management influences functional trait diversity and composition, and whether these results can be transferred to other functional groups besides insects (e.g. bacteria or nematodes) is still missing. To address these questions we used water-filled tree holes, which provide habitats for insect larvae and other aquatic organisms in forests, as a model system. We mapped all water-filled tree holes in 75 forest plots (1 ha) under different management intensity in three regions of Germany. We measured structural and climatic conditions at different spatial scales, sampled insect communities in 123 tree holes and bacterial and nematode communities in a subset of these. We found that forest management in terms of harvesting intensity and the proportion of non-natural tree species (species not part of the natural vegetation at the sites) negatively affected tree-hole abundance. An increased proportion of non-natural tree species had a positive direct effect on insect richness and functional diversity in the tree holes. However, a structural equation model showed that increasing management intensity had negative indirect effects on insect abundance and richness, operating via environmental variables at stand and tree-hole scale. Functional diversity and trait composition of insect communities similarly responded to changes in management-related variables. In contrast to insects, bacterial and nematode richness were not directly impacted by forest management but by other environmental variables. Our results suggest that forest management may strongly alter insect communities of tree holes, while nematodes and bacteria seem less affected. Most effects in our study were indirect and negative, indicating that management has often complex consequences for forest communities that should be taken into account in forest management schemes. (C) 2019 Elsevier B.V. All rights reserved.