Linking biodiversity to ecosystem functioning in forest ecosystems: biotic drivers of wood decomposition

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Massive declines in taxonomic diversity due to increasing pressure by land use can impair ecosystem functions. Decomposition of dead wood is a key process in forests with regard to, e.g., carbon and nutrient cycling, but the increasing need for land and forest products has led to decreasing populations of forest species worldwide. The overall process, driven by complex interactions between several decomposer taxa, mainly wood-decaying fungi and saproxylic insects, however, remains still poorly understood and particularly more specific questions such as the role of species richness of decomposers for wood decomposition has not been answered so far. Even for fungi, no clear overall relation between species richness and decomposition could be derived as both positive and negative relations have been observed. A further complicating factor is certain decomposer groups can contribute to wood decomposition directly, e.g. by consumption of wood by saproxylic beetles, but also indirectly via other decomposer groups, as e.g. saproxylic beetles vectoring wood-decaying fungi. To disentangle the role of species richness and functional diversity of saproxylic beetles, a mesocosm experiment has been set up successfully in the Bavarian Forest National Park in 2015, manipulating 190 saproxylic beetle communities spanning independent gradients of species richness and functional diversity (body size and excavating function). The proposed project aims at measuring the contribution of the manipulated beetle fauna to wood decomposition by an innovative method, namely X-ray computed tomography (CT). In contrast to traditional sample-based methods, a complete piece of wood can be scanned non-destructively within seconds and with a resolution sufficiently high to conduct quantitative analyses of wood density that allow detecting even small differences in wood decomposition. In addition, we aim to measure decomposition by wood-decaying fungi (ergosterol and enzyme contents) and to apply next-generation DNA sequencing of fungal communities on saproxylic beetles and inside experimental logs to record precisely the colonization of wood by fungi. By linking decomposition and wood-decaying fungi to characteristics of single beetle species and beetle communities, we will be able to disentangle direct and indirect effects of saproxylic beetles on wood decomposition. In specific, we want to test the following hypothesis: (H1) Functional diversity of saproxylic beetle communities is more important for the speed of wood decomposition than beetle species richness; (H2) The larger the beetle species and the deeper their galleries, the stronger their direct and indirect contribution to decomposition; (H3) The composition of fungal communities vectored by saproxylic beetles is beetle species specific, and (H4) wood decomposition by fungi starts from beetle galleries.