Management and tree dieback alter beetle assemblage structure and body size distribution in a temperate German forest

Changes in forest management and climate affect forest ecosystems, leading to changes in their structural characteristics. This subsequently affects the structure of forest beetle assemblages (Coleoptera). The underlying mechanisms of structural changes in an assemblage can be disentangled by investigating assemblage composition, diversity, species abundance distribution and body size distributions. However, information on how beetle assemblage structure and body size distributions are affected by combined effects of forest management and tree dieback is lacking. We assessed structural characteristics in 50 forest plots in Darmstadt, Germany, where forest management and tree dieback affected canopy openness, deadwood volume, the proportion of non-native trees, mean tree size, tree basal area and tree diversity. We analysed beetle assemblage structure in response to changes in forest characteristics using composition, abundance, diversity and three parameters describing species abundance distributions: rate of decline, the proportion of the most abundant species and species richness. Body size distributions calculated based on species presence/absence and all beetle individuals (both literature-based species mean body sizes) were examined separately to detect shifts in assemblage body size structure. Beetle assemblage composition was affected by all forest characteristics except for tree diversity. In forests with a higher canopy openness, we found less even abundance distributions with fewer but more dominant beetle species. Forest characterised by a lower mean tree size and a higher proportion of non-native trees showed a higher frequency of smaller-sized beetles. Different forest beetle assemblages require different structural characteristics. The dominance of individual beetle species and lower diversity in forests with a more open canopy could be attributed to lower resource diversity and the avoidance of such habitats by some species. Furthermore, forests with a high mean tree size and low proportion of non-native trees favour larger beetle species by providing a higher diversity and larger amounts of essential resources. Overall, the observed responses elucidate the mechanisms underlying changes in beetle assemblage structure in response to forest management and tree dieback.