Species composition of beetles grouped by host association in hollow oaks reveals management-relevant patterns

Veteran trees are keystone structures currently in decline worldwide. In Europe, veteran oaks (Quercus spp.) are important habitat trees for wood-living beetles. Still, we have limited knowledge of the drivers determining the composition of these communities and the relevant spatial and temporal scales for these drivers. We collected beetles from 32 hollow oaks in two regions in Southern Norway along a coast-inland gradient paralleling historical onset of oak harvesting. We focused on species with different host-association to oak and identified the relative importance of processes working on different spatial scales, ranging from tree-scale to region-scale, as well as effects of the coast-inland gradient. We found all the spatial scales to influence the species composition, with a response to the coast-inland gradient and differences between the regions for all beetle groups. The tree scale was the most important scale for species mainly associated with oak, generalist species and the overall species composition-with bryophyte cover on the stem, depth of bark crevices and amount of wood mould as the most important variables. The close surroundings and forest variables on the landscape scale was most important for the beetle species restricted to broadleaf trees. Our study confirmed established knowledge, but also revealed new relationships between oak-associated beetles and relevant drivers on the tree and regional scale. As important responses of species mainly associated with oak or broadleaf trees were concealed when analyzing the total community, care should be taken not to base management recommendations on such overall, general patterns.