Microclimate and habitat heterogeneity as the major drivers of beetle diversity in dead wood.
Resource availability and habitat heterogeneity are principle drivers of biodiversity, but their individual roles often remain unclear since both factors are usually correlated. The biodiversity of species dependent on dead wood could be driven by either resource availability represented by dead-wood amount or habitat heterogeneity characterized by dead-wood diversity or both. Understanding their roles is crucial for improving evidence-based conservation strategies for saproxylic species in managed forests. To disentangle the effects of dead-wood amount and dead-wood diversity on biodiversity relative to canopy openness (microclimate), we experimentally exposed different amounts of logs and branches of two different tree species representing a gradient of dead-wood diversity in 190 sunny and shady forest plots. During the 3 years after exposing dead wood, we sampled saproxylic beetles, which are together with fungi the most diverse and important taxonomic group involved in decomposition of wood. The composition of saproxylic beetle assemblages differed clearly between shady and sunny forest plots, with higher richness in sunny plots. Both dead-wood amount and dead-wood diversity positively and independently affected species richness of saproxylic beetles, but these effects were mediated by canopy openness. In sunny forest, species richness increased with increasing amount of dead wood, whereas in shady forest, dead-wood diversity was the prevailing factor. The stepwise analysis of abundance and species richness, however, indicated that effects of both factors supported only the habitat-heterogeneity hypothesis, as the positive effect of high amounts of dead wood could be explained by cryptic variability of dead-wood quality within single objects. Synthesis and applications. As canopy openness and habitat heterogeneity seem to be the major drivers of saproxylic beetle diversity in temperate forests, we recommend that managers aim to increase the heterogeneity of dead-wood substrates under both sunny and shady forest conditions. Intentional opening of the canopy should be considered in anthropogenically homogenized, dense forests. Specifically in temperate mixed montane forests, dead wood should be provided in the form of large logs in sunny habitats and a high diversity of different dead-wood substrates should be retained or created in shady forests.