Authors: Lindman, Ly; Öckinger, Erik;Ranius, Thomas;

While climate warming has sparked the studies of microclimatic conditions in many habitats, knowledge on the role of microclimate for insects inhabiting hollow trees, the severely decreasing habitat type, is still lacking. We explored relationships between tree characteristics and microclimate in tree hollows, and how these affect the occurrence and body size of an endangered beetle Osmoderma eremita.
We placed data-loggers in wood mould and surveyed O. eremita with mark-recapture in south-eastern Sweden. Tree diameter and entrance height were especially important for the microclimate in tree hollows, while wood mould volume, direction and angle of the entrance were also included in several models. Trees with larger diameter were warmer in autumn and winter but colder in summer with lower temperature fluctuations in summer. Hollows with higher entrances were warmer in spring and summer, but colder in autumn and winter.
O. eremita occurred more frequently in hollows situated higher from the ground with warmer and stable microclimate. Beetles were larger in trees with south-facing entrances with colder and had more stable microclimate. In summary, the occurrence and body size of O. eremita were strongly correlated with microclimatic conditions, which may be especially important near the northern range margin of the species.