Influence of tree hollow characteristics and forest structure on saproxylic beetle diversity in tree hollows in managed forests in a regional comparison

Tree hollows are among the rarest habitats in today’s Central European
managed forests but are considered key structures for high biodiversity in
forests. To analyze and compare the effects of tree hollow characteristics
and forest structure on diversity of saproxylic beetles in tree hollows in
differently structured managed forests, we examined between 41 and 50 tree
hollows in beech trees in each of three state forest management districts
in Germany. During the two-year study, we collected 283 saproxylic beetle
species (5880 individuals; 22% threatened species), using emergence traps.
At small spatial scales, size of hollow entrance and number of surrounding
microhabitat structures positively influenced beetle diversity, while
stage of wood mould decomposition had a negative influence, across all
three forest districts. We utilized forest inventory data to analyze the
effects of forest structure in radii of 50 to 500 m around tree hollows on
saproxylic beetle diversity in the hollows. At these larger spatial
scales, the three forest management districts differed remarkably
regarding the parameters that influenced saproxylic beetle diversity in
tree hollows. In Ebrach, characterized by mostly deciduous trees, the
amount of dead wood positively influenced beetle diversity. In the mostly
coniferous Fichtelberg forest district, with highly isolated tree hollows,
in contrast, only the proportion of beech trees around the focal tree
hollows showed a positive influence on beetle diversity. In Kelheim,
characterized by mixed forest stands, there were no significant
relationships between forest structure and beetle diversity in tree
hollows. In this study, the same local tree hollow parameters influenced
saproxylic beetle diversity in all three study regions, while parameters
of forest structure at larger spatial scales differed in their importance,
depending on tree-species composition.