Interacting effects of forest stratum, edge and tree diversity on beetles.

Edge effects are an important component of forest fragmentation, altering microclimatic conditions and species composition within forest remnants. Yet, major factors affecting strength and extent to which edge effects might penetrate into fragments have remained elusive. Here, we study for the first time how tree diversity and forest stratum alter edge effects and how these factors affect beetle communities.We sampled beetles over 7 months using 92 flight interception traps in the canopy and near the ground in the Hainich National Park; Germany's largest connected deciduous forest. Traps were exposed along 10 transects (0-500 m) from the forest edge into the forest interior, comparing transects with high or low abundance of beech (low or high tree diversity).Tree diversity had no influence on the range or strength of edge effects. In the understory, edge effects extended up to maximal transect length of 500. m into the forest interior. Edge effects were weaker in the canopy than in the understory, likely because of higher, edge-like microclimatic variability and harshness in the canopy. The edge response of beetle species richness was driven by habitat generalists while forest and saproxylic species responded less strongly.The richness of forest and saproxylic beetles peaked in the canopy, whereas habitat generalists and non-saproxylic beetles strongly dominated the understory. Pathways driving beetle species richness differed across forest strata. Structural equation modelling showed that tree diversity (+, positive effect) and overall dead wood volume (+) were the most important factors driving beetle species richness in the canopy. In contrast, tree diversity effects (+) were less strong and canopy openness (+) and distance from the forest edge (-) were more important in the understory.