Restoring structural complexity in temperate forests increases bat and bird diversity

Effective conservation and restoration rely on understanding how biodiversity responds to environmental change. Centuries of wood production have simplified forest structure and reduced biodiversity. In a large-scale, replicated landscape experiment, we tested how restoring structural complexity through canopy gaps and deadwood increases the diversity of bats and birds, both flying insectivorous vertebrates, across spatial scales. We compared diversity between structurally heterogeneous and homogeneous temperate beech forests across 11 experimental landscapes in Germany. Both taxa were recorded using autonomous acoustic monitoring and automatic species identification. We quantified taxonomic, functional, and phylogenetic diversity within forest patches (α-diversity), among patches (β-diversity), and across the landscape (γ-diversity). Our experimental enhancement of structural complexity increased γ-diversity for both taxa but through distinct mechanisms. In bats, increases in γ-diversity were primarily driven by β-diversity, indicating greater dissimilarity in assemblages among patches, while γ-diversity in birds only increased through local gains in α-diversity. Bat diversity increases were mainly taxonomic, suggesting functional similarity to control assemblages, whereas birds showed the highest gains in functional diversity, indicating greater trait dissimilarity in heterogeneous forests. These differences likely reflect variation in spatial ecology, with bats responding more to spatially distributed heterogeneity within landscapes, while birds benefit from increasing local habitat complexity. Our results provide evidence that γ-diversity can be shaped by different mechanisms, indicating that forest restoration must be taxon specific and scale dependent to effectively enhance biodiversity.