Drivers of biodiversity across Central European forest reserves: reserve size, landscape context and habitat structure

Protected areas play a central role in global forest biodiversity conservation. Improving their effectiveness needs informed site selection and management decisions - guided by robust ecological evidence. A better understanding is therefore needed how factors relevant for site selection affect biodiversity across taxonomic groups and forest types. Especially effects of reserve size and landscape context, as well as local habitat structures which are typically modified by management need to be better understood. To assess how reserve size, landscape forest share and local habitat characteristics – tree species composition, deadwood and tree-related microhabitats – influence biodiversity, we studied 44 protected forest areas of varying sizes across three major Central European forest types (riparian, upland and mountain forests). Within these reserves, we quantified coverage-based standardized diversity of beetles, spiders, birds and soil fungi. Biodiversity increased with reserve size for fungi, beetles and spiders. Increasing landscape-level forest share decreased beetle diversity. Interactive effects of reserve size and forest share were found for birds, indicating that diversity increased with reserve size more strongly in low-forested landscapes. Analysing forest specialists separately, we found no effects of reserve size and landscape-level forest share. The effect of local habitat characteristics varied across species groups and forest types. Beetle diversity was associated with deadwood volume and diversity, spider diversity with conifer share, canopy height variation and deadwood diversity, birds with canopy height variation and fungi with tree-related microhabitats. Overall, reserve size was the most consistent predictor of biodiversity across taxa, whereas landscape context and local habitat characteristics showed taxon- and forest-type-specific relationships. These findings suggest that forest conservation should aim at expanding the area under protection and promote structurally diverse habitats tailored to different taxonomic groups and forest types.