Unravelling Drivers of Forest Biodiversity: Contrasting Effects of Mean Environmental Conditions, Environmental Heterogeneity and Landscape Context

1. Understanding how biodiversity varies under different environmental conditions is one of the central aims of ecology. Mean environmental conditions and heterogeneity have an effect on biodiversity. Increased heterogeneity is generally associated with increased diversity, but mean conditions tend to have a stronger influence. Conditions on site are embedded into a landscape context, which adds another layer of complexity to be considered. Due to the rarity of multi-taxon data it remains unclear if resulting patterns are similar across taxa. Most European forests are managed, and management strongly influences both mean conditions and heterogeneity. How different species groups respond to variation in these forest characteristics is therefore crucial for testing ecological theories and designing effective conservation measures for management. 2. We assessed the effects of environmental conditions on biodiversity of seven taxonomic groups in a temperate mountain forest area in Central Europe. We analysed the responses of biodiversity (species richness, Shannon diversity, and $\beta$-diversity) to three groups of environmental variables: local mean conditions, local heterogeneity, and landscape. Our objectives were to determine which group of variables is most important in explaining biodiversity variation, and whether certain environmental conditions have consistent effects on the diversity of multiple taxonomic groups. 3. We found that the effects of environmental conditions varied substantially between taxa and aspects of biodiversity. The proportion of conifers had the largest number of significant effects overall, but the direction varied between taxa. None of the three groups of variables was more relevant in explaining the variation in biodiversity. While an increase in local heterogeneity and higher values in the landscape context were associated with increased diversity, an increase in mean conditions was mainly negatively associated with diversity. 4. Synthesis and applications Our results show that no single factor or group of factors affects biodiversity across different species groups in the same way. For management, this means that stand-level interventions, such as retention of old-growth elements, are likely not sufficient to promote the many aspects of biodiversity. Instead, different types of management are likely needed for objective-specific biodiversity conservation at the landscape scale.