Data for: Hierarchical drivers of understory functional diversity in managed temperate forests: contrasting effects of bedrock, canopy richness, and resource heterogeneity

Environmental heterogeneity is often hypothesized to promote plant diversity by increasing niche availability, yet empirical support from managed temperate forests remains inconsistent, particularly from a functional perspective. We examined how coarse-scale abiotic context (bedrock type), fine-scale resource heterogeneity (light, soil phosphorus, and C:N ratio), and overstory tree species richness relate to understory functional alpha diversity and within-stand functional beta diversity.We analysed vegetation and environmental data from 98 one-hectare forest plots in the southern Black Forest, Germany (434–1334 m a.s.l.), surveying herb-layer composition across 6–8 subplots per plot (192 understory species). Functional diversity was quantified using Rao quadratic entropy based on four plant traits (specific leaf area, vegetative height, seed dry mass, clonal growth form). Linear models related alpha and beta functional diversity to bedrock type, overstory richness, and resource heterogeneity. Functional alpha diversity was higher on base-rich bedrock and showed a positive association with overstory tree species richness, while light and soil resource heterogeneity were not significant predictors. Functional beta diversity, in contrast, increased with light heterogeneity, with other measures showing weak or no associations.Taken together, these results suggest that different environmental factors are associated with distinct components of understory functional diversity. While coarse-scale abiotic context and canopy composition appear to be linked to the breadth of functional strategies present at the plot scale, fine-scale variation in light availability may be more closely related to the spatial differentiation of those strategies within stands. The distinct drivers of alpha and beta functional diversity underscore the importance of considering multiple facets of functional diversity when assessing hierarchical drivers of plant diversity in managed forest ecosystems.