Scale-dependent biodiversity-biomass relationships vary among subtropical forest community types

Exploring how biodiversity influences forest biomass is critical to biodiversity-ecosystem functioning (BEF) research. However, the combined effects of species diversity and compositional heterogeneity among communities and spatial scale on aboveground biomass (AGB) remain insufficiently resolved. We quantified species alpha diversity, beta diversity, and AGB in 15 one-hectare permanent forest plots representing five forest community types across three spatial scales (10 m, 20 m, 50 m) in eastern China, then analyzed the relationships between diversity and AGB. The results showed that positive relationships between alpha diversity and aboveground biomass were consistently observed in evergreen broad-leaved forests and low-elevation evergreen and deciduous broad-leaved mixed forests, and these relationships generally became stronger with increasing spatial scale in these forest types. In contrast, high-elevation mixed forests exhibited weak or negative associations between alpha diversity and biomass. For beta diversity, significant positive associations with biomass differences were detected at the 10 m scale across all forest communities. At 20 m scale, these relationships remained predominantly positive but were less uniform among community types, with some variation in direction and strength. At 50 m scale, the relationships weakened and became more variable among communities, showing a mixture of positive, negative, and non-significant patterns depending on forest type. Collectively, our findings highlight that incorporating spatial scale and forest type is essential for interpreting biodiversity-biomass relationships, and conserving both local species diversity and compositional heterogeneity is very important for sustaining forest productivity, carbon storage, and ecosystem stability.