Moderate ground surface heterogeneity sustains higher aboveground biomass by enhancing structural diversity in fragmented forests

Forests are key components of global ecosystems, regulating nutrient cycling and energy flow. In karst landscapes, pronounced ground surface heterogeneity generates highly uneven resource distributions, thereby shaping forest community structure and biomass accumulation. Although multiple dimensions of biodiversity are known to influence aboveground biomass (AGB), how ground surface heterogeneity mediates biodiversity and AGB across fine spatial scales in fragmented forests remains unclear. In this study, we quantified ground surface heterogeneity using an entropy-based index across 212 subplots nested within 53 plots in a subtropical karst forest. By integrating multivariate biodiversity metrics and community-weighted traits, we evaluated the pathways through which ground surface heterogeneity regulates biodiversity and AGB. Our analyses revealed that structural diversity was the strongest predictor of AGB, whereas functional composition also contributed positively, suggesting that both selection and complementarity effects were involved. Moreover, at the plot scale, biodiversity and AGB exhibited unimodal responses to ground surface heterogeneity, indicating that AGB peaked at intermediate levels of heterogeneity, potentially reflecting enhanced structural differentiation and resource complementarity. Taken together, these findings demonstrate a clear hierarchical pathway that ground surface heterogeneity drives variations in biodiversity, which subsequently translates into enhanced stand structural diversity, ultimately maintaining higher AGB. This study underscores the central role of moderate ground surface heterogeneity in sustaining forest structural diversity and biomass accumulation, highlighting the importance of integrating habitat variability and structural metrics into forest management and restoration strategies in fragmented ecosystems.