Environmental filtering and species diversity shape the vertical aboveground biomass distribution in subtropical natural oak forests

The vertical distribution of aboveground biomass (AGB) across vegetation layers reflects the resource allocation strategies and structural resilience of forest ecosystems. However, how vertical AGB distribution in natural forests is jointly shaped by the site environment and internal community attributes remains unclear. Using data from 51 subtropical natural oak forests, we investigated the associations of site environment and community attributes with AGB distribution across the tree, shrub, and herb layers. Centered log-ratio (CLR) transformation was used to address the compositional nature of layer-specific AGB shares. Results showed that gravel content was the most consistent factor associated with vertical AGB distribution, with a contrasting tree–herb pattern. Higher gravel content was linked to reduced tree-layer dominance but to increased herb-layer contribution. In contrast, shrub-layer AGB share showed weaker relationships with the measured predictors. Species diversity PC1 and mingling degree were mainly linked to tree-layer dominance, whereas canopy closure showed limited independent associations with layer-specific AGB shares. In addition, stand age was linked to vertical AGB distribution mainly through its positive association with Species diversity PC1. These findings suggest that vertical AGB distribution in subtropical oak forests reflects the combined influence of environmental filtering, stand development, and community diversity. The study highlights the value of a layer-specific perspective for revealing vertical biomass organization that cannot be inferred from total AGB alone.