Branching hierarchy shapes branch-leaf allometry and trait coordination in mountain shrubs of Dongling Mountain, Beijing.

Branch-leaf allometry is fundamental to understanding biomass allocation, leaf deployment, and resource-use strategies in woody plants. However, most studies have focused on trees or terminal twigs, while the role of branching hierarchy in shaping branch-leaf relationships in shrubs remains poorly understood. In this study, we investigated 12 dominant shrub species from Dongling Mountain, Beijing, China, and examined how 11 branch-leaf traits, allometric relationships, and trait coordination varied among architectural life forms, elevational zones, slope aspects, and branch orders. The results showed: except for the total leaf biomass, all other branch-leaf traits showed non-significant phylogenetic signal, and phylogenetic PCA did not reveal clear separation among life forms, elevation zones, or slope aspects, suggesting that trait variation was not primarily structured by phylogenetic relatedness. Branch-leaf traits varied significantly across ecological and architectural groups. With increasing branch order, branch cross-sectional area, branch length, branch volume, total leaf biomass, and branch biomass decreased, whereas mass-based and volume-based leafing intensity increased. Branch-leaf allometric relationships were scale-dependent: relationships that were allometric at the whole-plant level tended to become isometric or near-isometric at the branch level. Individual leaf mass was negatively associated with leafing intensity, supporting a leaf size-number trade-off in shrubs. Plant trait networks further showed that lower-order branches were dominated by simple structural covariation, whereas higher-order branches exhibited stronger coordination among branch architecture, leaf number, and biomass allocation. These findings demonstrate that branching hierarchy is a key organizational level shaping branch-leaf allometry and trait coordination in mountain shrubs. Considering branch order can therefore improve our understanding of shrub resource allocation strategies and architectural adaptation in heterogeneous mountain environments.