Scale-dependent decoupling of leaf traits and phylogeny reveals dimension-specific environmental filtering in Polyspora chrysandra-associated montane forests

Understanding the mechanisms of community assembly is essential for elucidating biodiversity patterns. Polyspora chrysandra, a native tree that can achieve local dominance in certain montane forests of Southwest China and valued for its high ornamental appeal, provides an ideal model to explore how functional traits, phylogeny, and environment jointly shape associated communities—a linkage that remains underexplored and limits mechanistic understanding of regional biodiversity. We surveyed 16 plots (0.64 ha total) across Yunnan Province, collecting over 4,600 leaf samples from 155 co-occurring woody species in P. chrysandra-associated forests. Using an integrative multi-scale framework, we examined how trait coordination, evolutionary history, and environmental gradients jointly shape community assembly. Specifically, we assessed trait integration across elevation and precipitation gradients at both community and P. chrysandra levels, quantified the relative contributions of environment versus phylogeny to trait variation, and evaluated whether functional and phylogenetic diversity responded to distinct environmental filters. PTNs revealed scale-dependent trait coordination: at the community level, leaf area (LA), leaf length (LL), leaf fresh weight (LFW), leaf dry weight (LDW), and leaf tissue density (LTD) consistently emerged as core traits across elevation and precipitation gradients. In P. chrysandra, however, the composition of core traits varied with the environmental gradient: along elevation, the core trait suite comprised LA, LL, LW, LDW, and LTD, whereas along the precipitation gradient it comprised LA, LL, LW, LFW, and LTD. Functional richness (the volume of trait space filled by species in the community) increased with phylogenetic diversity, while functional dispersion (the mean distance of species to the community centroid in trait space) declined as communities became more phylogenetically clustered, indicating partial decoupling of functional and phylogenetic assembly processes. Most leaf traits showed weak but significant phylogenetic signals. After accounting for phylogeny, only leaf thickness (LT) responded significantly to the environment—increasing with soil pH, vapor pressure deficit (VPD), and available potassium (AK), but decreasing with soil gravel content. In contrast, at the community scale, canopy openness (CO) strongly predicted community-weighted mean LTD (CWM.LTD, adj R² = 0.81), highlighting light availability as a key environmental filter. Nonetheless, measured environmental variables explained little variation in overall functional or phylogenetic diversity. Assembly of P. chrysandra-associated communities exhibits patterns consistent with dimension-specific environmental filtering: CO, soil properties (SPs), and VPD selectively shape functional and phylogenetic structure, alongside intraspecific trait reorganization in P. chrysandra and a strong phylogenetic signal in LTD—the latter indicating evolutionary conservatism in this trait. Despite widespread trait–phylogeny decoupling, the conserved nature of LTD alongside context-dependent intraspecific reorganization highlights the interplay between environmental responsiveness and evolutionary conservatism. This work advances our understanding of montane forest assembly in Southwest China and suggests that Polyspora, as a phylogenetically distinctive genus with high ornamental value, warrants further investigation as a model for trait evolution, given its context-dependent intraspecific trait reorganization.