Tree species mixing shifts community-level leaf economics trait composition and functional diversity in phosphorus-limited plantations

Understanding how tree species mixing alters community-level functional strategies is critical for managing phosphorus-limited plantations, yet empirical evidence linking species composition to community functional composition remains scarce. Here, we investigated whether increasing tree species diversity shifts the community-weighted position on the leaf economics spectrum (LES) and enhances functional diversity in planted forests. We measured five leaf functional traits (specific leaf area, leaf dry matter content, leaf nitrogen,leaf phosphorus, and N:P ratio) across 54 plots, comprising monocultures, two-species mixtures, and three-species mixtures in the Yellow River Floodplain, China. Community-weighted means, non-metric multidimensional scaling (NMDS), functional dispersion (FDis), and structural equation modeling (SEM) were used to evaluate trait responses to species mixing and environmental gradients. Leaf phosphorus increased from 0.253 in monocultures ( n = 33) to 0.337 in two-species ( n = 15) and 0.467 in three-species mixtures ( n = 6; +84.6%; bootstrap 95% CI = 0.161–0.262). The three-species result should be interpreted with caution given the small sample size and limited species combinations. Functional diversity (FDis) increased from 0 in monocultures to 0.954 in two-species and 1.233 in three-species mixtures ( p < 0.001). NMDS revealed that mixed stands exhibited distinct community functional composition and occupied a significantly broader functional space than pure stands. SEM further demonstrated that the canopy–soil structure gradient (PC2) irectly and negatively associated with leaf phosphorus concentration ( β = −0.693, p < 0.001) and indirectly associated with it via decreased species richness, with the model explaining 67.5% of the variance in LTP. Collectively, our findings confirm that species mixing shifts the community-level leaf economic trait composition—particularly along nutrient-related dimensions—and enhances functional diversity in P-limited plantations, providing robust quantitative evidence that trait-based indicators can effectively guide biodiversity-oriented forest management in nutrient-poor alluvial soils.