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Both inter‐ and intraspecific trait variation are critical to species distribution along environmental gradients, but our understanding of these patterns predominantly relies upon species‐level trait means and variances. Trait integration, defined as how strongly multiple traits covary with one another, is a key indicator of the dimensionality of functional space for accommodating biodiversity in communities. As trait covariance can differ dramatically at the interspecific versus intraspecific levels, how intraspecific trait variability alters the strength of trait integration and eventually modulates biodiversity along environmental gradients has been rarely tested. Here, we measured nine functional traits (leaf area, specific leaf area, leaf and stem dry‐matter content, leaf nitrogen and phosphorus contents, specific stem length, Huber value and maximum height) paired with site‐specific soil fertility for 70 woody communities in subtropical Chinese forests. All species‐by‐site combinations were sampled to ensure a sufficient representation of intraspecific trait variation across sites. Community‐level trait integration was quantified from the variance of eigenvalues of the trait correlation matrix. The direct and/or indirect effects of soil fertility and trait integration on species richness and trait diversity were assessed through path analyses. Trait integration quantified from both inter‐ and intraspecific variances was on average 21.7% weaker than that from only interspecific variance, indicating a crucial role of intraspecific trait variability in promoting niche dimensionality. Whether accounting for intraspecific variation or not, less fertile sites had stronger trait integration, which in turn depressed both taxonomic and functional diversity, supporting the assumption that higher environmental stress demands stronger tradeoffs among multiple functions in viable strategies. Importantly, the negative association between trait integration and species richness became stronger when accounting for intraspecific variation, suggesting that species distribution and occurrence can be a consequence of intraspecific trait variability. This study highlights the importance of intraspecific trait variability in understanding functional tradeoffs underlying biodiversity patterns. [ABSTRACT FROM AUTHOR]

Leaf economics, hydraulic, and shade-tolerance traits involve different resource-use strategies, and are critical for interactive plants, yet it remains unclear whether co-occurring individuals modulate the coupling relationships between these trait dimensions. To address this knowledge gap, we measured four leaf economics, five hydraulic, and two shade-tolerant traits in leaf and wood tissues across 90 individuals from 26 tree species in two subtropical forests in Eastern China. Confirmatory factor analysis (CFA) and Principal Component Analysis (PCA) were employed to explore multi-trait relationships. With respect to leaf economics traits, leaf nitrogen concentrations were positively associated with leaf stomatal density and conductance, leaf-specific hydraulic conductivity, and twig vessel diameter. However, leaf phosphorus concentrations were negatively associated with leaf-specific hydraulic conductivity. The specific leaf area was negatively associated with the light exposure index, light saturation point, and leaf Huber value. Regarding hydraulic traits, twig vessel diameters were positively correlated with the light saturation point and light exposure index. Stomatal conductance was positively associated with the light saturation point, whereas leaf-specific hydraulic conductivity was positively associated with the light exposure index. The first two PCA axes explained 54.23% of the total trait variation. The first axis of the multi-trait variation was mainly explained by leaf habits and represented the trade-offs between acquisitive hydraulic economics and cheap leaf economics. The second axis of the multi-trait variation was mainly explained by plant growth form, and defined the trade-offs between weak shade tolerance and cheap leaf area construction. In conclusion, our results provide evidence for the coupling relationships between leaf economics, hydraulic, and shade-tolerant traits among co-occurring individuals. This study also highlights that bivariate trait relationships based on large-scale interspecific surveys may be the opposite of that within-community scales. • The coupling relationship among leaf economics, hydraulic and shade-tolerant traits hold at within-community scale. • The locally integrated trait dimensions capture variations in plant growth form and leaf habit. • Trait-trait trade-offs presented at local scale differ from those at the broad-spatial scale. • Evergreen plants with great hydraulic efficiency are particularly conservative in leaf phosphorus concentration. • Plants with a great capacity for light interception tend to construct low specific leaf area. [ABSTRACT FROM AUTHOR]