The strength of plant trait integration — the degree to which multiple traits co-vary with one another — is key to shape the dimensionality of functional niche space, thus maintaining species and functional diversity in plant communities. However, trait integration differs dramatically in terms of species-mean trait values vs. site-specific trait values. Currently, we still lack an understanding of how intraspecific trait variability, which has been a subject of great interest in recent years, may divert within-site trait covariance from across-site trait covariance and eventually modify the shift of species and trait diversity along environmental gradients. Here, we collected community composition, environmental variables (soil moisture contents, soil bulk density, and soil total carbon), and site-specific trait data (nine leaf and stem traits) across 70 woody plant assemblages in subtropical evergreen broadleaf forests of eastern China. We specifically test how trait integration regulates the distribution of trait diversity and species richness along environmental gradients, and how intraspecific trait variability regulates the multiple relationships between trait integration, trait diversity, and species richness.
Results/Conclusions
The observed strength of trait integration was significantly higher than randomly expected, implying the presence of deterministic processes underlying trait integration across the plant assemblages in the studied region. There was a rather weak positive correspondence between the strength of trait integration calculated from plot-specific trait vs. species mean trait values. The strength of trait integration increased significantly with increasing degrees of environmental stress, and both species richness and trait diversity decrease with increasing strength of trait integration. Importantly, the negative relationship between the strength of trait integration and trait diversity is evident only if intraspecific trait variability is accounted for, because trait covariance structure is poorly retained in species mean trait values. These results suggest that trait variance-covariance structure in individual communities is poorly retained while averaging trait values across communities. Consequently, it is crucial to consider intraspecific trait variability in building up the causal relationship between trait integration and trait diversity. Overall, our study provides evidence for the strong role of intraspecific trait variability and trait covariance structure in regulating the biodiversity distribution along environmental gradients.