Print PageEnvironmental filtering is a primary process shaping community assembly, but the extent to which it drives patterns in species-rich communities remains poorly understood. Analyses of the distributions of functional traits can provide novel insights to this fundamental question. Recent results from tropical forests suggest that allocation strategies are orthogonal among leaf and wood tissues. Here we take this analysis one step further to make predictions for not only the strength of environmental filtering in community assembly but also the direction of filtering; that is, how different plant tissue strategies are sorted among different habitats. In this way, we test the extent to which contrasting tissue strategies contribute to the extraordinarily high tropical plant diversity observed across habitat types, generating functional beta-diversity.
We established 74 0.5-ha plots in white-sand, clay terra firme and seasonally-flooded forests across northern Peru and French Guiana to represent the range of soil fertility, forest structure, and floristic compositions found throughout the Amazon region. We chose 13 plots representing these gradients in each region, in which we measured 17 functional traits describing leaf, stem and root tissues for all species, with 800 tree species measured.
Results/Conclusions
Principal component analysis revealed two orthogonal axes of coordinated traits: a first axis defined by leaf traits, corresponding to a ‘leaf economics spectrum’, and a second axis defined by covarying stem and root traits, corresponding to a ‘wood economics spectrum’. These axes were consistent among sites and when accounting for species evolutionary history with phylogenetically-independent contrasts.
Both univariate and multivariate analyses showed strong environmental filtering among habitat types, with contrasting results between leaf and wood tissues. White sand forests were characterized by tissues with high construction costs including leaves, stems and roots, whereas seasonally-flooded forests tended to have less dense wood and cheaper leaves than terra firme forests. These patterns were consistent between the two regions despite shifts in overall trait distributions between the two regions.
Our results represent an important integration of functional ecology and theoretical investigations of community assembly. In addition, we demonstrate how contrasting plant strategies contribute to high beta-diversity in tropical forests, and we provide a predictive framework for the response of these communities to global change.
