Trait dominance outweighs functional diversity in explaining carbon storage in tropical tree communities across environmental gradients

Abstract It is imperative to comprehend the mechanisms that regulate aboveground carbon stock (AGC) in tropical forests, as this knowledge is fundamental for predicting ecosystem responses to environmental change. Two non-mutually exclusive mechanisms have been proposed to explain how community functional composition influences carbon storage: the niche complementarity hypothesis (NCH) emphasizes functional diversity, and the mass-ratio hypothesis (MRH) highlights the role of dominant trait values. However, the simultaneous testing of these mechanisms under integrated climatic–edaphic gradients across contrasting tropical forests has rarely been undertaken. Using piecewise structural equation modeling with bootstrap confidence intervals for indirect effects, we disentangled the direct and indirect pathways linking an integrated environmental gradient, functional diversity, functional dominance, and AGC across dry and wet tropical forests in northeastern Brazil. We quantified community-weighted mean (CWM) traits and multiple functional diversity (FD) metrics from field-measured leaf area, leaf dry mass, specific leaf area, wood density, and tree height and evaluated competing causal models representing the NCH, MRH, and a joint framework. We found strong support for the MRH. The environmental gradient exerted an indirect influence on AGC through community-weighted mean tree height, thereby fully mediating the relationship between environmental conditions and AGC. Conversely, FD metrics exhibited no significant effects on AGC in either the independent or joint models. The results obtained demonstrate the importance of integrated climatic-edaphic gradients and suggest that environmental filtering exerts a primary influence on AGC by favoring communities dominated by tall-statured species, thus providing a trait-based framework for understanding and managing carbon dynamics in tropical forests.