1. Plant functional traits are thought to drive biomass production and biogeochemical cycling in tropical forests, but it remains unclear how nitrogen (N)-fixing legumes influence the functional traits of neighboring trees and forest-wide biomass dynamics. Further, the degree to which effects of N-fixers are density-dependent and may depend on stem size and spatial scale remain largely unknown.

2. Here, we examine 30-years of stem demography data for ~20,000 trees in a lowland tropical forest in Trinidad that span a wide range of functional traits thought to drive aboveground biomass (AGB) dynamics.

3. These forests show positive but decreasing long-term net AGB accumulation resulting from constant average productivity but increasing mortality of non-fixing trees over time. We find that high abundance of N-fixing trees is associated with compositional shifts in non-fixer functional traits that confer lower competitive performance and biomass accumulation. Across tree size classes, most interactions between N-fixers and non-fixers were negative, density dependent, and strongest at smaller spatial scales.

4. Synthesis. Overall, our findings suggest that local trait-based interactions between N-fixing and non-fixing trees can influence long-term carbon accumulation in tropical forests.