Stand structural attributes and functional trait composition overrule the effects of functional divergence on aboveground biomass during Amazon forest succession

Abstract

Two competing ecological hypotheses, i.e., the niche complementarity (NCH) and the mass ratio (MRH) hypotheses, have been proposed to explain how stand structural complexity, functional trait diversity and composition simultaneously determine aboveground biomass in natural forests. Here, we hypothesized that the effects of stand structural attributes and functional trait composition overrule the effects of functional diversity on aboveground biomass during tropical forest succession. We tested different linear mixed-effects models to determine the effects of abiotic (i.e. nutrients and soil texture), taxonomic (i.e. woody species richness and composition), stand structural (i.e. stem count and maximum tree diameter), and functional attributes (functional divergence and composition). The functional attributes were based on functional divergence (FDvar) and community-weighted mean (CWM) trait values of wood density (WD). We collected data using 45 sample plots (20 m × 50 m) established in four old-growth and second-growth forests with varied stand stages from three sites in the northern region of Amazonas State, Venezuela. The MRH model showed that CWM-WD had a strong direct positive effect on aboveground biomass, followed by a positive effect of maximum tree diameter, but had a weak positive relationship with soil fertility and FDvar-WD. Our study suggests that low trait diversity and high CWM-WD and maximum tree diameter determine high levels of aboveground biomass, which could be concentrated in species with larger diameters and high wood density. We conclude that stand structural attributes and functional dominance override the effects of FDvar of WD on aboveground biomass, and hence, it is important to test the mutual effects of functional diversity and composition when exploring the effects of functional traits on forest functioning.