Unraveling niche complementarity and mass ratio hypotheses along Amazon forest succession: Functional composition a key factor for restoration

Abstract

Understanding the relationship between biodiversity and aboveground biomass in tropical forests is a matter of global interest to mitigate climate change. Therefore, the niche complementarity (NCH) and mass ratio (MRH) hypotheses have been suggested to explain the simultaneous influence of functional trait diversity and composition on ecosystem functioning. The NCH emphasizes resource complementarity among multiple functional traits, while the MRH highlights the dominant role of traits from the most abundant species in driving ecosystem functions. We hypothesize that aboveground biomass increases with stand age, soil nutrients, and soil textural properties and that variation in aboveground biomass could be explained individually or jointly by NCH and MRH along Amazon forest succession. We evaluated different structural equation models to determine the direct and indirect effects of stand age, soil texture, fertility, and residual effects of functional diversity and community-weighted mean (CWM) trait values (tree diameter, wood density) on aboveground biomass. We collected data using 45 sample plots (20 m × 50 m) established in four old-growth and second-growth forests along a successional gradient from three sites in the northern region of Amazonas State, Venezuela. The MRH model showed that stand age had a strong direct positive effect on aboveground biomass, followed by a positive effect of CWM DBH and soil texture but a negative effect of soil fertility on aboveground biomass. The NCH model showed that stand age had the strongest direct positive effect on aboveground biomass, followed by the positive effects of soil texture and functional divergence and the negative direct effects of soil fertility. The integrative SEM for MRH and NCH showed that stand age had a positive direct effect on aboveground biomass, followed by the positive effect of soil texture and CWM DBH, but a negative direct effect on soil fertility. This emphasizes the need for management practices that promote the growth of larger trees within restored areas. Our research provides practical insights that can be applied in passive a active restoration.