Functional identity regulates aboveground biomass better than trait diversity along abiotic conditions in global forest metacommunities

Abstract

Although several studies have identified the effects of functional trait diversity (FTD) and/or identity, i.e. the community-weighted mean (CWM) of a trait, on aboveground biomass (AGB) along abiotic conditions, these effects on AGB in global forest metacommunities are still largely unexplored. Here, we modelled the effects of abiotic (i.e. climate, soil and plot physical conditions) and biotic [i.e. FTD, CWM of conservative traits (CWMCT), CWM of acquisitive traits (CWMAT) and functional dominance (FunDom; based on CWM of plant maximum height or diameter)] factors on AGB in 76 forest metacommunities (from 24 studies). Using multiple linear regression models and piecewise structural equation modeling (pSEM), we tested the hypothesis that both abiotic and biotic factors regulate AGB but that the mass ratio mechanism underpins AGB of metacommunities in global forests better than the niche complementarity mechanism. We found that abiotic and biotic factors contributed 45.39% and 54.07%, respectively, to the explained variance in AGB (R2 = 0.59), and as such, abiotic factors shaped FTD (R2 = 0.42–0.48), CWMCT (R2 = 0.33–0.36), CWMAT (R2 = 0.27–0.33) and FunDom (R2 = 0.59–0.61) through divergent effect sizes and directions. The final best-fitted pSEM showed that FunDom increased (β = 0.49) but CWMCT (β = −0.35) and CWMAT (β = −0.11) decreased AGB (R2 = 0.52) as compared to the negligible effect of FTD (β = 0.04). This study supports the mass ratio effect, specifically the overruling role of tall-stature or dominant trees on AGB, at a macroecological scale, and hence, suggests that a suitable species' functional strategy is important to promote carbon sequestration in forest metacommunities that underpins human well-being. We expect that our study will advance the field of biodiversity–ecosystem functioning at a macroecological scale by using the metacommunity concept and approach.