Abstract
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•Dense- and light-wooded species need to be separately considered.•The positive BEF relationships were observed for dense-wooded species only.•BEF relationships were obscured when mixing dense- and light-wooded species.•Species diversity promoted functional divergence across dense- and light-wooded species.•Homegarden agroforests should be included in the REDD+ initiative.
Recent studies have advanced our understanding regarding the niche complementarity and mass ratio effects on the ecosystem function, in both natural and experimental systems. However, biodiversity–ecosystem function (BEF) relationships may be fundamentally different across dense-wooded and light-wooded species, as they are clustered at two opposite extremes of the wood economics spectrum. Here we analyzed BEF relationships through mediations of functional dominance (i.e. community-weighted mean, CWM) and functional divergence (FDvar) of plant maximum height (H) while accounting for the effects of other characteristics of homegardens, across dense-wooded, light-wooded and all species, by using structural equation modelling (SEM) on 45 homegarden agroforestry systems in the dry zone of Sri Lanka. The dense-wooded SEM accounted for 69% variation in aboveground biomass through significant positive direct effects of CWM H (β = 0.51) and FDvar H (β = 0.20), and indirect effect of species diversity via FDvar H on aboveground biomass (β = 0.12). Although the light-wooded SEM accounted for 19% variation in aboveground biomass, the BEF relationships were non-significant. Regardless of SEMs, FDvar H but not CWM H was significantly positively related to species diversity. None of the BEF relationships was statistically significant when dense-wooded and light-wooded species were mixed. From a theoretical aspect, these positive BEF relationships are driven by both dominant and complementarity dense-wooded species. From a practical aspect, the positive BEF relationships support the feasibility of UN Reducing Emissions from Deforestation and Forest Degradation (REDD+) initiative in studied agroforests, which aims to enhance carbon storage in aboveground biomass while conserving biodiversity. Hence, this study suggests that wood density is a potential sustainability indicator for better management of agroforest-ecosystem while driving positive BEF relationships.