Both the selection and complementarity effects underpin the effect of structural diversity on aboveground biomass in tropical forests

Abstract Aim Despite mounting empirical evidence regarding the positive effects of forest structural diversity (STR DIV ) on forest functioning, the underlying biotic mechanisms and controlling abiotic factors remain poorly understood. This study provides the first assessment of the interactive effects of STR DIV and diversity in species and functional traits on aboveground biomass (AGB) in natural forests in West and East Africa. Location West and East Africa. Time period 2014–2020. Major taxa studied Woody plants. Methods Using data from 276 plots and 7993 trees of 207 species distributed across various types of natural forests and major climatic zones of Africa, linear mixed‐effects and structural equation models, we have evaluated how alternative causal relationships between STR DIV and taxonomic and functional diversity attributes influence AGB, while accounting for the effects of environmental covariates. We also assessed the consistency of these relationships across floristically and environmentally homogenous forest types. Results We found that the positive effects of STR DIV on AGB were underpinned by both the community‐weighted mean (CWM) of trait values (selection effects) and species richness (niche complementarity), but the relative importance of these effects varied depending on forest types. Across the forest types, STR DIV primarily mediated the effects of CWM of traits and species richness on AGB. We also found that STR DIV –AGB relationships were constrained by resource (water and nutrient) availability. Main conclusions Our findings provide novel insights into the role of functional traits as key determinants of the effects of STR DIV on AGB in tropical forests. We suggest that forest management and climate change mitigation strategies aimed at conserving biodiversity, and fostering biomass storage through increased STR DIV should focus on maintaining high levels of functionally dominant species while also increasing tree species diversity.