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

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.