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 (STRDIV) 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 STRDIV 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 STRDIV 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 STRDIV 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, STRDIV primarily mediated the effects of CWM of traits and species richness on AGB. We also found that STRDIV–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 STRDIV 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 STRDIV should focus on maintaining high levels of functionally dominant species while also increasing tree species diversity.

Methods
The dataset results from forest inventories conducted in 276 plots, distributed across four West African countries (Benin, Burkina Faso, Togo, and Ghana) and one East African country (Ethiopia) from 5°40ʹ W to 48°23 ʹE longitude and 3°18ʹ N to 15°03ʹ N latitude. The plots spanned various climate types of the region, including arid, semi-arid, dry sub-humid, and humid, and covered a broad range of topographic and edaphic gradients. The forest inventories were conducted between 2014 and 2020. The plot size was on average 0.1 ha and ranged from 0.02  to 0.25 ha. Within each plot, several key dendrometric parameters including height and diameter at breast height (DBH) were systematically recorded for all living individual trees that met specific criteria. Environmental factors were also either recorded in the field or downloaded from publicly available databses.