Decades of research have helped establish the positive relationships between species richness and ecosystem biomass. However, species richness or related taxonomic metrics do not fully capture the functional differences between species, nor are they sufficient to reflect overall stand complexity. Further, the relative importance of structural diversity, functional trait dominance and diversity for stand biomass is still controversial, especially in multispecies stands with differential intra- and interspecific functional and structural patterns. In this study, we used plot level floristic and functional trait data in mixed-species plantations in West Africa, to examine the relationship between multiple diversity metrics and AGB. Multi-model inference and subset regression analyses were performed to identify the most important diversity metrics for AGB. Structural equation models were also used to evaluate both direct and indirect responses of AGB. Overall, 23 species were enumerated in the mixed species stands, plot level values ranging from 1 to 12 species. AGB was in average 15.5 Mg/plot, and varied from 9.3 to 23.1 Mg/plot. The relationship between taxonomic diversity and AGB varied with specific metric. Among taxonomic diversity metrics, only species richness correlated positively with AGB. Neither the single trait-functional diversity nor the functional dominance played a significant role in predicting AGB. In contrast, functional evenness and structural diversity showed positive effects on AGB, and mediated its positive response to species richness. This work reveals that functional evenness showed a significant advantage in predicting and mediating AGB response to species richness, when compared with structural diversity. It also suggests that niche complementarity plays a crucial role in enhancing AGB in mixed plantations, and operates through functional evenness and structural diversity. Maintaining high structural diversity and functional diversity would contribute to preserve biodiversity and enhance stand biomass stock in mixed species plantations.