<p>Disturbance affects ecosystem composition and structure, influencing biomass accumulation and carbon regulation. However, limited research in West Africa has explored the role of species diversity and structural attributes in determining aboveground biomass (AGB), particularly the contributions of dominant and non-dominant species in forest and savannah ecosystems. This study examines how species richness, basal area, and tree density of each species category affect AGB accumulation across five vegetation types: semi-deciduous forest, swamp forest, gallery forest, woodland, and savannah. Data were collected from 186 plots (45.15&#xa0;ha) through a forest inventory. Biomass was estimated using species- and site-specific allometric models and analysed by vegetation type, tree category, and diameter class. Across all vegetation types, species richness and tree density decreased with increasing diameter class, regardless of species category. Dominant species generally had higher relative contribution to species richness than non-dominant species. Mean AGB peaked in the medium diameter class, reaching 208.98, 123.52, and 85.78&#xa0;t&#xa0;ha⁻¹ in semi-deciduous, swamp, and gallery forests, respectively, while woodland and savannah had the highest AGB in the smallest diameter class (39.98 and 34.77 t ha⁻¹, respectively). Large dominant trees significantly contributed to biomass accumulation in semi-deciduous (143.67 t ha⁻¹) and swamp forests (39 t ha⁻¹) but were negligible or absent in other vegetation types. Across all ecosystems, dominant species accounted for 60–90% of total AGB. Basal area and density of dominant species were the primary drivers of AGB, with species richness having a more variable effect (<i>p</i> &lt; 0.000, adjusted R² between 68.85 and 89.44%). Effective strategies should consider ecosystem-specific drivers of biomass accumulation, prioritising the protection of dominant species in forest ecosystems while ensuring tree composition and structure stability.</p>