Macroecological Patterns of Tree Species Richness and Functional Diversity Across Nigerian Vegetation Zones

Understanding macroecological patterns of tree species richness and functional diversity across Nigeria's vegetation gradient provides fundamental insights into the processes structuring plant communities at continental scales, yet this question has not been examined at the national scale for Nigeria. This study analysed macroecological patterns of tree species richness and functional trait diversity across Nigeria's vegetation zones, from mangrove and rainforest in the south to Sahel in the north, using a compiled dataset of vegetation plot records from the literature and national herbarium collections (n = 340 plots, distributed across 7 vegetation zones). Functional trait data (maximum plant height, specific leaf area, wood density, leaf nitrogen content, and seed mass) were collated from the TRY plant trait database supplemented with original measurements from Nigerian herbarium specimens. Tree species richness patterns were modelled using generalised linear models with climatic predictors (mean annual rainfall, temperature seasonality, and aridity index). Functional diversity metrics (functional richness, functional evenness, and functional divergence) were calculated for each vegetation zone. Tree species richness showed a significant positive relationship with mean annual rainfall (beta = 0.42, p < 0.001) and a negative relationship with aridity index (beta = -0.37, p < 0.001). Functional diversity peaked in the Guinea savanna transition zone rather than the species-richest rainforest zone, suggesting high functional redundancy in the latter. Environmental filtering dominated community assembly in dry vegetation zones, while competitive dynamics were inferred in wet zones. The study provides the first macro-scale quantitative analysis of tree functional diversity patterns across Nigerian vegetation zones. Keywords: macroecology, tree species richness, functional diversity, vegetation zones, Nigeria