Multiscale Drivers of Tree Diversity Patterns in the Nigerian Forest-Savanna Mosaic: Disentangling Climate, Soil, and Disturbance Effects

The forest-savanna mosaic of Nigeria represents a globally significant biome boundary where tree diversity patterns are shaped by interacting climatic, edaphic, and disturbance processes at multiple spatial scales. Disentangling these drivers remains a fundamental unresolved challenge with direct implications for conservation prioritisation and ecosystem resilience projections under climate change. This study applies a multiscale analytical design to characterise tree diversity patterns across 180 plots distributed systematically along two cross-boundary transects spanning Oyo State in the south to Niger State in the north. Plot-level diversity metrics were modelled against variables representing macroclimatic gradients, soil physicochemical properties, disturbance history reconstructed from aerial photograph chronosequences, and landscape context metrics. Hierarchical models accounting for spatial autocorrelation were used to estimate scale-dependent variance fractions attributable to each driver category. Beta diversity partitioning distinguished replacement from richness-loss components of compositional change across the mosaic. Results show that the primary drivers of alpha diversity shift with scale: at macroscales, rainfall seasonality dominates, while at local scales, soil cation exchange capacity and disturbance legacy become equally important. Beta diversity across the boundary was driven primarily by species replacement rather than richness loss, indicating genuine compositional differentiation rather than nested impoverishment. Fire history interacted significantly with soil type in mediating tree diversity, with fire effects more pronounced on sandy substrates. The study generates a mechanistically grounded, scale-explicit model of tree diversity determinants in the Nigerian forest-savanna mosaic, providing a theoretical foundation for biodiversity monitoring design and forest-savanna boundary conservation planning. Keywords: tree diversity, forest-savanna mosaic, multiscale analysis, Nigeria, drivers of diversity.