Disentangling the Relative Roles of Stochastic and Deterministic Processes in Structuring Tropical Forest Tree Communities in South East Nigeria

A central unresolved question in tropical forest ecology concerns the relative importance of stochastic neutral processes versus deterministic niche-based processes in assembling local tree communities. This study disentangles the relative roles of these processes in structuring tropical forest tree communities across twenty-four forest sites in South East Nigeria, spanning primary, secondary, and fragmented forest types in Cross River and Akwa Ibom States. Comprehensive tree inventories were conducted in 0.5-hectare plots at each site, recording all stems above five centimetres diameter at breast height to species level. Phylogenetic community structure analysis, trait-based assembly tests, and null model comparisons using standardised effect sizes were applied to assess community phylogenetic and trait dispersion relative to null expectations. Regional species pool characterisation and dispersal limitation tests using abundance-weighted beta-null deviation metrics distinguished stochastic dispersal limitation from ecological drift. Variation partitioning with measured environmental variables further decomposed deterministic environmental filtering effects. Results show strong evidence for deterministic environmental filtering as the primary structuring force in primary forest communities, with soil nutrient gradients and topographic moisture explaining 42 percent of compositional variation. In fragmented forest sites, stochastic processes including ecological drift and dispersal limitation gained proportionally greater importance, consistent with reduced immigration balancing local extinction. Secondary forests showed intermediate patterns reflecting mixed assembly dynamics during succession. The study provides the most rigorous test to date of assembly process dominance in Nigerian tropical forests and has direct implications for restoration prioritisation, arguing that functional trait-based planting is most critical in highly fragmented landscapes. Keywords: community assembly, stochastic processes, tropical forest, South East Nigeria, niche theory.