Biogeographic Determinants of Tree Species Distribution and Endemism in Nigerian Montane Forests

Nigerian montane forests, concentrated on the Obudu Plateau, Mambilla Plateau, and the highlands of the Cameroon-Nigeria border zone, harbour disproportionate levels of plant endemism relative to their limited area, yet the biogeographic processes and environmental determinants governing this endemism have not been systematically investigated, limiting conservation prioritisation decisions. This study investigates the biogeographic determinants of tree species distribution patterns and endemism in Nigerian montane forests, testing hypotheses drawn from island biogeography theory, climate niche conservatism, and Pleistocene refugia theory. A comprehensive tree inventory was conducted across thirty-two systematically located plots in montane forests of Taraba, Cross River, and Adamawa States. Voucher specimens were deposited at the University of Ibadan and Kew Royal Botanic Gardens herbaria for taxonomic verification. Floristic affinity analysis quantified relationships between Nigerian montane flora and Afromontane, Guineo-Congolian, and Saharo-Sindian elements. Species distribution modelling under current and Last Glacial Maximum climate reconstructions was used to test the refugia hypothesis. Biogeographic origin analysis confirmed a dominant Afromontane floristic element, with significant admixture from Guineo-Congolian lowland taxa, consistent with forest expansion during Holocene humid phases. Twenty-three tree species were identified as potentially endemic to Nigerian montane forests based on range analyses. Refugia modelling supports the Obudu-Mambilla corridor as a Pleistocene refugium, explaining elevated endemism. The study provides an evidence-based framework for prioritising montane forest parcels for legal protection. Keywords: biogeography, montane forest, endemism, Nigerian highlands, refugia