Evolutionary Ecology of Phenotypic Plasticity in Native Tree Species in Response to Changing Rainfall Regimes in Northwestern Nigeria

Phenotypic plasticity in native tree species represents a first-order adaptive mechanism mediating short-term responses to climate variability before evolutionary genetic change can occur, yet its ecological limits and evolutionary constraints in dryland tree species native to northwestern Nigeria are poorly characterised. This study investigates the phenotypic plasticity of morphological, physiological, and phenological traits in five native tree species (Vitellaria paradoxa, Faidherbia albida, Acacia senegal, Balanites aegyptiaca, and Ziziphus mauritiana) across a rainfall gradient spanning 400 to 900 millimetres mean annual precipitation in Sokoto, Zamfara, and Niger States. Individuals of each species were sampled at seven points along the gradient, with extensive trait measurement including leaf morphology, stomatal density, specific leaf area, wood anatomy, phenological timing, and xylem vessel dimensions. Common garden experiments with provenance material tested the genetic versus environmental components of observed trait variation. Reaction norm analysis quantified plasticity magnitude and direction for each trait-species combination. Among-population genetic differentiation (QST) was compared to neutral microsatellite differentiation (FST) to infer selection history. Results demonstrate species-specific plasticity architectures, with Vitellaria paradoxa showing the broadest plastic responses in leaf morphology while Faidherbia albida exhibited relatively canalized wood anatomy despite wide rainfall gradient exposure. QST-FST comparisons suggest divergent natural selection on stomatal density and specific leaf area in three of five species, implying local adaptation alongside plasticity. The study develops an evolutionary ecology framework for predicting native tree climate vulnerability in the Sahelian transition zone. Keywords: phenotypic plasticity, climate adaptation, dryland trees, rainfall gradient, northwestern Nigeria