Effects of Climate Variability on Savanna Tree Phenology in Northern Nigeria: A Multi-Decade Analysis

Savanna tree phenology is highly sensitive to climate variability, and documenting phenological shifts over multiple decades in Northern Nigeria provides critical evidence for climate change impact assessment and adaptive forest management. This study analysed the effects of climate variability on the leaf flush, flowering, fruiting, and leaf senescence phenology of six savanna tree species (Parkia biglobosa, Vitellaria paradoxa, Daniellia oliveri, Anogeissus leiocarpus, Terminalia avicennioides, and Faidherbia albida) across a 28-year observation period (1995 to 2023) using herbarium specimen collection dates and photographic records from Bayero University Kano and Usmanu Danfodiyo University herbaria, supplemented by 5-year direct field phenological monitoring in Kano, Katsina, and Sokoto States. Phenological event dates were regressed against mean monthly temperature, total annual rainfall, and onset of rains using generalised additive mixed models. Results showed that leaf flush timing advanced by an average of 8.4 days per decade across all six species, consistent with earlier onset of rains in the study period. Flowering was more strongly associated with temperature than rainfall in four of six species. Vitellaria paradoxa showed the largest phenological shift, with flowering advancing by 14 days over the study period. Fruiting season duration shortened significantly in Parkia biglobosa and Daniellia oliveri, potentially disrupting pollinator and seed disperser interactions. The study concludes that climate-induced phenological shifts in Northern Nigerian savanna trees are significant and recommends their integration into climate vulnerability assessments for major non-timber forest products in the region.

Keywords: savanna phenology, climate variability, Northern Nigeria, leaf flush, non-timber forest products