Habitat suitability dynamics of the Ethiopian endemic multipurpose Millettia ferruginea subspecies under current and future climate change scenarios

Millettia ferruginea (Hochst.) Hochst. ex Baker, is a fast-growing, multipurpose tree species endemic to Ethiopia, belonging to the Fabaceae family, comprising two subspecies: darassana and ferruginea. While the darassana subspecies is commonly found in southern Ethiopia, ferruginea mainly occurs in northern Ethiopia. Although M. ferruginea is an iconic agroforestry shade tree in Ethiopia’s coffee-growing landscapes, the impacts of climate change on its current and future ecological niche dynamics remain poorly understood. Hence, this study assessed the current and future suitable habitats of the two subspecies of Millettia ferruginea using ensemble species distribution models for the years 2050 and 2070 under SSP4.5 and SSP8.5 climate scenarios using outputs from the HadGEM3-GC31-LL global climate model. For this purpose, 70 and 60 geo-referenced occurrence points were used to model the suitable habitat distributions of darassana and ferruginea subspecies, respectively, based on 13 environmental variables with 10 replications. The results showed robust model performance (AUC ≥ 0.88; TSS ≥ 0.69). Key predictors for darassana included soil pH (0–5 cm), precipitation in the warmest quarter (Bio18), and precipitation seasonality (Bio15), while ferruginea was most influenced by mean diurnal temperature range (Bio2), the maximum temperature of the warmest month (Bio5), soil pH at a depth of 0–5 cm. Under the current climatic conditions, 21.81% of Ethiopia is suitable for darassana and 14.95% for ferruginea, with most areas classified as less suitable for both subspecies. Future projections indicate a significant reduction in the suitable geographical habitats for both subspecies. The currently overlapping habitats (i.e., ~ 64,000 km2) are expected to shrink, on average, to <21,000 km2 under future climate scenarios, raising significant concerns about hybridization and genetic erosion risks. These findings highlight the vulnerability of M. ferruginea to climate change and underscore the urgency of conservation measures, including both in situ and ex situ strategies, to safeguard its ecological, cultural, and economic roles.