LAUSR

Abstract The effect of tree species on understory vegetation, herbaceous biomass and soil nutrients were studied in a semi-arid savanna of Ethiopia. Twenty large trees, from each of the species: Acacia robusta, Ziziphus spina- christi, and Balanites aegyptiaca , a total of 60 trees, and 480 samples were used for measuring understory vegetation, herbaceous biomass, and soil nutrients during the study. The inside tree canopies had a higher speciesdiversityand plant abundance than the outside tree canopies. Acacia robusta had a higher number of species and plant abundance in the understory vegetation compared to other tree species. The biomass yield of herbaceous vegetation under the inside canopies of A. robusta was higher than the canopies of other tree species. Similarly, most soil nutrient contents were higher under A. robusta than other tree species, and the inside canopies had a higher soil nutrient contents compared to outside tree canopies. Hence, the presence of larger trees in semi-arid African savannas confirmed to maintain more species composition and diversity of understory vegetation, higher herbaceous biomass and improved soil nutrients. Therefore, conservation of larger tree species is crucial for proper utilization and ecological stability of semi-arid African savannas under the changing climate and global warming.