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(I) The effects of isolated, mature trees of Acacia tortilis and Adansonia digitata on their environments in a semi-arid savanna in Tsavo National Park (West), Kenya., were investigated by studying herbaceous-layer composition and productivity, site microclimate, and soil fertility under the canopy and in the rooting zones of individual trees and in the surrounding grasslands. Herbaceous-layer productivity was determined by clipping shoots monthly and by extracting roots from soil cores. Chemical and physical properties of soils were analysed by standard techniques. Plant species composition and soil nutrient data were ordinated by detrended correspondence analysis and principal components analysis, respectively. (2) Compared to open grassland, tree canopies. of both A. tortilis and A. digitala reduced solar irradiance by 45-65%, soil temperatures by 5-11 °C, and rainfall by 0-50%. Soil water content was higher in the open grass! and immediately after the start of the two rainy seasons, higher tinder tree canopies during the first rainy season, but was equal in the two areas during the second rainy season. (3) Patterns of herbaceous-layer composition and above-ground net primary productivity (ANPP) under and near trees of A. tortillis and A. digitata were similar, with significantly greater ANPP in their canopy zones (705±39 g m-2) than in their root (430 ± 23 g m-22) or grassland (361 ± 21 g m-2) zones. Herbaceous-root biomass (0-30cm depth) was similar in all three zones around both tree species (range= 528-659 g m-2). (4) Mineralizable N and microbial biomass were significantly higher in soils from the canopy than from the root and grassland zones, whereas organic matter, P, K, and Ca (but not Mg) declined in soils from the base of the trees towards the open grassland. [ABSTRACT FROM AUTHOR]