The effects of trees on their physical, chemical, and biological environments in a semi-arid savanna in Kenya.
Isolated mature trees of Acacia tortilis and Adansonia digitata in Tsavo National Park were studied for herbaceous-layer composition and productivity, site microclimate, and soil fertility under the canopy, in the rooting zone, and in surrounding grasslands. Compared with open grassland, tree canopies of both species reduced solar irradiance by 45-65%, soil temperatures by 5-11°C and rainfall by 50%. Soil water content was higher in open grassland immediately after the start of the two rainy seasons, higher under tree canopies during the first rainy season, but equal in the two areas during the second rainy season. Patterns of herbaceous-layer composition and above-ground net primary productivity (ANPP) under and near trees of the two species were similar. ANPP was significantly greater in their canopy zones (705±39 g/m2) than in their root zones (430±23 g/m2) or grasslands (361±21 g/m2). Mineralizable N and microbial biomass were significantly greater in soils under canopy than elsewhere, and organic matter, P, K and Ca (but not Mg) declined in soils from the base of trees towards the open grassland.