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To determine whether agroforestry and silvopastoralism might be introduced more successfully into xeric or into mesic environments, the effects of isolated, mature trees of Acacia tortilis (acacia) and Adansonia digitata (baobab) on herbaceous-layer composition and productivity, soil properties, and microclimate in a moderately mesic savanna (c. 750 mm annual rainfall) were investigated and compared with an earlier study of the effects of the same two species in a more xeric savanna (c. 450mm annual rainfall) in Tsavo National Park (West), Kenya. Similar to the more xeric site, solar radiation was reduced by 45–65% and soil temperatures were reduced by 5–12 C° under both tree species. Except for early in the growing season, soil-moisture values were similar under tree canopies and in open grasslands. Compared to the more xeric site, where herbaceous-layer productivity was 95% higher under trees than in the open, productivity in the mesic site was 52% higher under acacia canopies (a mean of 808 g m-2) than in the open (553 g m-2), but only 18% higher under baobab canopies (569 g m-2) than in the open (484 g m-2). Concentrations of organic matter, total N, 15N, P, K and Ca were significantly higher, and C:N ratios and soil bulk density significantly lower under tree canopies than in the open at both sites. Mg concentrations were significantly higher in the open than under tree canopies at the mesic, but not the xeric, site. The contrast noted in 3 above between the herbaceous layer productivity below tree canopies and that in areas beyond them was attributed higher soil-N concentrations and to reduced evapotranspiration in these N-and water-limited systems. The contrast in forage production under tree canopies between xeric and mesic sites may be due to the greater importance of shade in reducing temperatures and evapotranspiration in more arid environments. [ABSTRACT FROM AUTHOR]

(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]