COMPARATIVE EFFECTS OF ISOLATED TREES ON THEIR UNDERCANOPY ENVIRONMENTS IN HIGH-RAINFALL AND LOW-RAINFALL SAVANNAS

1. 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 herb-aceous-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. 450 mm annual rainfall) in Tsavo National Park (West), Kenya. 2. Similar to the more xeric site, solar radiation was reduced by 45-65% and soil temperatures were reduced by 5 - 12 C-degrees under both tree species. Except for early in the growing season, soil-moisture values were similar under tree canopies and in open grasslands. 3. 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 (533 g m-2), but only 18% higher under baobab canopies (569 g m-2) than in the open (484 g m-2). 4. Concentrations of organic matter, total N, N-15, 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. 5. 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. 6. 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.