Comparative effects of isolated trees on their undercanopy environments in high- and low-rainfall savannas.
Abstract
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 (about 750 mm annual rainfall) were investigated and compared with an earlier study of the effects of the same 2 species in a more xeric savanna (about 450 mm annual rainfall), also in Tsavo National Park (West), Kenya. Compared with 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/m2) than in the open (533 g/m2), but only 18% higher under baobab canopies (569 g/m2) than in the open (484 g/m2). Herbaceous-root biomass (0.30 cm) in the mesic site was higher in open grassland than under canopies. Herbaceous vegetation under the canopies of both tree species was dominated by Cynodon nlemfuensis, Panicum maximum and Enteropogon macrostachyus, while in open grassland the most common species, in descending order, were Sehima nervosum, Eragrostis cespitosa, Digitaria milanjiana, Eustachys paspaloides, Cymbopogon commutatus and Bothriochloa radicans. 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. Concentrations of OM, total N, 15N, P, K and Ca were higher, and C:N ratios and soil bulk density lower under tree canopies than in the open at both sites. Mg concentrations were higher in the open than under tree canopies at the mesic, but not the xeric, site. The higher herbaceous biomass production under trees was attributed to higher soil-N concentrations and to lower evapotranspiration. It was suggested that 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.