SAVANNA TREE INFLUENCE ON UNDERSTORY VEGETATION AND SOIL NUTRIENTS IN NORTHWESTERN KENYA

Abstract. Contrary to observations and models in which trees and herbaceous plants are viewed as competitors, we found that trees in an African savanna have positive impacts on herbaceous biomass production and composition, and on soil nutrient status. In the Turkana District of northwestern Kenya, we investigated vegetation and soil gradients along equi‐angular transects radiating from the boles of individual Acacia tortilis trees. Total herbaceous biomass averaged 260 ± 17(se) g/m2 at the bole and declined to 95 ± 8 g/m2 in the tree interspaces. Soil organic carbon and total nitrogen concentrations were greatest (0.72 % and 0.083 %, respectively) in shallow soils near the bole and declined rapidly toward the interspaces and with increasing depth. Transects were also established between tree pairs to assess effects of differential canopy proximities. Grass production averaged 220 ± 21 g / m2 below overlapping canopies, 150 ± 15 g / m2 under individual canopies, and 95 ± 8 g / m2 in interstitial areas. Detrended correspondence analysis revealed that shifts in species composition were correlated with distance from tree bole out to the edge of the canopy. Species response, in terms of relative cover, to increasing distance from the bole, seemed to fall into five general classes: 1) greatest at the bole, 2) increasing with distance from the bole, 3) greatest in the mid canopy zone, 4) least at the bole and 5) no response. Trees did not influence herbaceous compositionbeyondtree canopies. It is assumed that shade cast by the tree canopy with subsequent reductions of understory water stress and temperature and increased nutrient concentrations may be the most important factors affecting understory soil and vegetation. 1990 IAVS ‐ the International Association of Vegetation Science