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The Effects of Trees on Their Physical, Chemical and Biological Environments in a Semi-Arid Savanna in Kenya

A. J. Belsky, R. G. Amundson, J. M. Duxbury, S. J. Riha, A. R. Ali and S. M. Mwonga
Journal of Applied Ecology
Vol. 26, No. 3 (Dec., 1989), pp. 1005-1024
DOI: 10.2307/2403708
Stable URL: http://www.jstor.org/stable/2403708
Page Count: 20
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The Effects of Trees on Their Physical, Chemical and Biological Environments in a Semi-Arid Savanna in Kenya
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Abstract

(1) 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. digitata 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 grassland immediately after the start of the two rainy seasons, higher under 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. tortilis 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-2) or grassland (361 ± 21 g m-2) zones. Herbaceous-root biomass (0-30 cm 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. (5) Increased herbaceous-layer productivity was associated with the lower soil temperatures and greater soil fertility found under tree canopies. The soil nutrients that contributed to the greater fertility of the canopy-zone soils may have been transported to the canopy zone from surrounding soils by tree roots or deposited in dung by birds and large mammals that utilize the tree environment.

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