A. JOY BELSKY
Cornell Plantations, Cornell University, Ithaca, New York, U.S.A.
The effects of isolated savanna trees on nutrient concentrations, total nutrient content, cell-wall fibre content, and digestibility of gramineous forage growing in three zones (canopy, tree-root, and grassland) surrounding individual trees of Acacia tortilis (acacia) and Adansonia digitata (baobab) were compared in low- and high-rainfall sites in Tsavo National Park, Kenya.
The two tree species had similar effects on forage quality except that tissue-N was higher around the leguminous acacia trees. Concentrations of N, P, K, Ca, B, and Cu in the forage tended to increase from open grassland to tree understorey while concentrations of Mn, Zn, and Mo decreased. Since forage yield also increased from grassland to the canopy zone, total content of all nutrients per unit area increased towards the tree base. Concentrations of the forage fibres — neutral detergent, fibre, acid detergent fibre and cellulose — and of SiO2 decreased, while lignin increased, from grassland to canopy zone, but the digestibility of dry matter was slightly lower in the canopy zone. Trees in Kenyan savannas, therefore, increase the availability of nutrients in the herbaceous forage, but this is offset by a slight drop in digestibility.