Seasonal variation of nutrient concentrations in a semi-arid savanna ecosystem in Botswana.
Leaves, flowers and fruits (if present) were collected at 3-4 wk intervals during Sep. 1982-Dec. 1983 from the trees Acacia burkei, A. erubescens, A. fleckii, A. karroo, A. mellifera subsp. detinens, A. nilotica subsp. kraussiana, A. robusta, A. tortilis subsp. heteracantha and Terminalia sericea, the shrub Dichrostachys cinerea and the grasses Eragrostis rigidior and Panicum maximum in the cattle-free zone of Gaborone. All plant material was analysed to determine contents of N, P, K, Ca, Mg, Fe, Na, Zn and Mn and results for most elements are shown in graphs. Most trees were leafless at the end of the dry season. Seasonal variation in concentrations of foliar nutrients followed similar trends in all species. Concn. of N and P were greater in young leaves than in mature leaves, while Ca and Fe accumulated until leaf abscission. Mg and K showed no clear trend. T. sericea showed pronounced accumulation of foliar Mn. Generally, the nutrient concn. of P. maximum was higher than that of E. rigidior. If the grasses remained green for a longer period, their concn. of N and P varied in a way similar to that of the trees. Variations in foliar nutrients in flowering and nonflowering trees and total nutrients in flowers and fruits of trees are also described. It is concluded that P is the most limiting nutrient in this savanna because of the strong translocation from leaves to twigs before leaf abscission.