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Abstract

Variations in leaf mass per unit area (Ma) and foliar concentrations of N, P, C, K, Mg and Ca 40 were determined for 365 trees growing in 23 plots along a West African precipitation gradient ranging from 0.29 m a-1 to 1.62 m a-1. Contrary to previous studies, no marked increase in Ma with declining precipitation was observed, but savanna tree foliar [N] tended to be higher at the drier sites (mass basis). Generally, Ma was slightly higher and [N] slightly lower for forest vs. savanna trees with most of this difference attributable to differences in soil chemistry. No 45 systematic variations in [P], [Mg] and [Ca] with precipitation or between trees of forest vs. savanna stands were observed. We did, however, find a marked increase in foliar [K] of savanna trees as precipitation declined, with savanna trees also having a significantly lower [K] compared to those of nearby forest. These differences were not related to differences in soil nutrient status and were accompanied by systematic changes in [C] of opposite sign. We suggest an important 50 but as yet unidentified role for potassium in the adaption of savanna species to periods of limited water availability; with foliar [K] being also an important factor differentiating tree species adapted to forest vs. savanna soils within the ZOT of Western Africa.