Foliar trait contrasts between African forest and savanna trees: genetic versus environmental effects

Variations in leaf mass per unit area (Ma) and foliar concentrations of N, P, C, K, Mg and Ca were determined for 365 trees growing in 23 plots along a precipitation gradient ranging from 0.29 m a-1 to 1.62 m a-1. The transect extended from just south to the Sahara Desert in Mali to the forest-savanna transition zones (ZOT) of Ghana and Cameroon. 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 (both on a mass an area basis). Within the ZOT, 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 systematic variations in [P] with precipitation were observed, nor were there any differences in [P] observed for trees of forest vs. savanna stands. Although there was no systematic effect of vegetation type or precipitation on either [Mg] and [Ca], a marked increase in foliar [K] as precipitation declined was observed for savanna trees and with ZOT forest trees also having a significantly higher [K] compared to those of nearby savanna. 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 but as yet unidentified role for potassium in the adaption of savanna species to periods of limited water availability; with foliar [K] also an important factor differentiating tree species adapted to forest vs. savanna soils with in the ZOT of Western Africa.