Reverse phenology and dry-season water uptake by Faidherbia albida (Del.) A. Chev. in an agroforestry parkland of Sudanese west Africa

1. Faidherbia (Acacia) albida is a multipurpose tree widely distributed in semiarid Africa, notably in agroforestry parklands. It is in leaf during th e dry season and defoliated during the rainy season, displaying therefore a peculiar reverse phenology. The related water-use strategy, including leaf water potential, sapflow, hydraulic conductance and depth of uptake, were monitored on adult trees in a Sudanese west-African parkland. 2, Despite a severe drought in the superficial soil layers, the predawn lea f water potential of F. albida dropped only to ca. - 0.5 MPa during the end of the dry season, indicating only a moderate water stress. 3. Radial trunk growth ceased before the end of the dry season and could ha ve been affected by the moderate drought stress. However, leafiness remaine d constant during the dry season. Leaf shedding occurred after the first ra ins and was probably independent of drought. 4. Faidherbia albida displayed large transpiration rates under favourable c onditions but the ratio of sapflow to Penman evapotranspiration and the soi l-to-leaf-specific hydraulic conductance decreased severely towards the end of the dry season. 5, Roots of F. albida were distributed through the weathered rock, down to a depth of 7 m, and vanished in the vicinity of a permanent water-table. Th e isotopic composition of oxygen in the xylem sap (delta(18)O) remained ver y close to the values recorded in the water-table during the course of the year. Phreatophytism thus explained the maintenance of growth and transpira tion during the dry season. Nevertheless, during early rains, delta(18)O of sap switched towards the composition of the superficial soil layers, indic ating facultative phreatophytism. 6. Reverse phenology, low density and depth of water uptake of F. albida in dicated a low competition with annual crops for water; the fraction of annu al rainfall used by the trees was estimated to remain below 5%.