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

Abstract

1.Faidherbia (Acacia) albidais a multipurpose tree widely distributed in semiarid Africa, notably in agroforestry parklands. It is in leaf during the 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 leaf water potential ofF. albidadropped only toca.– 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 have been affected by the moderate drought stress. However, leafiness remained constant during the dry season. Leaf shedding occurred after the first rains and was probably independent of drought.4.Faidherbia albidadisplayed large transpiration rates under favourable conditions but the ratio of sapflow to Penman evapotranspiration and the soil‐to‐leaf specific hydraulic conductance decreased severely towards the end of the dry season.5.Roots ofF. albidawere distributed through the weathered rock, down to a depth of 7 m, and vanished in the vicinity of a permanent water‐table. The isotopic composition of oxygen in the xylem sap (δ18O) remained very close to the values recorded in the water‐table during the course of the year. Phreatophytism thus explained the maintenance of growth and transpiration during the dry season. Nevertheless, during early rains, δ18O of sap switched towards the composition of the superficial soil layers, indicating facultative phreatophytism.6.Reverse phenology, low density and depth of water uptake ofF. albidaindicated a low competition with annual crops for water; the fraction of annual rainfall used by the trees was estimated to remain below 5%.