Stable annual pattern of water use by Acacia tortilis in Sahelian Africa.

Water use by mature trees of Acacia tortilis (Forsk.) Hayne ssp. raddiana (Savi) Brenan var. raddiana growing in the northern Sahel was continuously recorded over 4 years. Water use was estimated from xylem sap flow measured by transient heat dissipation. Concurrently, cambial growth, canopy phenology, leaf water potential, climatic conditions and soil water availability (SWA) were monitored. In addition to the variation attributable to interannual variation in rainfall, SWA was increased by irrigation during one wet season. The wet season lasted from July to September, and annual rainfall ranged between 146 and 367 mm. The annual amount and pattern of tree water use were stable from year-to-year despite interannual and seasonal variations in SWA in the upper soil layers. Acacia tortilis transpired readily throughout the year, except for one month during the dry season when defoliation was at a maximum. Maximum water use of about 23 l (dm sapwood area)(-2) day(-1) was recorded at the end of the wet season. While trees retained foliage in the dry season, the decline in water use was modest at around 30%. Variation in predawn leaf water potential indicated that the trees were subject to soil water constraint. The rapid depletion of water in the uppermost soil layers after the wet season implies that there was extensive use of water from deep soil layers. The deep soil profile revealed (1) the existence of living roots at 25 m and (2) that the availability of soil water was low (-1.6 MPa) down to the water table at a depth of 31 m. However, transpiration was recorded at a predawn leaf water potential of -2.0 MPa, indicating that the trees used water from both intermediary soil layers and the water table. During the full canopy stage, mean values of whole-tree hydraulic conductance were similar in the wet and dry seasons. We propose that the stability of water use at the seasonal and annual scales resulted from a combination of features, including an extensive rooting habit related to deep water availability and an effective regulation of canopy conductance. Despite a limited effect on tree water use, irrigation during the wet season sharply increased predawn leaf water potential and cambial growth of trunks and branches.