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1. Prosopis velutina Woot. (Velvet Mesquite) at a site with limited groundwater availability derived a greater percentage of water from shallow soil at the onset of the summer rainy season than did trees at a site with greater availability of groundwater. Predawn leaf water potentials (Psi(pd)) were not a strong indicator of shallow water use for this species with roots in multiple soil layers.

2. We experimentally defoliated P. velutina plants to determine if reduced-canopy photosynthesis would alter vertical patterns of root activity. After natural rain events, hydrogen isotope ratios of xylem sap indicated that defoliated P. velutina took up a greater percentage of its water from shallow soils than did undefoliated plants.

3. Irrigation with deuterium-labelled water further demonstrated that undefoliated plants were able to use shallow soil water. Defoliation appeared to affect the ability of trees to use deep-water sources.

4. Reduced carbon assimilation limited water uptake from deep soil layers. These data highlight that there are internal physiological controls on carbon allocation that may limit water uptake from different soil layers. During periods of high vapour pressure deficit or soil drought, when leaf gas exchange and carbon assimilation decline, this may create positive feedbacks where plants are unable to forage for deep water due to carbon limitations.