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Prosopis flexuosa trees dominate woodlands in the Central Monte Desert (Mendoza, Argentina), with < 200 mm rainfall, exploiting the water table recharged by Andean rivers, and also growing in dunes with no access to the water table. Prosopis woodlands were extensively logged during development of the agricultural oasis, and surface and groundwater irrigation could lower the depth of the water table in the future. We evaluated tree populations with decreasing access to the water table: valley adult trees, valley saplings, and dune adult trees, in order to assess their ecophysiological response to water table accessibility. High and seasonally stable pre-dawn leaf water potentials (-2.2 +/- A 0.2 to -1.2 +/- A 0.07 MPa) indicated that valley adults utilize larger and more stable water reservoirs than valley saplings and dune adults (-3.8 +/- A 0.3 to -1.3 +/- A 0.07 MPa), with higher midday leaf conductance to water vapor (valley adults similar to 250; dune adults < 60 mmol m(-2) s(-1)), potentially higher CO2 uptake, and increased radial growth rate (valley adults 4.1 +/- A 0.07; dune adults 2.9 +/- A 0.02 mm year(-1)). Trees with poor access to the water table exhibited drought tolerance responses such as midday stomata closure, leaflet closure, and osmotic adjustment. Stomata density decreased in response to drought when leaf expansion was restricted. The combination of phreatophytism and drought tolerance would enlarge P. flexuosa habitats and buffer populations against changes in rainfall dynamics and water table depth.