Leaf conductance often decreases in response to elevated atmospheric CO2 concentration (Ca) potentially leading to changes in hydrology. We describe the hydrological responses of Florida scrub oak to elevated Ca during an eight-month period two years after Ca manipulation began. Whole-chamber gas exchange measurements revealed a consistent reduction in evapotranspiration in response to elevated Ca, despite an increase in leaf area index (LAI). Elevated Ca also increased surface soil water content, but xylem water deuterium measurements show that the dominant oaks in this system take up most of their water from the water table (which occurs at a depth of 1.5–3 m), suggesting that the water savings in elevated Ca in this system are primarily manifested as reduced water uptake at depth. Extrapolating these results to larger areas requires considering a number of processes that operate on scales beyond these accessible in this field experiment. Nevertheless, these results demonstrate the potential for reduced evapotranspiration and associated changes in hydrology in ecosystems dominated by woody vegetation in response to elevated Ca.