We determined how changes in environmental moisture affected leaf gas exchange in Nuttall (Quercus texana Buckley), overcup (Q. lyrata Walt.), and dominant and codominant swamp chestnut (Q. michauxii Nutt.) oak trees in Mississippi and Louisiana. We used canopy access towers to measure leaf level gas exchange rates, e.g., photosynthesis, transpiration, stomatal conductance, on the upper crowns. Leaf gas exchange rates of the dominant swamp chestnut oak tree increased in response to summer precipitation. Nuttall and overcup oak trees exhibited constant gas exchange rates throughout the measurement periods. Dominant swamp chestnut oak also maintained high water use efficiency during periods of low soil moisture. These responses and access to water sources during soil drying may buffer extreme fluctuations in leaf gas exchange rates in mature canopy oak trees. Seasonal fluxes in water use and transpiration among individual oak trees of various sizes and other bottomland hardwood species need to be determined to assess their physiological and ecological contributions on the water balance of southeastern bottomland hardwood forests.