Technical Abstract: Many transpiration studies on salt cedar (Tamarix ramosissima) have indicated that its water use is considerably greater than native trees; however these results have varied on temporal and spatial scales making it difficult to draw firm conclusions, especially in co-occurring communities. Using sap flow techniques, we quantified transpiration in co-occurring communities of native cottonwood (Populus fremontii) and Arizona ash (Fraxinus velutina) trees and invasive salt cedar during the dry and hot pre-monsoon season along an intermittent reach of the San Pedro River in southeastern Arizona. We also monitored meteorological variables and the declining groundwater levels to better understand their competition for water and the plant physiological response to environmental stressors. Riparian restoration efforts aimed at "water salvage" have focused on salt cedar removal to encourage return of native riparian forests. There is need to define environments and conditions in which salt cedar removal is unnecessary as its water use may not be significant.