Box elder (Acer negundo Sarg.) is a dominant dioecious riparian tree species that shows spatial segregation among the sexes: females are typically more common along streamside (high resource) environments than males. The spatial segregation of the sexes leads to the hypothesis that mature streamside female trees have significantly greater carbon gain than males, particularly where both water and nutrients are available. To test this hypothesis, leaf carbon isotope ratios and whole-plant transpiration (from sap flux) were measured on 8-year old male (n = 13) and female (n = 18) box elder trees cloned in a common garden. The trees were separated into two plots with artificial streams running through each plot. One stream contained free-flowing water plus 50 μmol·l ^-1 of ammonium nitrate while the other stream contained free-flowing water without nitrogen added. We combined measurements of carbon isotope ratios from extracted leaf non-structural carbohydrates with measurements of transpiration and leaf area to estimate total carbon gain of male and female trees in both plots. Counter to our hypothesis, males showed a stronger response to stream nitrate addition, in terms of improved water and carbon uptake than females. These initial results indicate that the observed patterns of high female to male ratios in streamside box elder populations are not related to high stream nitrogen content that often occurs during periods of high stream discharge and/or anthropogenic land use change.