Dioecious plants are frequently associated with different spatial distributions of the two sexes across resource gradients. Segregation between sexes might be expected to occur if the resource cost of reproduction is greater in females than in males. If so, females would be under stronger selection to increase rates of resource uptake. Currently we are investigating water and carbon relations of streamside and non-streamside Acer negundo, a common dioecious tree species that occurs in riparian zones throughout the western United States. Our goal is to determine if carbon, nutrient and water balances differ between genders. If so, male and female trees may differentially impact eco-hydrological processes at large spatial scales. Research is being conducted in Red Butte Canyon, above Salt Lake City (Figure 1), and at an artificial riparian area built on the campus of the University of Utah (Figure 2). Measurements include stem sap flux, carbon, and nitrogen isotope analysis of plant tissues, xylem function and anatomy, soil moisture, plant water status, and micrometeorology.