Air temperatures in the arid western United States are predicted to increase over the next century. These increases will likely impact the distribution of plant species, particularly dioecious species that show a spatial segregation of the sexes across broad resource gradients. On the basis of spatial segregation patterns, we hypothesized that temperature increases will have a greater negative impact on female plants compared with co-occurring male plants of dioecious species. This hypothesis was tested by examining the whole-plant carbon and water relations of 10-year-old female (n = 18) and male (n = 13) Acer negundo Sarg. trees grown in a common garden in Salt Lake City, UT. The trees were established from cuttings collected where the growing season temperature averaged about 6.5 °C cooler than at the common garden. During May and June, stem sap flux (Js) was similar between genders, but averaged 25% higher in males during the warmer months of July and August. Daytime canopy stomatal conductance (gs) per unit leaf area was 12% higher in females in May : June, but was 11% higher in males in July : August. We combined measurements of sap flux–scaled transpiration with measurements of tree allometry and δ13C of leaf soluble sugars to estimate whole-tree carbon assimilation (Atree) and water use efficiency (WUE) (Atree : Etree). Atree was similar between genders until late August when Atree was 32% higher in male trees. Atree : Etree was on average 7% higher in females than in males during the growing season. Patterns of Js, gs, Atree and Atree : Etree in the present study were in contrast to those previously reported for A. negundo genders under native growing season temperatures. Results suggest that the spatial segregation of the sexes could shift under global warming such that female plants lose their dominance in high-resource habitats, and males increase their dominance in relatively lower-resource habitats.