We examined the effects of elevated carbon dioxide concentration ([CO₂]) on the relationship between light-saturated net photosynthesis (A_sat) and area-based foliar nitrogen (N) concentration (N_a) in the canopy of the Duke Forest FACE experiment. Measurements of A_sat and N_a were made on two tree species growing in the forest overstory and four tree species growing in the forest understory, in ambient and elevated [CO₂] FACE rings, during early and late summer of 1999, 2001 and 2002, corresponding to years three, five and six of CO₂ treatment. When measured at the growth [CO₂], net photosynthetic rates of each species examined in the forest overstory and understory were stimulated by elevated [CO₂] at each measurement date. We found no effect of elevated [CO₂] on N_a in any of the species. The slope of the A_sat—N relationship was 81% greater in elevated [CO₂] than in ambient [CO₂] when averaged across all sample dates, reflecting a differential CO₂ effect on photosynthesis at the top and bottom of the canopy. We compared A_sat—N relationships in trees grown in ambient and elevated [CO₂] at two common CO₂ concentrations, during late summer 2001 and both early and late 2002, to determine if the stimulatory effect of elevated [CO₂] on photosynthesis diminishes over time. At all three sample times, neither the slopes nor the y-intercepts of the A_sat—N relationships of trees grown in ambient or elevated [CO₂] differed when measured at common CO₂ concentrations, indicating that the responses of photosynthesis to long-term elevated [CO₂] did not differ from the responses to a short-term increase in [CO₂]. This finding, together with the observation that N_a was unaffected by growth in elevated [CO₂], indicates that these overstory and understory trees growing at the Duke Forest FACE experiment continue to show a strong stimulation of photosynthesis by elevated [CO₂].