Photosynthetic light response curves (A/PPFD), leaf N concentration and content, and relative leaf absorbance (αr) were measured in 1-year-old seedlings of shade-intolerant Betula papyrifera Marsh., moderately shade-tolerant Quercus rubra L. and shade-tolerant Acer rubrum L. Seedlings were grown in full sun or 26% of full sun (shade) and in ambient (350 ppm) or elevated (714 ppm) CO2 for 80 days. In the shade treatments, 80% of the daily PPFD on cloud-free days was provided by two 30-min sun patches at midday. In Q. rubra and A. rubrum, leaf N concentration and αr were significantly higher in seedlings in the shade treatments than in the sun treatments, and leaf N concentration was lower in seedlings in the ambient CO2 treatments than in the elevated CO2 treatments. Changes in αr and leaf N content suggest that reapportionment of leaf N into light harvesting machinery in response to shade and elevated CO2 tended to increase with increasing shade tolerance of the plant. Shifts induced by elevated CO2 in the A/PPFD relationship in sun plants were largest in B. papyrifera and least in A. rubrum: the reverse was true for shade plants. Elevated CO2 resulted in increased light-saturated A in every species × light treatment combination, except in shaded B. papyrifera. The light compensation point (Γ) decreased in response to shade in all species, and in response to elevated CO2 in A. rubrum and Q. rubra. Acer rubrum had the greatest increases in apparent quantum yield (φ) in response to shade and elevated CO2. To illustrate the effects of shifts in A, Γ and φ on daily C gain, daily integrated C balance was calculated for individual sun and shade leaves. Ignoring possible stomatal effects, estimated daily (24 h) leaf C balance was 218 to 442% higher in the elevated CO2 treatments than in the ambient CO2 treatments in both sun and shade seedlings of Q. rubra and A. rubrum. These results suggest that the ability of species to acclimate photosynthetically to elevated CO2 may, in part, be related to their ability to adapt to low irradiance. Such a relationship has implications for altered C balance and nitrogen use efficiency of understory seedlings.