Seedling responses to elevated atmospheric CO2 concentration ([CO2]) and solar irradiance were measured over two growing seasons in shade-tolerant Acer saccharum Marsh. and Fagus grandifolia J.F. Ehrh. and shade-intolerant Prunus serotina, a J.F. Ehrh. and Betula papyrifera Marsh. Seedlings were exposed to a factorial combination of [CO2] (ambient and elevated (658 +mol molv1)) and understory shade (deep and moderate) in open-top chambers placed in a forest understory. The elevated [CO2] treatment increased mean light-saturated net photosynthetic rate by 63% in the shade-tolerant species and 67% in the shade-intolerant species. However, when measured at the elevated [CO2], long-term enhancement of photosynthesis was 10% lower than the instantaneous enhancement seen in ambient-[ CO2]-grown plants (P < 0.021). Overall, growth light environment affected long-term photosynthetic enhancement by elevated [CO2]: as the growth irradiance increased, proportional enhancement due to elevated [CO2] decreased from 97% for plants grownin deep shade to47% for plants grownin moderate shade. Results suggest that in N-limited northern temperate forests, trees grown in deep shade may display greater photosynthetic gains from a CO2-enriched atmosphere than trees growing in more moderate shade, because of greater downregulation in the latter environment. If photosynthetic gains by deep-shade-grown plants in response to elevated [CO2] translate into improved growth and survival of shade-intolerant species, it could alter the future composition and dynamics of successional forest communities.