Changes in light harvesting ability and other physiological responses could int1uence the competitive outcomes of tree species in a future elevated CO₂ atmosphere. Photosynthetic light response curves were constructed with a closed photosynthesis system (LI-COR, Inc. Lincoln, NB) in order to investigate the effect of growth in elevated CO₂ (746 μl⁻¹) and ambient CO₂ (379 μl⁻¹) on light responses, and seedlings were allowed to dry out to examine physiological changes to water stress. While drying out, photosynthesis was measured with a closed system (LI-COR, Inc. Lincoln, NB) and fluorescence was measured with a portable fluorescence measurement system (P.K. Morgan Instruments, Inc Andover, MA). No species showed significant increases In quantum yields or decreases in light-compensation points as a result of elevated CO₂. Photosynthesis declined in all species due to water but seedlings grown in elevated C0₂ maintained photosynthesis longer. Loblolly pine and red maple grown in e1evated CO₂ showed signs of photosynthetic acclimation. Photochemical efficiency of PSII declined with water stress in loblolly pine. Red maple and sweetgum showed no relationship between photochemical efficiency of PSII and simulated drought. Growth in elevated C0₂ did not influence this response in loblolly pine, but sweetgum started with a lower photochemical efficiency or PSII which increased significantly.