ECOPHYSIOLOGICAL RESPONSES TO SIMULATED CANOPY GAPS OF 2 TREE SPECIESOF CONTRASTING SHADE TOLERANCE IN ELEVATED CO2

1. One-year-old seedlings of shade tolerant Acer rubrum and intolerant Betula papyrifera were grown in ambient and twice ambient (elevated) CO2, and in full sun and 80% shade for 90 days. The shaded seedlings r eceived 30-min sun patches twice during the course of the day, Gas exc hange and tissue-wafer relations were measured at midday in the sun pl ants and following 20 min of exposure to full sun in the shade plants to determine the effect of elevated CO2 on constraints to sun-patch ut ilization in these species. 2. Elevated CO2 had the largest stimulatio n of photosynthesis in B. papyrifera sun plants and A. rubrum shade pl ants, 3. Higher photosynthesis per unit leaf area in sun plants than i n shade plants of B. papyrifera was largely owing to differences in le af morphology, Acer rubrum exhibited sun/shade differences in photosyn thesis per unit leaf mass consistent with biochemical acclimation to s hade. 4. Betula papyrifera exhibited CO2 responses that would facilita te tolerance to leaf water deficits in large sun patches, including os motic adjustment and higher transpiration and stomatal conductance at a given leaf-water potential, whereas A. rubrum exhibited large increa ses in photosynthetic nitrogen-use efficiency. 5. Results suggest that species of contrasting successional ranks respond differently to elev ated CO2, in ways that are consistent with the habitats in which they typically occur.