Abstract, Ecological Society of America Annual Meeting, Spokane, WA, July, 1999.

GUNDERSON*, C. A., S.J. SCHWEITZER, J.D. SHOLTIS, D.T. TISSUE, and R.J. NORBY. Oak Ridge National Laboratory, Oak Ridge, TN 37831-6422. Increased photosynthesis and photosynthetic nitrogen-use efficiency throughout a closed-canopy sweetgum stand in the first year of CO₂ enrichment.

Scaling photosynthesis to the canopy level requires a knowledge of the effects of microenvironment and nitrogen allocation patterns on photosynthetic responses. We are assessing these influences on responses to elevated CO₂ over time, by exposing a 10-year old stand of sweetgum (Liquidambar styraciflua L.) to free-air CO₂ enrichment. During the first season of enrichment, 1998, photosynthetic rates at the top of the canopy were generally 30-70% higher in the elevated (560 ppm) CO₂ treatment, and stomatal conductance was 25-35% lower, when measured at saturating light and treatment [CO₂]. A series of measurements at three canopy levels in early July indicated decreasing rates of photosynthesis, transpiration, and stomatal conductance accompanying the reduced irradiance and leaf temperatures lower in the canopy. Leaf mass and nitrogen content per unit area also declined with canopy depth, and chlorophyll concentration increased. CO₂ enrichment had no significant effect on these parameters. Photosynthetic enhancement was sustained, at all canopy positions, whether evaluated on an area, mass, nitrogen, or chlorophyll basis, and despite reduced stomatal conductance. Because prolonged exposure to CO₂ enrichment often causes changes in leaf mass per area and [N], and because a reallocation of N and leaf mass within the canopy is hypothesized to result as the stand adjusts to higher [CO₂], patterns from the first year will provide a baseline against which future canopy responses can be evaluated.