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

SHOLTIS, J., GUNDERSON, C., TISSUE, D., and NORBY, R.

Photosynthetic response of deciduous trees in a Free-Air CO₂ Enrichment (FACE) environment.

The effects of elevated CO₂ on the growth of ten-year-old sweetgum (Liquidambar styraciflua L.) trees were assessed at a FACE (Free-Air CO₂ Enrichment) site at the Oak Ridge National Laboratory, TN. Data were collected in this first season of enrichment to determine the extent of changes in gas exchange, and determine the key biochemical control points that regulate and integrate physiological processes that affect leaf-level carbon balance. Trees were exposed to elevated (56 Pa), control (36 Pa), or ambient CO₂ concentrations beginning in the spring of 1998. The net photosynthetic rates of canopy leaves at their growth CO₂ concentrations were measured and found to be 32% (summer) and 84% (autumn) higher in those trees grown in the enriched CO₂. Analyses of ACi curves indicated that reductions of 12% in summer and 39% in autumn photosynthetic capacities of leaves exposed to elevated CO₂ were the result of reductions in Rubisco carboxylation capacity (18% summer, 7% autumn) and electron transport capacity (16% summer, 35% autumn). Decreased photosynthetic capacity was accompanied by reductions in various photosynthetic components, including total chlorophyll (23% summer, 18% autumn) and mass-based leaf nitrogen concentration (21% summer, 6% autumn). These first-season trends are suggestive of both a physiological adjustment and seasonal differences in the photosynthetic capacity of these trees. Future research at this site should help elucidate the mechanisms at work in this system.