Abstract, Ecological Society of America Annual Meeting, Snowbird, UT , August, 2000.

SHOLTIS, J.D^.1, GUNDERSON, C.A^.2, and TISSUE, D.T.^{1 1}Texas Tech University, Lubbock, TX 79409 USA; ²Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA.

Seasonal changes in the photosynthetic response of an 11-year-old sweetgum stand in the second year of CO₂ enrichment at a FACE site in the southeastern United States.

Long-term photosynthetic responses of trees exposed to elevated CO₂ have been shown to vary due to biochemical, physiological and morphological feedback mechanisms that regulate carbon assimilation to meet plant growth demands. Because source-sink demands of deciduous trees change throughout the growing season, leaf gas exchange and related biochemical data were collected five times between May and September 1999 at the FACE facility at the Oak Ridge National Laboratory. Net photosynthetic rates of canopy leaves were found to be consistently higher in trees exposed to elevated (56 Pa) CO₂, with increases ranging from 23% in May to 97% in September. Analyses of ACi curves revealed seasonal trends in data related to photosynthetic capacity, which was increased by growth in elevated CO₂ at the beginning (8%) and end (7%) of the growing season, but was reduced (14%) in late July. Similarly, little difference existed in the Rubisco carboxylation and electron transport capacities, and total chlorophyll content early and late in the season in elevated CO₂, but were 21%, 13% and 13% lower, respectively, in late July. This decrease in photosynthetic capacity coincided with observed reductions in leaf production and expansion, and formation of branch terminal buds in late July, which possibly lowered sink strength. In general, it is expected that this deciduous species will generally show a positive photosynthetic response to elevated CO₂ with some physiological downregulation possible when growth is reduced.