PARENT SESSION
Contributed Oral Session 154: Photosynthesis and Water Relations: Climate Effects
Friday, August 12, 8:00 AM - 11:30 AM, Meeting Room 515 A, Level 5, Palais des congrès de Montréal

Inter- and intra-annual variation of CO₂ exchange in a Chesapeake Bay wetland during long-term elevated CO₂ exposure.

Delage, Francois^*,1, Erickson, John¹, Peresta, Gary¹, Drake, Bert¹, ¹ Smithsonian Environmental Research Center, Edgewater, MD, USA

ABSTRACT- Exposure to elevated atmospheric CO₂ concentration (Ca) commonly stimulates photosynthesis and growth in many plant species. However, our understanding of long-term responses and seasonal variation in natural plant communities is limited. Since 1987, data have been collected from Chesapeake Bay salt marsh communities of Schoenoplectus americanus (Pers.) Volk. (C₃) and Spartina patens (Aiton) Muhl. (C₄) grown in open top chambers at normal ambient or elevated (normal ambient plus 350 ppm) Ca. Data were collected on five replicate chambers per treatment for each community. We examined the elevated Ca effect on inter- and intra-annual trends in net ecosystem CO₂ exchange (NEE). During the first three years of exposure to the elevated Ca treatment (1987-1989), the response of NEE to elevated Ca was about 70%. Despite subsequent photosynthetic acclimation in the C₃ species, the long-term midday (10:00-15:00) stimulation of NEE at peak biomass remained positive at approximately 35%. In the C₄ community the stimulation was much less, averaging about 15% and was not statistically significant in many years. Interestingly, seasonal patterns of CO₂ stimulation of NEE in both plant communities were similar: during early and mid-season (May-August) the stimulation was near that reported for peak biomass above, but increased significantly in both communities late in the season (September and October). The most likely explanation for this increase in the elevated Ca stimulation of NEE is earlier senescence of the vegetation in the ambient chambers relative to elevated chambers. Our results point to a long-term increase in carbon cycling due to elevated Ca in the C₃ community as well as delayed senescence which resulted in a longer photosynthetically active season. Thus, elevated Ca could affect ecosystem carbon uptake and species interactions in the salt marsh through at least three mechanisms: increased leaf area, increased photosynthetic activity and longer growing season.

Key words: Elevated CO₂ exposure, Net ecosystem exchange, Salt marsh ecosystem