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François Delage - CO2 Lab

IUFR-Bordeaux, France

Inter- and intra-annual variation of CO2 exchange in a Chesapeake Bay wetland during

long-term elevated CO2 exposure.

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.