Studying the effects of Increased Atmospheric CO2
on Plant Communities in a Brackish
Salt Marsh Ecosystem in Maryland.

Experimental enrichment chambers like those shown here with Dr. Bert Drake allow scientists to regulate both the concentration of CO2 around wetland plants and the duration of CO2 exposure. The species being studied belong to two photosynthetic types that represent most of the plant life on earth. The sedge, Scirpus olneyi, belongs to a class called C3 plants, which includes more than 95 percent of the plant species on earth. (Trees, for example, are also C3 plants.) C4 plants, such as the common marsh grass, Spartina patens, and other herbaceous plants, are abundant in arid, hot environments. They include such crop plants as sugar cane, corn, and soybeans, and are the second most prevalent photosynthetic type.

Since May of 1987, we have operated 30 open top chambers in a Maryland wetland ecosystem on the Kirkpatrick Marsh, Rhode River , a sub-estuary of the Chesapeake Bay . This is the longest running study of ecosystem response to elevated CO2 attempted to date.

Responses of the mechanisms of carbon cycling to elevated CO2 have been observed at virtually all levels in this ecosystem. Our data show the importance of inter-annual variations in rainfall and salinity in the regulation of responses to elevated CO2 on the wetland. This finding is in contrast to the expectation based only on studies of gas exchange which have suggested that improved water balance, resulting from reduced transpiration, would increase carbon assimilation at elevated CO2. We have found there is a general stimulation of soil microbial respiration, which significantly increased the amount of carbon cycled through this ecosystem. How much of this additional carbon remains in place in the marsh is not clear: this will be the subject of our continuing work. Support for our finding that elevated CO2 acting first on photosynthesis and subsequently on soil carbon pools has been found in the comparison between the responses measured in the C3- dominated sedge community and C4-dominated grass community. The responses found in the C4-dominated vegetation were either absent or much smaller in the than those in the C3-dominated community.

In some respects, the effects of elevated CO2 have been surprising. The importance of rainfall in the responses to elevated CO2 and a shift from relatively labile or short-lived to more recalcitrant (long-lasting) soil carbon were unexpected. More stable carbon compounds may mean a long-term shift in the rate of decomposition. Expansive growth, represented as shoot density, has shown a relatively large stimulation during periods of high salinity, which accompanies low rainfall. Important ecosystem functions have been altered by long-term exposure to elevated atmospheric CO2, including species composition (a change in the relative dominance of the C4 plants in the mixed community), nitrogen fixation, and palatability of foliage for herbivores.

Research Questions