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Andrea Martin - Biogeochemistry

University of North Carolina, Chapel Hill

Wetland Plant Productivity in the Context of Global Change

Due to its location at the interface between the land and sea, the wetland ecosystem is especially vulnerable to the rising atmospheric CO2 concentration, sea level rise, and increased amount of nitrogen loading due to agricultural and municipal runoff. Increased productivity of the wetland can mitigate the effects of these anthropogenic changes through elevation gain and carbon sequestration. It is known that the peak biomass productivity of the wetland sedge Scirpus olneyi increases under elevated CO2 concentration and nitrogen addition. These productivity estimates, however, do not consider the turnover rate of stems throughout the growing season. This study investigated the turnover rate of Scirpus stems throughout the season, as well as the effect of elevated CO2 concentrations and nitrogen loading on this turnover rate. Total biomass was determined by direct measurements of stem density and average stem mass in open top in situ chambers, and the turnover rate was evaluated by tagging cohorts of stems and monitoring their mortality throughout the season. The peak standing biomass productivity of Scirpus significantly increased under a combination of elevated CO2 and nitrogen addition, but was not affected by nitrogen addition or elevated CO2 concentration alone. With the incorporation of turnover rate to this productivity estimate, however, the total productivity of the nitrogen and elevated CO2 combination treatment, as well as of the nitrogen without elevated CO2 treatment, significantly increased. These results suggest that the increased productivity response to elevated CO2 seen in past studies may be limited by nitrogen availability, thus indicating that the ability of the wetland ecosystem to keep pace with sea level rise under future atmospheric CO2 concentration may also depend upon this nitrogen availability.