Background
The authors write that "wetlands evapotranspire more water than other ecosystems, including agricultural, forest and grassland ecosystems," but they say that "effects of elevated atmospheric carbon dioxide concentration on wetland evapotranspiration (ET) are largely unknown."

What was done
To help fill this data void, Li et al. present the results of what they describe as "twelve years of measurements of ET, net ecosystem CO2 exchange (NEE), and ecosystem water use efficiency (EWUE, i.e., NEE/ET) at 13:00-15:00 hours in July and August for a Scirpus olneyi (C3 sedge) community and a Spartina patens (C4 grass) community exposed to ambient and elevated (ambient + 340 ppm) CO2 in a Chesapeake Bay wetland."

What was learned
The four researchers report that "although a decrease in stomatal conductance at elevated CO2 in the S. olneyi community was counteracted by an increase in leaf area index (LAI) to some extent, ET was still reduced by 19% on average over 12 years," while "in the S. patens community, LAI was not affected by elevated CO2 and the reduction of ET was 34%." With respect to NEE, they found that it "was stimulated about 36% at elevated CO2 in the S. olneyi community but was not significantly affected by elevated CO2 in the S. patens community." And merging the ET and NEE responses of the two species, they determined that "EWUE was increased about 83% by elevated CO2 in both the S. olneyi and S. patens communities."

What it means
In light of their several findings, Li et al. conclude that rising concentrations of atmospheric CO2 "could have significant impacts on the hydrologic cycles of coastal wetlands," noting that "reduced ET could increase carbon uptake by mitigating the effects of drought on carbon uptake (Rasse et al., 2005)," and that it "could also facilitate ground water recharge to counteract salinity intrusion in coastal areas caused by rising sea levels from global warming," stating that salinity intrusion has been identified as "a serious problem in the United States" based on the work of Stevenson et al. (1988) and Day et al. (2000).

References
Day, J., Shaffer, G., Britsch, L., Reed, D., Hawes, S. and Cahoon, D. 2000. Pattern and processes of land loss in the Mississippi Delta: a spatial and temporal analysis of wetland habitat change. Estuaries 23: 425-438.

Rasse, D.P., Peresta, G., Saunders, C.J. and Drake, B.G. 2005. Seventeen-years of elevated CO2 exposure in a Chesapeake Bay Wetland: sustained but contrasting responses of plant growth and CO2 uptake. Global Change Biology 11: 369-377.

Stevenson, J., Kearney, M. and Pendleton, E. 1988. Sedimentation and erosion in a Chesapeake Bay brackish marsh system. Marine Geology 67: 213-235.