Background
The authors write that "tidal wetlands experiencing increased rates of sea-level rise (SLR) must increase rates of soil elevation gain to avoid permanent conversion to open water." As for how that might happen, they note that "root zone expansion by accumulation of plant material is essential to maintaining a constant surface elevation relative to rising sea level."

What was done
In Kirkpatrick Marsh -- a microtidal subestuary of Chesapeake Bay, where each of several 200-m² plots was outfitted with a surface elevation table (SET) to measure soil elevation change -- Langley et al. exposed half of the plots to an extra 340 ppm of CO2 for a period of two years, while "data from a greenhouse mesocosm experiment (Cherry et al., 2009) were used to examine how elevated CO2 might affect elevation response under simulated SLR scenarios."

What was learned
The five researchers report that the extra CO2 of their marsh experiment increased fine root productivity by an average of 36% over the two-year study, and that aboveground biomass production was increased by as much as 30%, "consistent with a 20-year record of elevated CO2 treatment in a previous CO2 study on the same marsh (Erickson et al., 2007)." In addition, they say the elevated CO2 caused an increase in root zone thickness of 4.9 mm/year compared with only 0.7 mm/year in the ambient CO2 treatment, with the result that there was "a slight loss of elevation in ambient CO2 (-0.9 mm/year) compared with an elevation gain (3.0 mm/year) in the elevated CO2 treatment." Furthermore, they report that the greenhouse mesocosm experiment of Cherry et al. (2009) "revealed that the CO2 effect was enhanced under salinity and flooding conditions likely to accompany future SLR."

What it means
Langley et al. conclude that "by stimulating biogenic contributions to marsh elevation, increases in the greenhouse gas, CO2, may paradoxically aid some coastal wetlands in counterbalancing rising seas." In this regard, they say their findings "bear particular importance given the threat of accelerating SLR to coastal wetlands worldwide," citing the recent EPA report of Reed et al. (2008), which suggests that "a 2-mm increase in the rate of SLR will threaten or eliminate a large portion of mid-Atlantic marshes." Once again, however, the proven and positive growth-promoting effect of atmospheric CO2 enrichment more than compensates for its hypothetical and negative global-warming effect.

References
Cherry, J.A., McKee, K. and Grace, J.B. 2009. Elevated CO2 enhances biological contributions to elevation change in coastal wetlands by offsetting stressors associated with sea-level rise. Journal of Ecology 97: 67-77.

Erickson, J.E., Megonigal, J.P., Peresta, G. and Drake, B.G. 2007. Salinity and sea level mediate elevated CO2 effects on C-3-C4 plant interactions and tissue nitrogen in a Chesapeake Bay tidal wetland. Global Change Biology 13: 202-215.

Reed, D.J. et al. 2008. Site-Specific Scenarios for Wetlands Accretion as Sea Level Rises in the Mid-Atlantic Region. Section 2.1. Background Documents Supporting Climate Change Science Program Synthesis and Assessment Product. Titus, J.G. and Strange CO2, E.M. (Eds.). EPA 430R07004, U.S. Environmental Protection Agency, Washington, DC.