While higher CO2 levels drive sea levels to rise, they also stimulate wetland-sustaining plant growth.
By Noreen Parks
From the Daily Planet
Publication Date (Web): April 22, 2009
Coastlines are already bearing the brunt of rising sea levels, and a recent government report warns that the mid-Atlantic seaboard, with its bountiful wetlands, is particularly vulnerable. However, new research published in the March 23 issue of the Proceedings of the National Academy of Sciences U.S.A. (2009, DOI 10.1073/pnas.0807695106) suggests that the escalating CO2 concentrations driving climate change also stimulate plant growth that counteracts the inevitable erosion from higher seas.
Researchers investigated CO2’s effects on wetlands in the Chesapeake Bay watershed, where they manipulated conditions in experimental chambers.
Coastal wetlands must continuously build upward to “keep their heads above water”, or they would wash out with the tides. Plant growth and the accumulation of soil-building organic litter normally preserve this equilibrium, but the pace of sea-level rise threatens to upset the balance. Ecologist Adam Langley, at the Smithsonian Environmental Research Center in Edgewater, Md., and his colleagues investigated how changes in plant productivity influence soil elevation in Chesapeake Bay tidelands. They enclosed marshland plots in chambers, half of which received a constant supplement of CO2 to simulate atmospheric levels predicted for 2100. CO2 in the other chambers was maintained at present levels. The team also added soil nitrogen to some chambers in both groups to mimic polluted estuaries. From 2006 to 2007, they measured total plant growth and tracked changes in soil height.
The results revealed that by the end of the experimental period, exposure to elevated CO2 alone had increased the rate of elevation gain by 3.9 millimeters per year. To explain the pattern, the authors looked belowground. Root growth had doubled, pushing the soil surface upward. “This also explains why nitrogen-fertilized enclosures didn’t boast the same gains,” Langley says. “With added nitrogen, the plants didn’t have to produce as many roots to obtain nutrients.” In companion greenhouse experiments that simulated salinity and flooding conditions likely to accompany rising sea levels, the CO2 effect was even more pronounced.
The findings sound a hopeful note that mounting CO2 concentrations may boost the capacity of wetlands to weather advancing seas, at least in the short term. However, “it’s important to remember that by warming the planet and accelerating sea-level rise, increasing CO2 is precisely what threatens marshes the most,” Langley warns.