Giacomini Wetland Restoration Project: Restoration: What's the Long-Term Future of the Restored Wetlands?: Carbon Sequestration and Interactions of Carbon with Marsh Vegetation Communities

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There can be interaction between coastal wetlands and atmospheric carbon, as well as nitrogen. However, in the case of carbon, marshes may perform a valuable function in mediating effects of climate change, just as do forests. In fact, tidal marshes may be more efficient than forests at carbon sequestration, as it is called, on per unit area (Trulio et al. 2007). Tidal marshes, in particular, are extremely productive habitats that capture significant amounts of carbon from the atmosphere (Trulio et al. 2007). Tidal marshes can remove carbon as carbon dioxide from the atmosphere by producing up to 8,000 metric tons of plant material per year (Mitsch and Gosselink 2000 in Trulio et al. 2007). Upon decomposition, much of this material is stored long-term in soils, unlike forests, where carbon is stored in trees. Some rates estimated for south San Francisco Bay wetlands were as high as 54 g to 385 g of carbon per square meter per year (Chmura et al. 2003). In a review on North American wetlands, Brigham and colleagues noted that, "estuarine wetlands sequester carbon at a rate about 10-fold higher on an area basis than any other wetland ecosystem due to high sedimentation rates, high soil carbon content, and constant burial due to sea level rise" (Bridgham et al. 2006). In addition, unlike freshwater wetlands, tidal marshes release only negligible amounts of methane, which is a powerful greenhouse gas (Choi et al. 2004). The carbon sequestration rates of undisturbed wetlands were lower (15 percent for mangrove and 55 percent for saltmarsh) than disturbed wetlands, although the carbon store was higher (65 percent for mangrove and 60 percent for saltmarsh), suggesting that "wetland rehabilitation has positive benefits for regulation of atmospheric carbon concentrations, in addition to more broadly accepted ecosystem services" (Howe et al. 2009).

Ultimately, carbon sequestration could change the character of coastal marshes by favoring species with enhanced carbon uptake rates over species with lower rates. Essentially, increasing CO₂ is like adding fertilizer. For some species, increased CO₂ uptake results in higher shoot density and biomass and higher root biomass (Langley et al. 2009). In higher CO₂ environments, so-called C₃ plants appear to have an edge over C₄ plants. C₃ plants, which are so named because the first step of photosynthesis involves a three carbon-molecule, is the original and more common style of photosynthesis, which evolved more than 600 million years ago when atmospheric CO₂ was extremely high (Baskin 1994). C₃ plants include key crops such as wheat, rice, and beans and virtually all trees. At today's CO₂ concentrations, this carbon-capturing mechanism is somewhat inefficient, with plants losing as much as half the carbon they garner, but under higher levels, this loss is eliminated (Baskin 1994). C₄ plants, on the other hand, have a more efficient form of photosynthesis that creates a four-carbon molecule in the first stage and prevents carbon loss (Baskin 1994). This type of photosynthesis evolved later, probably at the end of the Cretaceous Period (some 65 million years ago), when CO₂ began to drop (Baskin 1994). These plants do not seemingly respond to increases in atmospheric CO₂. Though fewer in number, C4 plants also include important crops—corn, sorghum, sugarcane, pineapples—as well as prairie and savanna grasses and many shrubs (Baskin 1994).

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Should C₃ species have an evolutionary advantage over C₄ plants in richer carbon environments, the communities within coastal marshes could shift. One early experiment in Maryland's Chesapeake Bay tracked C₃ sedges and C₄ cordgrass growing in a coastal salt marsh in open-top chambers that exposed them to double current levels of CO₂ for at least five years (Baskin 1994). With nutrients in marshes relatively plentiful, the C₃ sedges responded with increased photosynthesis and growth, while the C₄ cordgrass showed little response to the increased gas and lost ground to the sedges (Baskin 1994). Many brackish marshes and mangrove swamps are dominated by C₃ plants (Langley at al. 2009). C₃ plants, at least in the Chesapeake Bay region, also tend to be more flood tolerant than C₄ plants, which are more salt tolerant (Langley et al. 2009). While some have theorized that this carbon enrichment of C₃ plants is not long-term and that there is an acclimation or leveling off in plant growth after a few years, others dispute this theory, pointing to sustained increases in biomass over longer periods of time with no leveling off (Johnson et al. 1993). Certainly, even if carbon uptake and photosynthesis, biomass increases can be inhibited by other factors, such as water, sunlight, nitrogen, and phosphorous, such that the factor limiting growth–and there always is one–simply shifts.

Biomass increases are not just limited to aboveground portions of plants, but can extend into the soil, and, as discussed earlier, this factor may help to alleviate some of the impacts from sea level rise in marshes. While there was a slight loss in elevation (-0.9 mm/yr) in the ambient CO₂ plot, there was a net gain (3.0 mm/yr) in the elevated CO₂ treatment (Langley et al. 2009). Increases in root zone thickness appeared to account for most of this elevation gain, and, not surprisingly, vertical changes in elevation were positively correlated with subsurface shoot volume of the C₃, but not C₄, species (Langley et al. 2009). Interestingly, higher levels of nitrogen, as might occur in increased nitrogen deposition scenarios as described above, appeared to potentially affect elevation negatively (Langley et al. 2009). Elevation of tidal marsh soils in the vicinity of plants with higher CO₂ uptake rates may enable these species or communities to better persist in face of sea level rise than those with lower CO₂ uptake rates.

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What's the Long-Term Future of the Restored Wetlands?
Page 1. Introduction
Page 2. Sea Level Rise
Page 3. Precipitation, Run-Off, and Sedimentation
Page 4. Coastal Winds and Temperature
Page 5. Coastal Fog
Page 6. Coastal Upwelling
Page 7. Acidification of Coastal Waters
Page 8. Changes in Estuarine Salinity with Sea Level Rise
Page 9. Nitrogen Deposition
Page 10. Carbon Sequestration and Interactions of Carbon with Marsh Vegetation Communities
Page 11. Conclusions and References

 

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restoration includes:

Restoration Water, Water, and More Water How Long Will It Take For Tidal Wetlands To Develop? Two Years Later: Already Dramatic Improvements in Water Quality Giacomini Wetlands: A Haven for Ducks

Spring is in the Air--and in the Water Untangling the Food Web: Changes in Prey Base Following Restoration What's the Long-Term Future of the Restored Wetlands? How Can We Learn About the Restoration? How Can We Be A Part of the Restoration Process? Kids in Wetlands - Learning Through Experience Where Can We View the Restored Wetlands?

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