Marsh Elevation Change and Carbon Sequestration

Science Center Objects

Tidal marsh vegetation grows in a narrow elevation zone between sea level and the upland behind it. These plant communities have evolved to accumulate sediment over time and maintain their relative elevation with gradual rates of change in sea level. It is uncertain which marsh vegetation communities will be able to accumulate sediment at a rate that keeps pace with accelerated sea level rise.

Aerial map of modeling area in the Nisqually River Delta

Aerial map of modeling area in the Nisqually River Delta. Managed freshwater marsh at the southern end of the refuge is excluded. Inset: Black point on terrain map shows the location of Billy Frank Jr. Nisqually National Wildlife Refuge within Washington State (grey outline).

(Public domain.)

 

 

 

We will model change in mudflat, tidal marsh (restored and reference), and tidal forested wetlands for multiple management scenarios using a suite of modeling tools (MEM, Delft3D, MOSAICS; Davis et al. 2019, Grossman et al. 2018, Morris et al. 2002), which together can estimate change in elevation and vegetation communities. Our marsh habitat change models will show where habitats (mudflat, low marsh, high marsh, brackish marsh, tidal forest and upland vegetation) may shrink or grow under different amounts of sea level rise.

 

 

 

 

 

 

 

 

 

 

Carbon Sequestration in Tidal Vegetation

A photo of researcher collecting on vegetation health

Tidal marsh vegetation captures carbon from the air and stores it in soils. Three researchers collect data on the vegetation to monitor marsh health.

(Credit: Ryan Munes, USFWS. Ryan Munes' image used with permission)

Wetland vegetation removes carbon dioxide from the atmosphere and stores it in the plant tissues. Litter and root material are buried in the soil and break down very slowly due to low oxygen conditions. This process makes tidal marshes and forested wetlands effective sinks for carbon, particularly in places where the decomposition process does not produce methane, another potent greenhouse gas. We will compare the current annual rates of carbon sequestration in the Delta’s restored marsh and historically unaltered marsh. For valuing carbon sequestration as a service to society, we will use the Social Cost of Carbon (EPA 2017), which is an estimate of the dollar value of damages avoided from having the equivalent amount of carbon in the atmosphere as a greenhouse gas.

Additionally, our habitat change models (specifically MEM; Marsh Equilibrium Model V.8.6) estimate how much carbon tidal marshes and tidal forests wetlands can accumulate in the soil. When combined with methane measurements taken from flux towers in restoring and reference marshes and information on mudflat carbon storage, we can examine how carbon sequestration capabilities of the marshes will change in the next 100 years. We will use the Social Cost of Carbon to estimate the value of this sequestration to society.

 

 

Sea level rise and tidal forested wetlands (tidal forests)

A photo of a tidal forest of the Nisqually River Delta

The tidal forests of the Nisqually River Delta become inundated to just below the trunk during the highest tides of the year.

(Credit: Monica Moritsch, USGS. Public domain.)

Tidal forested wetlands or tidal forests, also known as swamps, grow in locations where the water table rises and falls with the tide. While their present-day area is relatively small compared to emergent tidal marshes, they provide important habitat to species above and below the water, such as birds, fish, and beavers. In the Pacific Northwest, much of these forests have been lost to human activities including agriculture, logging, and development (Brophy 2019). Sea-level driven changes to the water table or salinity can increase tree mortality and eventually contribute to a transition from forest to emergent vegetation (Kirwan and Geden 2019). We aim apply the same habitat modeling tools to project how these forests and their services will change under different sea level rise scenarios.

 

 

 

 

 

References

Brophy, L.S. 2019. Comparing historical losses of forested, scrub-shrub, and emergent tidal wetlands on the Oregon coast, USA: A paradigm shift for estuary restoration and conservation. Prepared for the Pacific States Marine Fisheries Commission and the Pacific Marine and Estuarine Fish Habitat Partnership. Estuary Technical Group, Institute for Applied Ecology, Corvallis, Oregon, USA.

Davis, M. J., I. Woo, and S. E. W. De La Cruz. 2019. Development and implementation of an empirical habitat change model and decision support tool for estuarine ecosystems. Ecological Modelling 410:108722.

Kirwan, M. L., and K. B. Gedan. 2019. Sea-level driven land conversion and the formation of ghost forests. Nature Climate Change 9:450-457.

Morris, J. T., P. Sundareshwar, C. T. Nietch, B. Kjerfve, and D. R. Cahoon. 2002. Responses of coastal wetlands to rising sea level. 8:2869–2877.

United States Environmental Protection Agency (EPA). 2017. Regulatory Impact Analysis for the Review of the Clean Power Plan: Proposal.

 

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