Climate Change and the Future of Southern Wetlands

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Wetlands like Suggs Mill Pond in the coastal plain of North Carolina provide valuable breeding habitat for amphibians. Photo courtesy of NC Wetlands, via Flickr.

The Southeast hosts an impressive network of forested wetlands. These wetlands improve water quality, reduce flooding, store excess carbon, and provide important habitat for wildlife. They are also particularly vulnerable to changes in climate and land use.

With researchers from North Carolina State University, USDA Forest Service scientist Peter Caldwell designed a model to assess the impacts of climate change on wetland boundaries along the east coast.

The researchers derived future daily temperature and precipitation values from the Hadley climate forecast model. These data were analyzed in another model called DRAINMOD along with soil and hydrology records to predict levels of current and future water tables.

The team analyzed site data from an isolated depressional wetland in Pitt County, North Carolina. Analyses were repeated for sites in Portland, Maine; Easton, Maryland; and Miami, Florida. The modeling results were published in the journal Wetlands.

The team applied this definition of a wetland-hydrology boundary: when the water table is within 30 cm of the soil surface for two weeks or more during the growing season. Land below this point of the boundary is considered a wetland, while land upslope is not.

The drainage intensity needed to meet this definition of a wetland boundary for both present and future conditions is called the Threshold Drainage Intensity (TDI).

The difference in TDI for present and future conditions was used to determine elevation changes in wetland boundaries from 2012 to 2070. The researchers concluded that wetland boundaries would move downslope at all sites by 2070.

Specifically, the wetland boundary in North Carolina is characterized by a water table that persists at a depth at or below 30 cm for two weeks. For future conditions, the water table will persist at the same depth for only one week. This indicates that the current wetland boundary will become drier over time. Over the modeling period, the wetland boundary in Pitt County, North Carolina is predicted to decrease in elevation by 17 cm.

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Loss in wetland area in the modeling study area of Pitt County, NC between two time periods, historic and future, is shown in this frequency diagram. Image by authors Michael Vepraskas, Richard Skaggs, and Peter Caldwell.

The same trend was predicted at all other sites, with water tables lowering from 5 to 25 cm. This change is mostly driven by hotter temperatures and increased evapotranspiration.

“Essentially, this work is showing that there will be less wetland acreage in the future as a result of climate change,” says Caldwell.

Reduced wetland acreage could significantly impact biological process. Increases in temperature may lead to increased methane and carbon emissions, as well as heightened drought and wildfire risk. Additionally, drying wetlands may lead to changes in wildlife habitat. Aquatic species — such as cricket frogs, swamp snakes, and many species of waterfowl — may become isolated. Dispersal, recolonization, and genetic exchange may also be impacted.

In addition to increased temperatures and evapotranspiration, wetlands are influenced by changes in vegetation, inter-wetland relationships, and surrounding land use. Regional differences in these factors will influence changes to the water table.

“Wetlands are not static. They move and change over time, and we need to consider these dynamics when deciding how to manage them,” adds Caldwell.

Read the full text of the article.

For more information, contact Peter Caldwell at peter.v.caldwell@usda.gov.

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