Report

Adventures in the Science and Policy of Coastal Wetland Restoration

10/04/2010

On a sunny September day in 2005, I traveled with my doctoral advisor to coastal North Carolina to visit a wetland restoration project. As we drove eastward, rolling hills and suburbs turned to fields of tobacco, cotton, and corn. Prior to 1970, however, this landscape would have featured many wetlands: in the 1970’s-1980, 13 percent of North Carolina’s wetlands were converted to agriculture, mainly in the coastal plain (Dahl 1990; Heimlich et al. 1998). Yet signs of historic and remnant wetlands were all around: roadside canals and ditches filled with water, forested swamps dominated by cypress, blackwater rivers meandering towards the sound (Figure 1). We were going to visit one of the largest wetland mitigation sites in the eastern U.S., to help answer a seemingly simple question:  What is a stream?  But we got hooked by a lot of other equally important and exciting questions in coastal wetland restoration.

Wetlands
Figure 1. Although 13 percent of North Carolina’s wetlands were converted to agriculture during the 1970-1980's, remnant wetlands still can be found in the coastal plain.

Wetlands are protected under various laws, because they provide important benefits to people and ecosystems, including habitat for wildlife and protection from flooding (Figure 2). The loss of over half of the nation’s historical wetlands and increased agricultural fertilizer use have led to water pollution and negative impacts on fisheries (Zedler and Kercher 2005). Even a popular bumper sticker makes the connection between wetlands and healthy fisheries, saying simply, “No wetlands, no seafood.”

Yet drainage or filling of wetlands is still permitted by regulatory agencies, in part because a remedy known as wetland mitigation was invented to offset “unavoidable” wetland losses. A developer can get a permit to destroy existing wetlands if he mitigates these impacts by restoring or creating other wetlands. The same goes for development that impacts streams: restoring other streams makes new impacts to streams permissible. A booming industry has thus emerged for wetland and stream “credits,” with mitigation “banks” making acres of restored wetlands or linear feet of restored streams available to developers, to compensate for permitted environmental impacts. This policy hinges on the idea that created and restored streams and wetlands are essentially equivalent to naturally occurring ecosystems, provided certain monitoring criteria are fulfilled. Despite much scientific evidence showing that restored ecosystems lack many of the fundamental characteristics of natural systems, mitigation policies continue to promote such unbalanced substitutions. (More information on mitigation banking at U.S. EPA’s website.)

Wetlands
Figure 2. Wetlands are protected by law because they provide benefit to people and ecosystems, including habitat for wildlife and protection from flooding.

As we peered across Timberlake Restoration Project, with its 1000 acres of newly planted wetland tree seedlings still littered with corn stalks from the last harvest, the landscape looked far removed from a wetland or a stream (Figure 3, left side). But the massive drainage pump and canals that were keeping the water table low would be removed under the restoration plan, passively reflooding the landscape and reinstating the wind-tide hydrology. While other colleagues tackled the definition of a stream in the coastal plain, I wondered about the big picture of this restoration project from a biogeochemist’s perspective, considering the cycling of biologically important elements such as carbon, nitrogen, and phosphorus through ecosystems. One simple assumption is that wetlands remove both nitrogen and phosphorus from water, providing such an improvement to water quality that wetland nutrient reduction credits are tradeable commodities (for example, NC’s Nutrient Offset Program). Yet the conditions that promote nitrogen removal in wetlands can cause the release of phosphorus.  So would the restored wetland actually improve water quality downstream by decreasing both nitrogen and phosphorus levels, or would phosphorus fertilizers leftover from decades of farming be released to the estuary, causing additional phosphorus pollution? An additional question was whether the build-up of nitrogen fertilizers in the farmed soils would cause an increase in greenhouse gas emissions from the restored wetland, compared to drained farm fields or natural wetlands? These questions became the focus of my dissertation research.

Wetlands
Figure 3. Timberlake Restoration Project before reflooding (2006, grayscale) and three years later (2009, in color). Photoshopped by Ian Breckheimer, University of North Carolina, from original photos by the author.

After three years of environmental monitoring and experiments at Timberlake and nearby reference sites, my colleagues and I found a mixed bag of environmental consequences from reflooding this former agricultural wetland. While there is no question that this restored wetland is a haven for birds and wildlife, and most of the planted trees are thriving, we found that the site actually released phosphorus following reflooding, while retaining most of the nitrogen inputs (Ardón et al., in 2010). Concerns over increased greenhouse gas emissions did not materialize during this period, as we found no overall difference in greenhouse gas emissions across all the sites (Morse et al., in review). Aside from monetizing these ecosystem costs and benefits, it is difficult to calculate the net environmental result of this ecosystem restoration in non-monetary terms. Yet I feel confident that the restoration is a net environmental gain, especially with the cessation of water pollution from agricultural activities across the site.

But on the horizon are environmental and political realities that threaten the future of the Timberlake Restoration Project. Politics and delays in the mitigation banking system, along with higher prices for corn, have caused the property owners to consider restarting the drainage pump and plowing under the 750,000 trees they planted. A slower but more certain agent of change is the rising sea: saltwater intrusion from the estuary has begun to alter the freshwater habitat that the wetland trees or crops require, such that the restored forested wetland or farm field may be closer to a saltmarsh in the next century. In this regard, Timberlake Restoration Project is not unique: human-dominated and natural environments close to sea level are already experiencing major environmental changes and how these ecosystems will respond to these changes over the next century is yet to be determined.


References:

  • Ardón, M, J.L. Morse, E.S. Bernhardt, and M.L. Doyle. 2010. The water quality consequences of restoring wetland hydrology to a large agricultural watershed in the southeastern coastal plain. DOI: 10.1007/s10021-010-9374-x.
    Dahl, T. E. 1990. Wetlands losses in the United States: 1780's to 1980's. Washington, D.C . U.S . Department of the Interior, Fish and Wildlife Service.
  • Heimlich, R. E., K. D. Wiebe, R. Claassen, D. Gadsby and R. M. House. 1998. Wetlands and Agriculture: Private Interests and Public Benefits. Agricultural Economic Report. Washington, DC, US Department of Agriculture.
  • Zedler, J. B. and S. Kercher. 2005. Wetland resources: status, trends, ecosystem services, and restorability. Annual Review of Environment and Resources 30: 39-74.

Keywords:

coastal wetlands, greenhouse gas, mitigation, nitrogen, phosphorus, ecosystem restoration, sea level rise

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