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Forested Wetlands

Primary Question (chapter 20): What are the history, status, and likely future of forested wetlands in the South?

The South has numerous types of forested wetlands, which provide a diverse array of water-quality, flood attenuation, and wildlife habitat benefits. Wetlands of various types have experienced significant structural changes and permanent losses. While the rate of wetland loss is declining, the South has lost more than half of its wetlands between colonial times (1780) and the 1990s.

In the late 1990s, the 10 Southeastern States (the Assessment region minus Virginia, Texas, and Oklahoma) contained approximately 32.6 million acres of forested wetlands (chapter 20). More than half of the region’s wetlands are contained in the Coastal Plain provinces.

Forested wetlands are highly diverse but can be placed into three major categories: riverine, depression, and flatwoods.

Riverine wetlands—They represent the vast majority of forested wetlands in the South. They are located on floodplains and riparian corridors associated with streams and include deepwater swamps and alluvial floodplains that range in size from narrow riparian strips to broad alluvial valleys. The major water sources are overbank flooding and subsurface connections with stream channels. Riverine forested wetlands store water, intercept and cycle nutrients and toxicants, and provide environments for wetland flora and fauna. They may mitigate flood damage, enhance water quality, and support characteristic ecosystems. Because they provide linear connections across landscapes often dominated by other land uses, these types of wetlands also serve as important corridors for the dispersal of wildlife species.

Depression wetlands—They include pocosins, Carolina bays, pond cypress swamps, and mountain fens. These wetlands derive from topographic depressions and receive water from a variety of sources including precipitation, overland flow, and ground water. As with riverine types, benefits are associated with water retention and filtration and provision of wildlife habitat. Most depression wetlands are located low in a watershed, allowing them to play an especially important role in removing sediments, nutrients, and pollutants. Because their vegetative structure is typically very heterogeneous, these areas support highly diverse plant and animal communities. Carolina bays are located throughout the Atlantic Coastal Plain, and cypress domes are located primarily in Florida. Both provide especially important breeding grounds for imperiled amphibians.

Mineral soil flats—They are predominately wet pine flats, shaped by a combination of fire and water regimes. They are commonly found on areas between rivers—called interfluves—on extensive lake bottoms or on large floodplain terraces. The main sources of water are precipitation and slow drainage associated with a landscape of low relief. If subjected to periodic fire, these flats have very few trees. Frequent fire and an uninterrupted hydrologic regime yield a highly diverse and unusual floral assemblage. Their herbaceous species richness is considered to be among the highest in the Western Hemisphere.

Riverine wetland types dominate in the South, representing 91 percent of forested wetland area, compared to approximately 2.5 million acres of mineral soil flats and less than 1 million acres of depression wetlands. We found that (chapter 20):

• All three types of forested wetlands have been and continue to be influenced directly and indirectly by development. According to the National Wetland Inventory, 3.5 million acres of southern forested wetland underwent changes between 1986 and 1997. Ninety percent of the changes were conversions to another wetland or aquatic habitat type. Of these conversions 95 percent were to scrub-shrub or emergent wetlands. During the same time period, approximately 119,000 acres of forested wetland went into urban and rural development, 112,000 acres to agriculture, and 102,00 acres to intensive silviculture. Draining and filling of wetlands impairs water storage capacity and therefore flood protection capacity and water filtration functions.

• Upstream urban development and certain agricultural and silvicultural practices can alter wetland functions.

• Changes in vegetation, species composition, soils, and wildlife use resulting from forest management activities are often transitory, especially when stands are naturally regenerated following harvest.

• The effects of silvicultural treatments on nutrient cycling and pollutant sequestration within wetlands are uncertain.

Drainage and bedding of wetlands can impact soils, hydrologic function, and some species of wildlife. This is an especially important concern for some amphibian species.

An examination of wetland history and status revealed:

• The 10 Southeastern States (the Assessment region minus Virginia, Texas, and Oklahoma) contain approximately 32.6 million acres of forested wetlands (chapter 20). They represent 64 percent of the total in the coterminous United States.

• Forested wetland losses have been widespread, but have been concentrated in the Lower Mississippi Alluvial Valley, where losses were primarily to agriculture, and Coastal Flats of North and South Carolina, where losses have been to urbanization, agriculture, and silviculture.

• Rates of wetland losses have declined since the 1970s, largely as a result of Section 404 of the Clean Water Act, which regulates dredging and filling of wetlands. However, functional changes continue to occur in some forested wetlands as a result of continued urbanization, intensification of forest management, fire exclusion, and alteration of hydrologic regimes.

• Wetland restoration efforts pursuant to the Wetlands Reserve Program are concentrated in the Lower Mississippi Alluvial Valley (fig. 47). Tree planting success has often been acceptable, but restoration of the ecological functions of these wetlands is difficult to determine and presently unknown.

• Information on mitigation and restoration pursuant to Section 404 of the Clean Water Act is not systematically compiled and reported. Thus, successful implementation of required mitigation is not reportable.