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4 Data Sources

Status and trends of southern forested wetlands were derived from National Wetland Inventory (NWI) reports for the United States and the Southeast (Dahl 1990, Dahl 2000, Hefner and Brown 1985; Hefner and others 1994). These reports also provided information on the causes of forested wetland loss. The NWI was undertaken by the U.S. Fish and Wildlife Service to provide a comprehensive inventory of the Nation's wetlands. The NWI is conducted at 10-year intervals. Gains and losses of wetlands are estimated using aerial photographs, soil surveys, topographic maps, and field work on a permanent set of randomly selected points (Shepard and others 1998, Dahl 2000). These photos are analyzed for a selected 10-year interval to detect changes in wetlands. Quality control is included throughout the data collection and analysis stages, and 21 percent of the plots are field verified (Dahl 2000). Studies have been completed for the 1950s to 1970s, 1970s to 1980s, and 1980s to 1990s.

Since NWI is used as the primary source of status and trends data for this chapter, terminology used by NWI in reporting changes in forested wetlands (Dahl 2000) is important to understand. Terms regarding wetland types and land-use definitions can be found in Dahl (2000). However, two pivotal terms are defined here. "Conversion" is a change in vegetative cover on an area that is still a wetland. In other words, when a forested wetland is "converted" it remains a wetland (i.e., soils and hydrology remain intact) but the dominant vegetation is changed. Wetland "loss" is a change in which an area no longer has the hydrologic characteristics of a wetland. "Losses" involve the detection on high resolution aerial photographs of: (1) significant hydrologic alterations such as large ditches and levees, (2) soil alterations such as filling or leveling, and (3) upland vegetation indicating the wetland character of a site has been removed.

The National Resources Inventory (NRI), prepared by the Natural Resources Conservation Service, is an inventory of multiple natural resource conditions on non-Federal land in the United States (Shepard and others 1998). The purpose of the NRI is to provide information for policymaking in natural resource conservation programs at State and Federal levels. The NRI is based upon stratified random samples distributed throughout the country. Data are collected using aerial photographs and ancillary data and by making select field visits.

Forest Inventory and Analysis (FIA) data gathered by the USDA Forest Service also were used in this report. The purpose of FIA is to provide information on forest resources at the local, State and national levels. The evaluations are State-by State multiple resource inventories of land use, timber, wildlife, range, recreation, water and soils completed on a 7- to 10-year cycle. Data in this report were collected between 1989 and 1998 during the forest surveys in Virgina, North and South Carolina, Georgia, and Florida from field plots that met Federal wetland criteria (areas having wetland soils, plants and hydrology) (Brown and others in press).

Scientific literature including HGM models for low gradient riverine wetlands (Ainslie and others 1999, Smith and Klimas in press), pine flatwood wetlands (Rheinhardt and others 2001), hardwood flat wetlands (Smith and Klimas in press) and forested depressions (Smith and Klimas in press), were reviewed as a means to hypothesize the effects of conversion on the structure and function of forested wetlands. Information on land ownership and timber harvests came from FIA data and Brown and others (in press). Wetland restoration literature and university studies on the extent and success of wetland restoration also were reviewed.