Evaluation of Restored Wetlands in the Prairie Pothole Region of the United States
Background and Justification
The PPR is a major producer of cereal grains and is the most important region in the United States for the production of dabbling ducks (Smith et al. 1964; Batt et al. 1989). The value of this region for agriculture has led to the loss and degradation of wetlands important to waterfowl (Stewart and Kantrud 1973, 1974; Kantrud et al. 1989), shorebirds, and other wetland-dependent wildlife (Duebbert 1981; Kantrud and Stewart 1984; Kantrud et al. 1989). Because of agricultural conversion, wetland losses have been severe in the PPR; less than 50% of the original 8 million ha of wetlands remain (Dahl 1990; Dahl and Johnson 1991).
Concern over wetland loss coupled with an increased knowledge of the functions and values of wetlands (Sather and Smith 1984) has resulted in considerable wetland restoration activity in the PPR. Based on number of wetland basins, more wetlands have been restored in the PPR (more than 40,000) than in any other region of the United States. This is due, in part to the vast number of ditch- and tile-drained wetlands that readily lend themselves to restoration; the importance of this region for waterfowl production; and the unprecedented opportunities to restore wetlands on private lands through the CRP, the Wetlands Reserve Program, and the North American Waterfowl Management Plan. Overall, the goal of wetland restoration has been to restore ecological functions, biological integrity, and diversity to approximate predrainage conditions to the extent practical. However, little scientific information was available to guide restoration activities to date and field personnel have used best professional judgement to facilitate restorations. Further, there has been little monitoring to determine if the physical and hydrologic characteristics of restored wetlands resemble their nondrained analogues. Similarly, the biological functions performed by restored wetlands have received little attention from the scientific community, especially in relation to their nondrained analogues. Documentation of the functions of restored wetlands relative to the pre-restoration conditions and to specific restoration techniques is needed to facilitate development and improvement of techniques for restoring wetlands.
Earlier Monitoring Efforts
The few studies conducted to examine the success of wetland restorations generally have shown that the success of wetland restorations is highly variable. LaGrange and Dinsmore (1989), and Sewell (1989) concluded that plant and invertebrate communities quickly recolonize after wetlands are reflooded; however, these studies did not include reference wetlands. More recent work including reference wetlands has shown that certain plant communities (e.g., wet meadow vegetation) may not recover following restoration (Galatowitsch 1993; Galatowitsch and van der Valk 1994, 1996). Further, lower breeding bird diversity in restored wetlands relative to their nondrained wetland analogues has been related to the lack of vegetative zone development in recently restored wetlands (Delphey and Dinsmore 1993).
The technology to restore PPR wetlands has changed little over the past several decades, and generally involves plugging ditches or tile drains to restore hydrologic regimes. A major assumption of restoration methodology is that once the hydrology is restored, recolonization of plant and invertebrate communities will occur naturally (i.e., from relic plant seed and invertebrate egg banks, or inoculum from nearby wetlands). However, anthropogenic disturbances to wetland basins and the surrounding landscape may make this assumption of natural recolonization untenable (Galatowitsch and van der Valk 1994). Due to extensive drainage of wetlands, nearby wetlands may not be available to provide an inoculum for restored wetlands. Further, most wetlands targeted for restoration are usually in previously farmed fields and most have substantial overburdens of sediments that greatly reduce the potential water depth of restored basins relative to predrainage conditions (Martin and Hartman 1987). Excessive sediment overburdens may also impede recolonization by covering relic seed and invertebrate egg banks. In addition, prolonged drainage coupled with years of cultivation can decimate relic seed (Erlandson 1987) and egg banks (Environmental Monitoring and Assessment Program; Euliss and Mushet unpublished data).
Wetland biologists have an incomplete understanding of the effect of prior land-use on the success of restored wetlands and how restoration techniques might be modified to enhance the success of specific restorations. Because most wetlands were restored without consideration of prior land-use impacts and without scientific guidance (Dan Stinnett, FWS-Region 3, pers. comm.), a fruitful approach appears to be one of determining the outcomes of restorations relative to land-use histories, site characteristics, and the restoration techniques employed. Knowledge of the ecological outcome of restored wetlands will provide valuable information to land-managers and help foster development of improved technology.
Approach and Objectives:
The study approach will consist of gathering available information on wetlands restored by state, federal, and private resource agencies in the United States PPR. This information will be used to develop a region-wide database on restorations to facilitate temporal and spatial characterization of restored wetlands. Additionally, we will conduct a one-time extensive survey to characterize the outcomes of various restorations in the United States PPR. Because of limited funding, the extensive survey will be restricted to seasonal and semipermanent restored wetlands on CRP lands or in similar grassland habitats; seasonal and semipermanent wetlands in CRP type habitats were determined as the most representative type of restoration completed by state and federal agencies (Restoration Planning Meeting 1996). The information provided by our extensive survey will be used to provide federal, state, and private collaborators who are conducting intensive studies with the linkage information they need to extrapolate their findings to regional perspective. The specific objectives of our study are:
- Develop a region-wide database of restored wetlands to facilitate temporal and spatial characterizations of wetland restorations in the United States PPR.
- Determine the effect of restoration age and prior land-use history (e.g., row crops, small grains, length of time drained or farmed) on recolonization, floristic quality, and diversity of wetland vegetation; and to compare the floristic composition of vegetation in restored wetlands to that of drained and nondrained wetlands in CRP habitats.
- Provide collaborators who are conducting intensive investigations with background information and linkage variables that can be used to extrapolate results from intensive studies to the larger regional sample.
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