Compensating for Wetland Losses Under the Clean Water Act (2001) / Chapter Skim
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2 Outcomes of Wetland Restoration and Creation
2 Outcomes of Wetland Restoration and Creation
Pages 22-45
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Wetland Types That Have Been Restored and Created The committee evaluated restored and created wetlands from around the United States, including coastal and inland projects, by reviewing numerous scientific studies (see Appendix A) and by visiting several wetland mitigation sites.
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Mixed results have been reported in the restoration of wet prairies and sedge meadows (Galatowitsch and van der Valk 1996; Ashworth 1997~. Certain floristic assemblages, such as sedge meadows visited by the committee in the Chicago area, require extensive planting and intensive management in order to maintain the desired species composition.
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. · Microbial and animal components of these ecosystems are readily dispersed by the widely circulating ocean waters that connect distant seagrass beds and/or salt marshes.
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Another problem is that the uplands surrounding vernal pools also are in short supply and support rare species. Thus, as the available land decreases, vernal pool creation and restoration increasingly conflict with efforts to preserve habitat for other endangered plants and animals.
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Bog drainage exposes the organic soil to aeration, accelerating decomposition and fundamentally altering organic carbon compounds in the soil. Agricultural uses of bogs further alter soil chemistry and structure through tillage, fertilizer inputs, and subsidence as soils compact and oxidize.
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Thus, a site that has wetland structure in terms of its vegetation assemblage might not provide the function of denitrification if these four requirements are not met. FIVE WETLAND FUNCTIONS The following sections discuss five major wetland functions that warrant attention in evaluating wetland restoration and creation: hydrological functions, water-quality functions, support of vegetation, support of habitat for fauna, and soil functions.
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Hydrological Function Hydrology is most often cited as the primary driving force influencing wetland development, structure, function, and persistence. Consequently, establishment of the appropriate hydrology is fundamental to wetland mitigation through either restoration or creation.
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Since wetland hydrology is fundamental to wetland structure and function, those who implement restoration and mitigation projects have also tended to take a conservative stance, that is, to err toward the wet end of the transition zone (12.5% inundation or saturation during the growing season; see Box 2-1~. The structure and character of a wetland that is inundated or saturated 5% of the growing season differ greatly from those of one that is inundated or saturated 12.5% of the growing season.
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It is entirely possible for the restoration or creation site to have water-quality functions superior to those of the impact site. If the impacted wetland is a mineral or organic soil flat (Brinson's 1995 classification)
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Mitigation sites that receive nutrientrich surface-water runoff are well situated to perform water-quality-improvement functions, but biodiversity-support functions may suffer in the process. Wetlands that are designed to maximize the water-treatment function typically become monotypes of invasive species within a few years, even if they are initially planted to multiple species (Kadlec and Knight 1996~.
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Soils that are high in heavy metals may release toxic forms, such as methylmercury and selenium, when creation of a wetland induces anaerobic conditions. In wetland restoration and creation projects, soil is generally viewed as merely a rooting medium for the plants that are desired (the first function listed above)
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The vertical distribution of organic matter was also different in the profiles of created versus reference wetlands in Pennsylvania, with greater horizonation in the reference wetland soils (soil organic matter content significantly greater at depths of 5 centimeters (cm) than at 20 cm)
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reported lower concentrations of plant-available nutrients in a created salt marsh than in a reference salt marsh, which may have implications for plant primary productivity (Zedler and Langis 1991~. Differences in the activity of soil microbes that control nutrient availability may also occur in created versus restored wetlands (Groffman et al.
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The committee concludes that in a degraded wetland situation, many wetland functions are difficult to restore to their pre-disturbance condition. The ability to replace wetland functions depends on the particular function, the restoration-creation approach used, and the degree of degradation at the compensation site.
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The return of natural water levels, however, might not restore soil chemistry or the ability to support calciphilic species, as discussed earlier. Important groundwater parameters are often inadequately characterized at reference and mitigation sites (Hunt 1996; Hunt et al.
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1999~. Position in the landscape affects a number of wetland functions, such as water quality (Johnston et al.
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Thus, the mitigation design should recognize and accommodate hydrological variability and extremes caused by climate. Wetland restoration and enhancement should be preferred over creation in such areas.
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. Unless actively controlled at the outset, exotic and weedy plant species often dominate restoration sites.
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Created wetlands are generally not designed to meet the needs of animals found in the impacted wetland; hence, animal species' richness is often low in mitigation sites. Amphibians are perhaps the best-studied group of organisms from the standpoint of wetland dependency both in terms of association with a particular wetland and the landscape pattern of wetland connectivity.
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The animal components of communities may develop at quite different rates than do plants at mitigation sites. Mobile species may migrate in and colonize with the first floodwaters.
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Results from a variety of trajectory analyses of soil, plant, and animal communities for mitigation sites are given in Appendix C and summarized in Table 2-2. Plant cover and biomass for some marshes may reach TABLE 2-2 Time Toward Equivalency for Soil, Plant, and Animal Components in Wetland Restoration Projects Compared with That of Natural Reference Wetlands Structural Component Number of Sites Range (years)
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However, because there has · = Soil organic matter ·= Soil total Kjeldahl nitrogen 0 = Total stem length of cordgrass as a surrogate for above-ground biomass ~ = Number of cordgrass stems taller than 90 cm FIGURE 2-2 Long-term data for salt marshes constructed in San Diego Bay. Values are in relation to the adjacent natural salt marsh.
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will often need to be increased to achieve functional equivalency, rather than simply matching wetland area. The above review of wetland restoration and creation outcomes has not differentiated projects on the basis of their starting conditions.
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In conclusion, the literature and long-term trajectories reported therein suggest that wetland restoration and creation sites do not often achieve functional equivalency with reference sites within 5 years; indeed, up to 20 years may be needed for functional attributes to be determined or assessed correctly. RECOMMENDATIONS On the basis of its evaluation of wetland structure and function, the committee makes the following recommendations for compensatory mitigation: 1.
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