Wetland mitigation banking is the practice of creating, restoring, enhancing, or preserving large, off-site wetlands to compensate for authorized impacts to natural wetlands. By 2002, there were 219 active mitigation banks in the United States, encompassing 50,000 hectares in 29 states. This study is the first systematic analysis of the ecological quality of these ecosystems; the objective is to determine if mitigation banks are successfully supporting native wetland vegetation and if success differs by mitigation method (created, restored, or enhanced), geomorphic class, age, or area. I obtained monitoring reports from 45 randomly selected mitigation bank wetlands in 21 states to evaluate three measures of ecological status: the prevalence of wetland vegetation, the pervasiveness of non-native species, and plant species richness. Sites range from less than one ha to over 560 ha and include 17 created wetlands, 19 restored wetlands, and 9 enhanced wetlands. Prevalence Index scores (PI; 1.0 for obligate wetland vegetation to 5.0 for upland vegetation) do not differ by wetland area but are significantly lower in created wetlands and significantly decrease from one- and two-year-old created wetlands (PI=2.37±0.15; mean±SE) to those five to seven years old (PI=1.96±0.12). Created and restored wetlands support 12.4 and 12.2 species per 10 m2 respectively, nearly four times more than the 3.2 species in 10 m2 of enhanced wetland. This is in part attributable to a greater incidence of non-native species in created and restored wetlands. The vegetative cover in created mitigation bank wetlands is 18.9±2.8 percent non-native–statistically similar to that of restored (17.6±2.9) but significantly greater than that of enhanced systems (8.7±2.7). Within mitigation methods, there are clear differences among geomorphic and vegetation classes. Depressional systems with a single vegetation class support highly hydrophytic, highly non-native communities with low species richness, while restored and enhanced riverine systems have a greater prevalence of native species. For mitigation bank wetlands in this study, the prevalence of wetland vegetation, the representation of native species, and the plant community homogeneity increase with age, indicating a period of self-organization and a potential trend toward vegetative equivalence with natural wetlands.
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Vol. 25 • No. 1