Vegetation development in created, restored, and enhanced mitigation wetland banks of the United States

Wetlands (Impact Factor: 1.44). 03/2005; 25(1):51-63. DOI: 10.1672/0277-5212(2005)025[0051:VDICRA]2.0.CO;2

ABSTRACT 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 10m2 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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