permit requirements often suggest the need to consider area and function, structural characteristics (usually the amount of vegetation cover) may be used as a criterion to judge whether functional replacement is achieved (e.g., Kentula et al. 1992b). Vegetation structure (e.g., percent cover) is a pervasive example of one structural attribute that is often the default indicator of wetland function. One example of such a singular criterion is the Floristic Quality Assessment developed by Swink and Wilhelm (1979, 1994) for wetlands in the Chicago region and several Midwest states1 (Andreas and Lichvar 1995; Taft et al. 1997; Herman et al. 1996; Mack et al. 2000). The floristic and similar approaches basically characterize a mitigation site solely on the vegetation present. The assumption underlying this approach is that wetland vegetation is a comprehensive indicator of the hydrological and ecological status of the site, and specific vegetation parameters can be used to indicate the functions of the mitigation site. The reasoning behind this approach is that if the vegetation community is healthy and has “natural” species diversity, the ecological components (e.g., physical, biological, and biochemical) that support the vegetation must be present.

In Swink and Wilhelm's (1979, 1994) application of Floristic Quality Assessment, indicators are based on the site in question; thus, a riparian system would have completely different vegetation parameters than a coastal salt marsh. In the Floristic Quality Assessment, each plant species was assigned a coefficient of conservatism (C) ranging from 0 (ubiquitous species) to 10 (species having narrow habitat tolerances), based on the authors' knowledge of the flora of the Chicago region (Swink and Wilhelm 1979, 1994). Other indicators used in past evaluations include percent canopy and/or ground cover, percent survival of specific indicator species, tree height, and species diversity. In many areas, floristic assessment has been the method of choice because vegetation parameters are easy to measure, provide a dramatic visual indicator of compliance (full canopy, tall trees), and allow resource agencies to write well-defined performance criteria for the mitigation sites.

However, the assumptions and premises of the floristic approach are often unclear or incompletely specified when examining the regional spectrum of wetland types. Low plant diversity is not always characteristic of “inferior” hydrogeological and geochemical settings, and high plant diversity is not necessarily a de facto indicator of the multitude of wetland functions (e.g., NRC 1995). Systematic assessment of more than just floristic quality indicators reduces dependence on such speculative assumptions.

1  

D.M.Ladd. The Missouri Floristic Quality Assessment system. The Nature Conservancy, St. Louis, MO, in preparation.



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