WETLAND BOUNDARY DETERMINATION IN THE GREAT DISMAL SWAMP USING WEIGHTED AVERAGES¹

Abstract

A weighted average method was used to analyze transition zone vegetation in the Great Dismal Swamp to determine if a more uniform determination of wetland boundaries can be made nationwide. The method was applied to vegetation data collected on four transects and three vertical layers across the wetland‐to‐upland transition zone of the swamp. Ecological index values based on water tolerance were either taken from the literature or derived from local species tolerances. Wetland index values were calculated for 25‐m increments using species cover and rankings based on the ecological indices. Wetland index values were used to designate increments as either wetland, transitional, or upland, and to examine the usefulness of a provisional wetland‐upland break‐point. Most increments were designated wetland or transitional when all species were used. Removal of three or five ubiquitous species either gave a wider range of wetland index values with a more variable designation of increments or caused designation of increments to be similar for all layers. The use of locally‐derived rankings showed the sensitivity of the weighted averages method to ecological indices of species with large importance values. The weighted average method did not provide for an objective placement of an absolute wetland boundary, but did serve to focus attention on the transitional boundary zone where supplementary information is necessary to select a wetland‐upland breakpoint.

Use of created Wetland delineation and weighted averages as a component of assessment

“…Weighted averages have been described as useful for assessing wetland status of some vegetation types along a moisture gradient for wetland delineation under natural conditions (Carter et al 1988, Wentworth et al 1988, and Scott et at. 1989.…”

“…For direct gradient analysis of vegetation along a single environmental gradient, such as moisture, weighted averages (WA) may be used (Whittaker 1978). Weighted averages have been described as useful for assessing wetland status of some vegetation types along a moisture gradient for wetland delineation under natural conditions (Carter et al 1988, Wentworth et al 1988, and Scott et al 1989. Weighted averages are also used in the "Comprehensive Onsite Determination Method" described in the Federal Manual, where it is referred to as a "'prevalence index" and is an optional method for determination of the vegetation parameter.…”

Vegetation Sampling for Wetland Delineation: A Review and Synthesis of Methods and Sampling Issues

“…An alternative approached for determining whether a plant community is hydrophytic is the "weighted average" method, also called the "prevalence index" (Carter et al 1988;Wentworth et al 1988). All species in a community (not just a subset of dominants) are used to calculate a weighted-average wetland indicator status by multiplying indicator status category ratings (OBL = 1, FACW = 2, FAC = 3, FACU = 4, and UPL = 5) by the relative abundance of species in the sample.…”

Correspondence between Vegetation and Soils in Wetlands and Nearby Uplands

“…1987, and Carter et al 1988. Intraspecific morphological variation along such gradients also has been identified and suggested as a way of separating wetland from upland (Seliskar 1983(Seliskar , 1985a.…”