Js. Wakeley et Rw. Lichvar, DISAGREEMENTS BETWEEN PLOT-BASED PREVALENCE INDEXES AND DOMINANCE RATIOS IN EVALUATIONS OF WETLAND VEGETATION, Wetlands, 17(2), 1997, pp. 301-309
Methods for wetland identification and delineation require the investi
gator to determine whether vegetation is hydrophytic. Two widely used
techniques for making hydrophytic vegetation decisions involve dominan
ce ratios (i.e., the percentage of dominant species that are rated obl
igate (OBL), facultative wetland (FACW), and facultative (FAG)) and pr
evalence indices (i.e., the weighted-average wetland indicator status
of all plants present). We sampled 338 vegetation plots on sites throu
ghout the United States and calculated the dominance ratio and a plot-
based prevalence index for each plot. We found that hydrophytic vegeta
tion decisions based on the two methods disagreed on 16% of field plot
s. Analysis of simulated plot data (n = 80,000) indicated that frequen
cies of disagreement increase as vegetation complexity (i.e., number o
f strata and number of species per stratum) increases. We conclude tha
t the two methods for hydrophytic vegetation decisions disagree too of
ten to be considered equivalent. Additional studies are needed in diff
erent biogeographic regions and plant community types to determine the
conditions under which prevalence indices, dominance ratios, or some
other treatment of vegetation data provide more reliable indicators of
wetland vegetation.