DISAGREEMENTS BETWEEN PLOT-BASED PREVALENCE INDEXES AND DOMINANCE RATIOS IN EVALUATIONS OF WETLAND VEGETATION

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.