Discrepancies in Hydrophytic Determinations Produced by Three Vegetation Formulas Used for Wetland Delineations

Abstract

We examined disagreement among three methods used in the USA to make hydrophytic vegetation determinations during wetland delineations: the Dominance Ratio (DR), the Prevalence Index (PI), and the FAC-neutral Dominance Ratio (FN-DR). We had two objectives: to determine whether the number of dominant species in a plot affects the percentage of hydrophytic vegetation determinations made by each of the three methods and, if so, to explain the mathematical origin of disagreements among the methods. We compared the percentage of hydrophytic vegetation determinations produced when each method was applied to 200,000 simulations. The PI was the most consistent method for making hydrophytic vegetation determinations. We found that the DR is biased toward nonhydrophytic vegetation when there is an even number of dominant plant species in a plot; it is biased toward hydrophytic vegetation when there is an odd number of dominants. As the number of dominant species and strata increased, there were >20% more hydrophytic determinations made when we used DR than when we used PI. The FN-DR was also biased; it consistently produced fewer hydrophytic determinations than the other methods. When the DR disagrees with hydric soil and hydrology indicators, delineators should re-examine vegetation using the plot-based PI approach.