Examining discrepancies among three methods used to make hydrophytic vegetation determinations for wetland delineation purposes

Abstract

Three methods for making vegetation determinations—the Prevalence Index (PI), the Dominance Ratio (DR), and the Hydrophytic Cover Index (HCI)—were compared using national wetland delineation data. The PI and the DR produced significantly fewer hydrophytic determinations (69%, p < 0.01, and 76%, p = 0.04) than the HCI (80%). The three methods disagreed 16% of the time. The HCI produced hydrophytic determinations only in plots where hydrophyte (Facultative, Facultative Wetland, and Obligate species) cover was greater than 50% of the total cover and never produced hydrophytic determinations in plots where hydrophyte cover was 50% or less. The PI disagreed with the HCI 12% of the time, producing nonhydrophytic determinations in plots where hydrophyte cover was greater than 50%. The DR disagreed with the HCI 9% of the time. Disagreements included nonhydrophytic determinations in plots where hydrophyte cover was greater than 50%, hydrophytic determinations in plots where hydrophyte cover was 50% or less, and a nonhydrophytic bias in plots dominated by even numbers of plant species. These results demonstrate that HCI determinations are more accurate and consistent than those of the PI and the DR. The HCI method is recommended for making vegetation determinations during wetland delineations in future revisions of the Corps delineation manual and its supplements.