Field testing new plot designs and methods for determining hydrophytic vegetation during wetland delineations in the United States

Abstract

Recommendations made by the National Technical Committee on Wetland Vegetation (NTCWV) for vegetation sampling and for assessing hydrophytic vegetation were tested for possible inclusion in the update of the Army Corps of Engineers Wetland Delineation Manual. A new method, the Hydrophytic Cover Index (HCI), was used to determine if vegetation was hydrophytic. Plot shape and size, rectangular (10 × 2 m) vs. nested circular (9 m and 2 m radii used in the Corps 1987 Manual), were tested for differences in hydrophytic vegetation outcomes. HCI determinations produced from calculations using 100%, 90%, and 80% of the total cover in each plot were also tested for differences. Differences in plot size or shape had no effect on the percentage of hydrophytic vegetation determinations produced by the HCI. Calculating the HCI using 100%, 90%, and 80% of the total cover in delineation plots also had no effect on the outcome. Therefore, to determine if hydrophytic vegetation is present, it is not necessary to identify more than 80% of the total cover to the species level. To apply the 80% approach, the NTCWV suggests ranking the species cover in descending order and selecting the 20% from the least common species in the plot. Once 80% of the total cover in a plot has been positively identified, the remaining cover may be disregarded. If the 80% approach determines that the vegetation is nonhydrophytic (≤50%) but hydric soil and wetland hydrology indicators are present, Chapter Five in the appropriate Regional Supplement should be consulted to determine if problematic vegetation is present (e.g., USACE 2007).