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Influence of Wetland Indicator Status Ratings on the Extent of Hydrophytic Vegetation during Wetland Delineations


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This study evaluated how far and how often hydrophytic vegetation exceeded the wetland boundary in three U.S. Army Corps of Engineers wetland regions. Wetland boundaries were delineated using methods described in the Corps of Engineers Wetland Delineation Manual and appropriate Regional Supplements. Hydrophytic vegetation determinations were made using the Dominance Ratio, the Prevalence Index, and the Hydrophytic Cover Index. Distances and frequencies were tested using nonparametric statistics. Hydrophytic vegetation exceeded the wetland boundary by 549 m, in 65% of the upland plots. Differences in distance (p = 0.88) or frequency (p = 0.14) among the three methods were not significant. Most (83%) of the species with hydrophytic ratings occurred in wetland and upland plots, suggesting that NWPL ratings may overestimate wetland frequency in some circumstances. Hydrophytic vegetation determinations in uplands were driven by 38% of FACW and 54% of FAC species, which exhibited an upland abundance greater than or equal to that of wetlands. These data indicate that hydrophytic vegetation is a poor indicator of the wetland boundary when used without considering hydrology and soils. Periodic review of NWPL ratings is recommended to improve accuracy of delineated boundaries.



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