Swamp Stomp

Volume 14, Issue 16

Last month the US Army Corps of Engineers (Corps) published several documents about how wetland vegetation is determined. They are reexamining the calculations and methods that we use to make wetland determinations. Some of the factors that the Corps considered for these studies include the importance of dominance (or not), plot sizes, the impact of stratum, and plot shape. All of these observations and conclusions are intended to be included in a new Corps wetland delineation manual.

This week we will discuss the document that focuses on the comparison between the formulas that are used to determine the presence or absence of a wetland plant community. The document is entitled,” Examining Discrepancies Among Three Methods Used to Make Hydrophytic Vegetation Determinations for Wetland Delineation Purposes,” (Lichvar and Gilrich, 2014).

The purpose and need statement for this document is quite clear. In the past there have been discrepancies between the use of the dominance test (DR) and the newer prevalence index (PI). The dominance test dates back to the original Corps Wetland Delineation Manual (1987). The prevalence index is now used in all of the regional supplements; however it first made its appearance to the regulatory scene in the 1989 Federal Manual. It was buried deep in the Federal Manual appendix as an advanced wetland delineation method.

There is a new method under consideration for replacement of both of these calculations. It is called Hydrophytic Cover Index (HCI). At present this method remains unpublished and is not to be used with any current wetland delineations. It will most likely be published in the Federal Register as part of the new Corps Wetland Delineation Manual. However, it is an interesting approach that appears to have much promise. Chief among its features is the elimination of the 50/20 rule for determining dominance. Have you ever tried to explain to a non-wetland person at a cocktail party what the 50/20 rule is? Well I have and it is not fun. For some reason, I do not get invited to many cocktail parties anymore.

There are several variables that were discussed in the Corps report that are interesting. Factors like number or strata, odd or even numbers of species, and plot size all had impacts on the traditional methods of determining a wetland plant community. It seems that the more strata you have the more likely you are going to find a wetland plant community. It also appears that plant communities that have odd number of species were more likely to be a wetland community than even numbered populations. In theory, these factors should have no bearing on the presence or absence of a wetland plant community. However, it appears that it does, hence the need to fix the problem.

The Hydrophytic Cover Index (HCI) method was compared to both of the existing in use methods. There were three tests. The first one compared bottom line results. In this test each method was run on data collected by the Corps in the field. The first test showed that the use of the HCI resulted in more areas being called hydrophytic. The PI (69%) and the DR (76%) produced significantly smaller percentages of hydrophytic vegetation determinations than the Hydrophytic Cover Index (HCI) (80%) produced.

The second test compared all three tests with the total number of all species found in the field. In situations where greater than 50% of the species were identified as Obligate (OBL), Facultative Wet (FACW) or Facultative (FAC) the HCI tested positive for hydrophytes 100% of the time. When the numbers of species with OBL. FACW and FAC indicators were less than 50% the HCI tested negative for hydrophytes 100% of the time.

The third test looked at the even versus odd number of species that were calculated using all three methods. The DR test seems to show that if you have an even number of species examined, there is a 10% decrease the area passing the DR test. The HCI and the PI tests did not seem to show this bias.

The actual procedure for using the HCI test was not described in the report. The formula was provided and is:

HCI = (Sobl + Sfacw + Sfac)/(Sobl + Sfacw + Sfac + Sfacu + Supl) × 100

All of the species identified in the study area are to be counted and run though this equation. It does not appear that the vegetation is separated into stratum (tree, shrub, herb, & woody vine). Dominance is also not calculated.

You should be able to use the data that you currently collect for The PI test to run this equation. It is fairly simple and very quick.

If the Corps and perhaps EPA decide to use this equation, it will be part of a new manual and put out for public comment. Until then the HCI method is not to be used for jurisdictional determinations. However, it might be worth your while to try it out on a few projects to see if there is any significant difference in the results. It would also be interesting to correlate the presence or absence of hydric soils and hydrology indicators with the HCI result. This would make comments to the new method a lot more meaningful and scientific when the time comes for the public to respond.

Have a great week!

Marc