Field Survey & Data Collection Flashcards

In the Corps of Engineers three-parameter method, dominant species in each vegetation stratum are identified using:

Answer: The 50/20 rule based on cumulative cover

The 50/20 rule designates as dominant any species contributing to the first 50% of cumulative cover (ranked highest to lowest) plus any additional species with ≥20% cover individually.

A plant assigned a National Wetland Indicator (NWI) status of 'OBL' is defined as one that:

Answer: Almost always occurs in wetlands (greater than 99% probability)

OBL (Obligate Wetland) plants occur in wetlands under natural conditions more than 99% of the time and are the strongest indicators of hydrophytic vegetation.

For the vegetation parameter to support a positive wetland determination using the Dominance Test, what threshold of dominant species must be hydrophytic (OBL, FACW, or FAC)?

Answer: More than 50% of dominant species

Under the Dominance Test, a positive vegetation indicator is established when more than 50% of dominant species across all strata are hydrophytes (OBL, FACW, or FAC).

Under the 50/20 rule, a species qualifies as dominant if it meets which condition?

Answer: It is among species whose cumulative cover reaches 50% when ranked, plus any species individually exceeding 20% cover

Species are ranked by decreasing cover; those that push cumulative cover to ≥50% are dominant, and any additional species with ≥20% individual cover are also included as dominants.

Which vegetation stratum is generally NOT assessed as a separate layer in a standard Corps of Engineers wetland determination?

Answer: Moss and lichen (cryptogamic crust) stratum

Standard Corps methodology assesses tree, shrub/sapling, herb, and woody vine strata; mosses and lichens are not evaluated as a separate stratum in routine determinations.

A plant listed as 'FACW' under NWI indicator status occurs in wetlands at what frequency under natural conditions?

Answer: Usually — between 67% and 99% of the time

FACW (Facultative Wetland) plants usually, but not always, occur in wetlands, with an estimated frequency of 67–99% of occurrences in wetland environments.

Which of the following best describes why the 'prevalence index' is used as an alternative to the Dominance Test in some vegetation assessments?

Answer: It provides a weighted calculation that accounts for the full range of NWI statuses across all species, not just dominants

The Prevalence Index weights cover values of all species by their NWI status scores, providing a more nuanced determination when dominant species alone give ambiguous results.