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WETLANDS, Vol. 26, No. 2, June 2006, pp. 624–634
2006, The Society of Wetland Scientists
NOTE
LISTS OF POTENTIAL HYDROPHYTES FOR THE UNITED STATES: A
REGIONAL REVIEW AND THEIR USE IN WETLAND IDENTIFICATION
Ralph W. Tiner
U.S. Fish and Wildlife Service
National Wetlands Inventory Program, Northeast Region
300 Westgate Center Drive
Hadley, Massachusetts, USA 01035
E-mail: ralph tiner@fws.gov
Abstract: The U.S. federal government has developed lists of plant species that occur in wetlands. The
initial purpose of these lists was to enumerate plants that grow in wetlands and that could be used to identify
wetlands according to the U.S. Fish and Wildlife Service’s wetland classification system. The first list was
generated in 1976 by the Service, and since that time, the list has undergone several iterations as more
information was reviewed or became available through field investigations and scientific research. Two lists
are currently published and available for use: a 1988 list and a 1996 draft list. The latter list represents an
improvement based on nearly 10 years of field work by the four signatory agencies plus comments from
other agencies, organizations, wetland scientists, and others. The national list was generated from 13 regional
lists. These data have not been summarized previously; this note provides an interregional summary of vital
statistics. The 1988 list included 6,728 species, while the 1996 list has nearly 1,000 additions for a total of
7,662 species (a 14% increase). Roughly one-third of the nation’s vascular plants have some potential for
being hydrophytes—plants growing in water or on a substrate that is at least periodically deficient in oxygen
due to excessive wetness. Each species on the list is assigned an indicator status reflecting its frequency of
occurrence in wetlands: 1) obligate (OBL; 99% of time in wetlands), 2) facultative wetland (FACW; 67–
99% in wetlands), 3) facultative (FAC; 34–66%), 4) facultative upland (FACU; 1–33%), and 5) upland
(UPL; 1%). From 1988 to 1996, the regional lists of potentially hydrophytic species increased by more
than 39 percent in three regions: Caribbean, North Plains, and Central Plains. The percent of OBL, FACW,
and FAC species on the lists decreased in the Northeast and Hawaii. The percent of OBL and FACW species
also decreased in the Southeast and Northwest. The number of OBL species declined in all but three regions,
whereas the number of FACU species added to the lists increased in all regions except Hawaii. The regional
‘‘wetland plant’’ lists have been used to help identify plant communities that possess a predominance of
wetland indicator plants (i.e., a positive indicator of hydrophytic vegetation) and to identify wetlands that
can be recognized solely based on their vegetation.
Key Words: wetland plant lists, hydrophytes, hydrophytic vegetation, wetland ecotypes, National Wetlands
Inventory, prevalence index, wetland identification, wetland delineation
INTRODUCTION
When the U.S. Fish and Wildlife Service (FWS)
initiated its National Wetlands Inventory Program
(NWI) in the mid-1970s, one of the first tasks was to
develop a wetland classification system to serve as the
standard for mapping wetlands across the country. The
classification system went through a few versions and
field testing prior to its publication as ‘‘Classification
of Wetlands and Deepwater Habitats of the United
States’’ (Cowardin et al. 1979). This classification sys-tem
has been used for wetland mapping for over 25
years. On December 17, 1996, it was adopted by the
Federal Geographic Data Committee as the national
technical standard for wetland classification (FGDC-STD-
004) when reporting on wetland status and trends
and for geospatial data entered into the national geos-patial
database (http://www.fgdc.gov/standards/status/
swgstat.html; http://www.nwi.fws.gov/fgdc/certificate.
pdf). The FWS’s wetland definition listed the predom-inance
of hydrophytes and undrained hydric soils as
two main indicators of wetlands. Moreover, the doc-ument
referenced that the FWS was preparing a list of
‘‘hydrophytes and other plants occurring in wetlands
of the United States.’’
The initial list compiled in March 1976 by the FWS
contained only 1,626 species and was considered ‘‘ob-
