Vegetation Classification and Mapping Project Report - USGS

USGS-NPS Vegetation Mapping Program
Colonial National Historical Park
The USNVC is currently maintained and updated by NatureServe. Additional support has come
from federal agencies, the FGDC, and the Ecological Society of America. Refinements to the
classification occur in the process of application, leading to ongoing proposed revisions that are
reviewed both locally and nationally. A two volume publication (Grossman et al. 1998)
presenting the standardized classification, and providing a thorough introduction to the
classification, its structure, and the list of vegetation types found across the United States as of
April 1997 can be found on the Internet at: http://www.NatureServe.org/publications/library.jsp.
Volume II of this publication (the classification listing) has been superceded by the online
database server, NatureServe Explorer®, http://www.NatureServe.org/explorer 1 , which provides
regular updates to ecological communities in the United States and Canada.
The USNVC is a hierarchical system that classifies vegetation using physiognomic (structural)
features at the highest levels of the hierarchy and floristic features at the lower levels. The
physiognomic units have a broad geographic perspective and the floristic units have local and
site-specific perspectives (The Nature Conservancy and Environmental Systems Research
Institute 1994a; Grossman et al. 1998). The USNVC includes most existing vegetation, whether
natural or cultural, but attention is focused on natural vegetation types. Natural vegetation, as
defined in The Nature Conservancy and Environmental Systems Research Institute (1994a),
includes types that “occur spontaneously without regular management, maintenance, or planting
and have a strong component of native species”. Cultural vegetation includes planted/cultivated
vegetation types such as orchards, pastures, forestry plantations, and vineyards.
The physiognomic-floristic classification includes all upland terrestrial vegetation and all
wetland vegetation with rooted vascular plants. The hierarchy has five physiognomic levels and
two floristic levels (Table 1). The basic unit of the physiognomic portion of the classification is
the formation, a type defined by dominance of a given growth form in the uppermost stratum and
characteristics of the environment (e.g., cold-deciduous alluvial forests). The physiognomic
portion of the classification is based upon the United Nations Educational, Scientific, and
Cultural Organization world physiognomic classification of vegetation, which was modified to
provide greater consistency at all hierarchical levels and to include additional types (UNESCO
1973).
The floristic levels include alliances and associations. The alliance is a physiognomically
uniform group of plant associations that share dominant or diagnostic species, usually found in
the uppermost strata of the vegetation. The association is the finest level of the USNVC. An
association is defined as a plant community type that is relatively homogeneous in composition
and structure and occurs in a uniform habitat. For example, Mid-Atlantic Mesic Mixed
Hardwood Forest (Fagus grandifolia - Quercus (alba, rubra) - Liriodendron tulipifera / (Ilex
opaca var. opaca) / Polystichum acrostichoides Forest) is a common forest type on well drained,
acidic midslopes in the Piedmont and Coastal Plain of Virginia. Associations are also assigned
global rarity ranks that indicate their conservation status and relative risk of extirpation
(Grossman et al. 1998). Associations from the USNVC are cross-referenced to communities in
1 As of this writing, cultural, early successional / ruderal, and exotic (i.e. nonnative plot species)
dominated vegetation units defined in the USNVC are not served on NatureServe Explorer. Additional
information on these types can be obtained from a NatureServe regional or national vegetation ecologist.
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