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Progressing Toward a Standardized Classification of Vegetation for the U.S.

Michael D. Jennings
National Gap Analysis Program, Moscow, Idaho

Since 1989, GAP land cover mappers have been in the unenviable position of applying a vegetation classification system before it was well developed. During this time, GAP funded The Nature Conservancy (TNC) regional ecologists to produce first-ever regional classifications of alliance vegetation types. These classifications are now complete for the Northeast (Sneddon et al. 1994), Southeast (Weakley et al. 1997), and Midwest (Drake and Faber-Langendoen 1997). The Western region classification (Bourgeron and Engelking 1994) is being redone by Marion Reid and colleagues (TNC Western Regional Office) to provide full descriptions of each alliance, including California as well.

In Bulletin No. 5, I reported the establishment of the Ecological Society of America’s Vegetation Classification Panel (ESA-Veg.). The panel’s mission is to provide a standardized, scientifically credible classification of vegetation for the U.S. in partnership with TNC and the Federal Geographic Data Committee (FGDC). This article provides a bit more background about vegetation classification in the U.S. and an overview of the ESA-Veg. status. Some of the following is based on material from a draft ESA-Veg. manuscript discussed below.

Today, the United States is close to having its first fully functional, widely applied vegetation classification, that, based on the tradition of systematics, will be improved upon as our knowledge expands. On October 22 of this year, the Secretary of the Interior, acting as Chair of the FGDC, gave final approval to the Vegetation Information and Classification Standard (FGDC 1997). This is now the standard vegetation classification for U.S. federal agencies and their cooperators. Those already familiar with the classification will recall its two-part structure of classifying vegetation by physical and environmental attributes in one part and by floristic assemblage in the other part (Table 1). While the FGDC’s "Vegetation Information and Classification Standard" includes a full classification of the physiognomic part, it only describes the floristic classification in concept because this part of the classification is far from being fully developed and requires a very large effort (Loucks 1996).

The ESA has joined with TNC, federal agencies, and others to meet the need for a unified floristically-based classification for the U.S. The intent is in strengthening the existing classification system by providing a mechanism for its refinement through scientific review and by disseminating its standards. These objectives are being accomplished in phases, with the initial focus having been on review and improvement of the original FGDC proposal. The second phase, now in progress, is to provide the basic standards needed to support a floristically-based classification. In addition, a framework for ongoing review of the described types and the system’s structure is needed, both initially and as changes are proposed. The common purpose is to provide a recognized vegetation classification system of broad utility for incorporating ecological science in conservation, natural resource management, planning, and research.

Recent History of Vegetation Classification in the U.S.

The United States was late to recognize the need for a unified national vegetation classification. As early as 1973, international scientists working for the United Nations Educational Scientific Cultural Organization developed and adopted a worldwide vegetation classification system based largely on physiognomy (UNESCO 1973). Many countries also had successful vegetation classifications completed or under way (Rodwell et al. 1995). The UNESCO system was not generally adopted in the United States because it was perceived as being not detailed enough to be useful at a local level. At that time, there was little recognition of the parallel importance of regional context with localized content, and the value of a single national classification system was not acknowledged. Some federal land management agencies produced vegetation classifications, maps, and other information about public lands that they administered, but these projects typically were limited in scope and geography (Ellis et al. 1977).

In the late 1970s, TNC initiated state natural heritage programs. Inventories of rare species and community "element occurrences" were produced as part of state agency surveys, and some state-specific community classification systems were developed. Although many state natural heritage programs began collecting descriptions of plant communities by the early 1980s, the development of state-level classifications was not widespread until the mid-1990s, driven largely by a coordinated effort to map existing vegetation.

In the early 1980s, five federal agencies tried to develop a national classification system integrating vegetation, soils, water, and landform (Driscoll et al. 1984). The vegetation criteria were based on a potential taxonomic hierarchy. One obstacle was identifying potential vegetation types for inventory sites when taxonomies and keys for the floristic-based levels were available for only a few regions in the country. A second obstacle was that the system was designed primarily for aggregating plot data; little attention was paid to mapping systems and methods for mapping land cover. Third, the classification could deal only with potential, not existing, vegetation.

In the late 1980s, the U.S. Fish and Wildlife Service initiated a research project that became the National Gap Analysis Program (GAP). In 1990, GAP began supporting TNC’s effort to develop a national system (Jennings 1993) which started by compiling and standardizing regional (multistate) classifications from state natural heritage programs. Supported by GAP and the National Park Vegetation Mapping Program, Grossman et al. (1994) drafted the basis for what is now recognized as the National Vegetation Classification (NVC).

The requirement for national spatial information standards, including vegetation, was implemented by the federal government with the 1990 revised Office of Management and Budget Circular A-16, Coordination of Surveying, Mapping, and Related Spatial Data Activities. The goals of this circular were to develop the National Spatial Data Infrastructure (NSDI), to reduce duplication, to reduce the expense of developing new geographically based data, and to increase the benefits of using available data through coordination and standardization of federal geographic data. The circular established the Federal Geographic Data Committee (FGDC) to promote development of distributed database systems, information standards, exchange formats, and guidelines, and to encourage broad public access.

Interagency commitment to coordination under Circular A-16 was strengthened in 1994 under Executive Order 12906, Coordinating Geographic Data Acquisition and Access: the National Spatial Data Infrastructure. It instructed the FGDC to involve state, local, and tribal governments in development, and to use the expertise of academia, the private sector, and professional societies in implementing the order. Under this mandate, the FGDC established a subcommittee to develop standards for mapping vegetation.

The ESA established its Panel on Vegetation Classification in 1994 (Barbour 1994, Peet 1994, Loucks 1995, 1996), and the panel began by reviewing the draft FGDC Vegetation Classification and Information Standards. Completion of the review and publication of the FGDC proposed national classification (the overall framework and details for physiognomic levels; see http://www.fgdc.gov/Standards/Status/sub2_1.html ) concluded the first phase of the ESA Panel’s work. However, the bulk of the ESA panel’s work is to deal with the issues of a floristically defined vegetation classification. This is the focus of the second phase of the ESA Panel’s activities.

A Point of Departure

An early step in the ongoing cooperative process to develop a national vegetation classification is to recognize the present baseline classification as a point of departure. This must be followed by recommended standards that both require and provide a mechanism for further improvements and complete documentation of named units. To improve these standards, work groups will be established, composed of academic, agency, and private-sector scientists.

Both TNC’s classification (Nature Conservancy Ecology Working Group 1997) and the FGDC Vegetation Classification and Information Standards must now be recognized as a single, integrated system. A complete review of the floristic levels is required, but a consensus on basic standards is needed first. In August of this year, the ESA panel distributed for review a draft version of An Initiative for a Standardized Classification of Vegetation in the United States. This document proposes the first approximation of standards for the categories shown in Table 2. These standards lay the foundation for an enduring taxonomy of vegetation that can be improved upon, in a collaborative way, as our body of knowledge expands.

Literature Cited

Barbour, M. 1994. Special Committee on Vegetation Classification. Bulletin of the Ecological Society of America 75: 252-253.

Bourgeron, P.S., and L.D. Engelking, editors. 1994. A preliminary vegetation classification of the Western United States. A report prepared by the Western Heritage Task Force and The Nature Conservancy for the University of Idaho Cooperative Fish and Wildlife Research Unit. Nature Conservancy Western Regional Office, Boulder, Colorado.

Drake, J., and D. Faber-Langendoen. 1997. An alliance-level classification of the vegetation of the Midwestern United States. A report prepared by The Nature Conservancy Midwest Conservation Science Department for the University of Idaho Cooperative Fish and Wildlife Research Unit. The Nature Conservancy Midwest Regional Office, Minneapolis, Minnesota.

Driscoll, R.S., D.L. Merkel, R.L. Radloff, D.E. Snyder, and J.S. Hagihara. 1984. An ecological land classification framework for the United States. U.S. Department of Agriculture, Forest Service Miscellaneous Publication Number 1439. Washington, D.C.

Ellis, S.L., C. Fallat, N. Reece, and C. Riordan. 1977. Guide to land cover and use classification systems employed by western governmental agencies. FWS/OBS-77/05. U.S. Fish and Wildlife Service, Arlington, Virginia, U.S.

FGDC. 1997. Vegetation information and classification standard. Federal Geographic Data Committee, U.S. Geological Survey, Reston, Virginia.

Grossman, D., K.L. Goodin, X. Li, D. Faber-Langendoen, and M. Anderson. 1994. Standardized national vegetation classification system. Report prepared by the Nature Conservancy for the National Biological Service. The Nature Conservancy, Arlington, Virginia.

Jennings, M.D. 1993. Natural Terrestrial Cover Classification: assumptions and definitions. GAP Analysis Technical Bulletin 2. U.S. Fish and Wildlife Service.

Loucks, O.L. 1995. Special Committee on Vegetation Classification: Annual report. Bulletin of the Ecological Society of America 76: 221-223.

______. 1996. 100 years after Cowles: a National Classification Vegetation. Bulletin of the Ecological Society of America 77 (2): 75-76.

Peet, R.K. 1994. Vegetation classification in contemporary ecology. Ecological Society of America Vegetation Section Newsletter, 2:5.

Rodwell, J.S., S. Pignatti, L. Mucina, and J.H.J. Sohamined. 1995. European vegetation survey: Update on progress. Journal of Vegetation Science 6: 759-762.

Sneddon, L., M. Anderson, and K. Metzler. 1994. A classification and description of terrestrial community alliances in The Nature Conservancy’s Eastern region: First approximation. A report prepared by The Nature Conservancy Eastern Regional Office for the University of Idaho Cooperative Fish and Wildlife Research Unit. The Nature Conservancy Eastern Regional Office, Boston, Massachusetts.

The Nature Conservancy Ecology Working Group. 1997, in preparation. International classification of ecological communities: Terrestrial vegetation of the United States. The Nature Conservancy, Arlington, Virginia.

UNESCO (United Nations Educational, Scientific, and Cultural Organization). 1973. International classification and mapping of vegetation. Series 6. Ecology and conservation. Paris, France.

Weakley, A.S., K. Patterson, S. Landaal, and M. Gallyoun. 1997. An alliance-level classification of the vegetation of the southeastern United States. A report prepared for the University of Idaho Cooperative Fish and Wildlife Research Unit by The Nature Conservancy Southeastern Regional Office, Chapel Hill, North Carolina.

Table 1. The U.S. National Begetation Classification's structure.

National Vegetation Classification Structure

Physiognomic criteria:
Class          Woodlands
     Subclass     Mainly Evergreen Woodlands
          Group     Evergreen Needle-leaved Woodlands
               Subgroup     Natural/Seminatural
                    Formation     Evergreen Coniferous Woodlands with Rounded Crowns

Floristic criteria:
                         Community Alliance     Juniperus occidentalis
                              Community Type     Juniperus occidentalis /Artemesia tridentata

Table 2. Categories for vegetation classification standards

Terminology. - If the classification system is to be fully understood and available for all to use, associated terminology must be defined consistently.

Inventory methods and plot data. - Information from vegetation stands and plots must be collected and managed using standards, such that data from different plots and sources can be analyzed for description and classification of vegetation units.

Nomenclature. - Names of the vegetation units must be applied through standard nomenclature rules for each hierarchical level of the classification system.

Classification, documentation, and description. - Procedures for classifying, documenting, and describing floristic units are the core of systematic taxonomy.

Peer review, dissemination, and information management. - A credible system must incorporate peer review of its structure and named units, must be continuously revisable to allow efficient reconstruction of its elements from prior dates, and must be relatively easy to disseminate, update, and maintain.

Review and refinement. - We must begin with a clear understanding of the initial structure and set of units, and then implement processes for review and incorporation of newly described units. We must develop and maintain a database to cross reference the units of other classifications.

Institutional structure for maintaining and developing the classification. - The classification can only succeed through cooperation of professional ecologists and their institutions, whose roles must be formally articulated in a Memorandum of Understanding now being developed.

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