Bulletin 7 - Vegetation Alliance Descriptions of the Western U.S.

Vegetation Alliance Descriptions of the Western U.S.

Marion Reid and Patrick Comer

The Nature Conservancy, Western Conservation Science Department, Boulder, Colorado

Background

The Nature Conservancy (TNC) and the network of state Natural Heritage Programs have developed a national, hierarchical, vegetation classification to further their shared mission of conserving biological diversity (Grossman et al. 1994, 1998). This classification can serve many purposes, including inventory, mapping, resource and ecosystem management, and scientific research. It has received widespread support from state, federal, academic, and international partners (Jennings 1993, Greenall 1996, Loucks 1996, FGDC 1997). Jennings (1997) discussed the progress being made toward further development of this system through the cooperative efforts of TNC, the Ecological Society of America, the Gap Analysis Program (GAP), the Federal Geographic Data Committee (FGDC), and many others. In this classification, vegetation of all types, whether dwarf-shrub bogs, mixedgrass prairies, or pinyon-juniper woodlands, are treated together in one system.

Ecologists in each of the Conservancy’s four regions and the national office have worked closely together to coordinate the development of this classification across the United States. Since 1985 TNC’s Western Conservation Science Department has worked with state Heritage Programs to develop a regional vegetation classification. A 1994 draft included vegetation types for all the coterminous western states except California, and was distributed for review and testing (Bourgeron and Engelking 1994). Since then the western regional vegetation classification (WRVC) has undergone several revisions. The WRVC has now been integrated with other regional classifications into a single national list of vegetation types (Anderson et al. 1998).

The National Vegetation Classification System

Details of the background, structure, and development of the classification are presented in a series of documents (Grossman et al. 1998, Anderson et al. 1998). The system is a physiognomic-floristic classification of existing vegetation, structured hierarchically, with physiognomic criteria at the highest (coarsest) levels of the hierarchy and floristic criteria at the lower (finer) levels. The formation concept, with units modified from UNESCO (1973), guides the definition of the physiognomic units, and the association and alliance concepts define the floristic units (Figure 1).

Figure 1. TNC’s National Vegetation Classification System (Example)

The basic unit of the physiognomic portion of the classification is the "formation," a "community type defined by dominance of a given growth form in the uppermost stratum (or the uppermost closed stratum) of the community, or by a combination of dominant growth forms" (Whittaker 1962, see also Schrader-Frechette and McCoy 1993). In practice, formations are defined by varied, conventionally accepted combinations of growth-form dominance and characteristics of the environment (e.g., cold-deciduous alluvial forests, evergreen desert shrublands, alpine meadows).

The alliance is a physiognomically uniform group of plant associations sharing one or more dominant or diagnostic species which, as a rule, are found in the uppermost strata of the vegetation (see Mueller-Dombois and Ellenberg 1974). Dominant species are often emphasized in the absence of detailed floristic information (such as quantitative plot data), whereas diagnostic species (including characteristic species, dominant, differential, and other species groupings based on constancy) are used where detailed floristic data are available (Moravec 1993).

Using a combined physiognomic/floristic system allows the identification of vegetation types from both a "top-down" (divisive) and "bottom-up" (agglomerative) approach. The top-down approach allows the use of physiognomic distinctions to help map vegetation, stratify sampling, and where floristic information is lacking, delimit vegetation units. A bottom-up approach employs plot sampling and floristic analysis as the primary means for defining associations. Where physiognomy is variable, the bottom-up approach can also be used to help determine the important physiognomic distinctions. The relationships between physiognomy and floristics are not always simple; when they do not correspond, precedent may be given to the floristic relationships over the physiognomic structure.

Describing Alliances in the Western United States

GAP develops vegetation maps with mapping units comparable to the level of the national classification’s alliance, or aggregations of these into cover types (e.g., Driese et al. 1997). A series of cooperative agreements between the four TNC regions and GAP has provided support to Conservancy ecologists to develop descriptions of each alliance (see Sneddon et al. 1994, Drake and Faber-Langendoen 1997, Weakley et al. 1997). The Western Conservation Science Department began this work in late summer of 1997 and continues to develop these descriptions for all alliances in the WRVC. This work is being accomplished by gathering information over the range of distribution of each alliance, primarily through the review of literature and other data sources available for the plant associations that make up an alliance. This information on the associations is then synthesized into a rangewide description for each alliance. At this time, 733 natural and semi-natural alliances are included in the WRVC (for AZ, CA, CO, ID, MT, NM, NV, OR, UT, WA, and WY ). The region covers roughly 1/3 of the contiguous U.S. For comparison, 219 alliances are described from TNC’s Midwest Region, and over 670 are described for the southeastern United States.

The descriptions are being completed by TNC’s Western Conservation Science ecology staff and are projected to be complete in late March 1999. Information being developed for each alliance includes:

Placement in the classification hierarchy;
Relationship to other classification systems;
Range of distribution;
Vegetation structure and composition;
Dominant and diagnostic species;
Environmental features with which it is associated;
Ecological dynamics and successional status; and
Literature references.

Additional data being developed include attributing alliances to section-level ecoregions (Bailey 1995) and recording the specific environmental variables (elevation range, aspect and slope, geologic parent material, geomorphic processes, landscape type, soil drainage, pH, and depth) with which the alliance is associated.

Applications

An alliance level classification helps to guide vegetation mapping programs as vegetation maps are a special application of a vegetation classification (Küchler 1988). For mapping applications supported by GAP it is important that the vegetation classification be based on the similarity of structural, floristic, and ecological characteristics of vegetation. The vegetation units are used to label relatively homogeneous polygons or signatures of vegetation to make a map that portrays spatial relationships of vegetation across the landscape.

Alliances, primarily based on dominant species in the top or dominant strata of the vegetation, are particularly amenable to mapping. However, because a map is at a fixed scale and the scale and pattern of alliances are variable, the mapping units may not have a one-to-one relationship to the alliance units. Depending on the map scale desired, vegetation often will form a number of mosaic patterns. A mapping convention can be adopted to attribute a map polygon with multiple alliances and, because the classification system is hierarchical, it is possible to produce map units at different levels of the classification hierarchy.

Current techniques do not permit all alliances to be delineated using remotely sensed data, therefore documenting distributions of alliances by Bailey’s Provinces and Sections will be valuable to GAP projects. Documenting which alliances occur in each section should increase the accuracy of image interpretations and speed the development of more accurate image processing techniques.

As is being done in this project, the development of specific environmental data fields within the alliance description eases the application of the data for predictive modeling of vegetation patterns by linking alliance descriptions to other databases and spatial data sets. This information would be invaluable for analysis, mapping, and modeling of abiotic characteristics of plant associations within each alliance. This creates additional opportunities to improve methods to validate and enhance the utility of GAP vegetation maps. GAP vegetation mapping efforts should feed back to make further improvements to the vegetation classification.

Using the classification by GAP projects also supports the development of new partnerships to make these improvements. Natural Heritage Program ecologists have the expertise and field experience needed for field validation of vegetation maps (at all scales of mapping), and the data being developed for vegetation alliances should prove useful in efforts to create predictive models and maps at the scale of the plant association. Natural Heritage Programs are currently working with various state and federal agencies (e.g., NPS, BLM, USFS, NRCS, EPA) on vegetation mapping and classification projects that could be linked directly to GAP for map validation and establishing sites for long-term environmental monitoring.

Literature Cited

Anderson, M., P. Bourgeron, M.T. Bryer, R. Crawford, L. Engelking, D. Faber-Langendoen, M. Gallyoun, K. Goodin, D.H. Grossman, S. Landaal, K. Metzler, K.D. Patterson, M. Pyne, M. Reid, L. Sneddon, and A.S. Weakley. 1998. International classification of ecological communities: Terrestrial vegetation of the United States. Volume II. The National Vegetation Classification System: List of types. The Nature Conservancy, Arlington, Virginia.

Bailey, R.G. 1995. Description of the ecoregions of the United States. 2nd edition. USDA Forest Service Miscellaneous Publication 1391, Washington, D.C. 108 pp. with separate map at 1:7,500,000.

Bourgeron, P.S. and L.S. Engelking. 1994. A preliminary vegetation classification of the Western United States. Unpublished report prepared by the Western Heritage Task Force for The Nature Conservancy, Boulder, Colorado.

Drake, J., and D. Faber-Langendoen. 1997. An alliance level classification of the vegetation of the midwestern United States. Unpublished report by The Nature Conservancy Midwest Conservation Science Department, Minneapolis, Minnesota.

Driese, K.L., W.A. Reiners, E.H. Merrill, and K.G. Gerow. 1997. A digital land cover map for Wyoming, USA: A tool for vegetation analysis. Journal of Vegetation Science 8:133-146.

FGDC. 1997. National Vegetation Classification Standard. Federal Geographic Data Committee, Vegetation Subcommittee, U.S. Geological Survey, Reston, Virginia.

Greenall, J. 1996. Manitoba’s terrestrial plant communities. Manitoba Conservation Data Centre MS Report 96-02. Winnipeg, Manitoba, Canada. 32 pp.

Grossman, D.H., K.L. Goodin, and C.L. Reuss, editors. 1994. Rare plant communities of the conterminous United States: An initial survey. The Nature Conservancy, Arlington, Virginia. 620 pp.

Grossman, D.H., D. Faber-Langendoen, A.W. Weakley, M. Anderson, P. Bourgeron, R. Crawford, K. Goodin, S. Landaal, K. Metzler, K.D. Patterson, M. Pyne, M. Reid, and L. Sneddon. 1998. International classification of ecological communities: Terrestrial vegetation of the United States. Volume I. The National Vegetation Classification System: Development, status, and applications. The Nature Conservancy, Arlington, Virginia.

Jennings, M. 1993. Natural terrestrial cover classification: Assumptions and definitions. Gap Analysis Technical Bulletin 2. U.S. Fish and Wildlife Service, Idaho Cooperative Fish and Wildlife Research Unit, University of Idaho, Moscow, Idaho.

Jennings, M. 1997. Progressing toward a standardized classification of vegetation for the U.S. Gap Analysis Bulletin 6. U.S. Geological Survey, Biological Resources Division, National Gap Analysis Program, University of Idaho, Moscow, Idaho.

Küchler, A.W. 1988. Vegetation mapping. In A.W. Küchler and I.S. Zonneveld, editors. Vegetation Mapping. Handbook of Vegetation Science, vol. 10. Kluwer Academic Publishers, Dordrecht, Netherlands.

Loucks, O. 1996. 100 years after Cowles: A national classification for vegetation. Bulletin of the Ecological Society of America 77:75-76.

Moravec, J. 1993. Syntaxonomic and nomenclatural treatment of Scandinavian-type associations and sociations. Journal of Vegetation Science 4:833-838.

Mueller-Dombois, D. and H. Ellenberg. 1974. Aims and methods of vegetation ecology. John Wiley and Sons, New York. 547 pp.

Schrader-Frechette, K.S., and E.D. McCoy. 1993. Methods in ecology: Strategies for conservation. Cambridge University Press, New York. 328 pp.

Sneddon, L., M. Anderson, and K. Metzler. 1994. A classification and description of terrestrial community alliances in The Nature Conservancy’s Eastern Region, Boston, Massachusetts. 102 pp.

UNESCO. 1973. International classification and mapping of vegetation. Series 6 Ecology and Conservation. United Nations Education, Scientific, and Cultural Organization, Paris. 93 pp.

Weakley, A.S., K.D. Patterson, S. Landaal, M. Pyne, M. Gallyoun. 1997. An alliance level classification of the vegetation of the southeastern United States. A report to the University of Idaho Cooperative Fish and Wildlife Research Unit and National Gap Analysis Program. The Nature Conservancy, Southeast Conservation Science Department, Community Ecology Group, Chapel Hill, North Carolina.

Whittaker, R.H. 1962. Classification of natural communities. Botanical Review 28:1-239.