Ecological Subregions of the United States


The USDA Forest Service adopted a policy of ecosystem management on June 4, 1992, that applied to national forests, grasslands and research programs. By July, an Ecological Classification and Mapping Task Team (ECOMAP) was formed in the Washington Office to develop a consistent approach to ecosystem classification and mapping at multiple geographic scales. This was identified by the Chief as a critical first step in providing field units with an essential tool and scientific basis to plan for and implement ecosystem management. Soon afterwards a subgroup of ECOMAP was formed with representatives from all Forest Service Regions, two Research Stations, the USDA Soil Conservation Service, and The Nature Conservancy. They met in September in Lincoln, NE, to begin development of a land classification system. The structure of the National Hierarchical Framework of Ecological Units (Table 1) was formulated at this meeting and was adopted by the Forest Service on November 5, 1993 (ECOMAP 1993).

Briefly, as described by ECOMAP (1993), the Framework "\dots is a regionalization, classification, and mapping system for stratifying the Earth into progressively smaller areas of increasingly uniform ecological potentials. Ecological types are classified and ecological units are mapped based on associations of those biotic and environmental factors that directly affect or indirectly express energy, moisture, and nutrient gradients which regulate the structure and function of ecosystems. These factors include climate, physiography, water, soils, air, hydrology, and potential natural communities."

In November 1992, the subgroup began the process of producing a national map of ecological units at the Section level of the subregion planning and analysis scale. During the process of delineating Sections, ecoregion boundaries were revised. The map "Ecoregions and Subregions of the United States" was compiled by December 1993 and printed in June 1994 (Bailey and others 1994). The Section map unit descriptions in this text were produced after the map was compiled. A new, revised ecoregion map was also printed in June 1994. Bailey's publication (Bailey, 1980), which describes the Domains, Divisions, and Provinces of the United States is being revised (Bailey, In prep.).

Work is underway by the Forest Service and other agencies to subdivide Sections into Subsections, the next lower level in the hierarchy. In addition, maps are being developed at landscape and land unit scales on national forests and other selected areas in the United States to provide detailed information for project implementation. Thus, delineation and description of ecosystems at all levels in the hierarchy are components of an ongoing process that will result in a series of maps and explanatory texts to meet planning and analysis objectives (Figure 1). Each map and each descriptive text documents our current knowledge and provides a basis for study and communication among natural resource managers and planners.

In summary, the National Hierarchical Framework provides a scientific basis for regionalization of ecosystems into successively smaller, more homogeneous units. At the Section level, these units allow managers, planners, and scientists in the Forest Service, and in cooperation with other agencies, to study management problems on a multi-forest and statewide basis; organize data collected during broad-scale resource inventories; and interpret these data among regions.

Table 1.--The Forest Service National Hierarchical Framework of Ecological Units.

Planning and
analysis scale
Ecological Units Purpose, objectives, and general useGeneral size range



Broad applicability for modeling and sampling, strategic planning and assessment, and international planningMillions to tens thousands of square miles
Forest, area-wide planning and wathershed analysisThousands to hundreds of acres
LandscapeLandtype associationForest, area-wide planning and wathershed analysisThousands to hundreds of acres
Land unit.Landtype association
Landtype phase.
Project and management area planning and analysis.Hundreds to less than ten acres.

Figure 1.--The upper four levels of ecological units in the Forest Service National Hierarchical Framework consist of Domain , Division, Province, and Section. Selected ecological units of the Humid Temperate Domain, in the eastern United States, are progressively revealed to the Section level to illustrate the hierarchical structure, the identification system, and relative sizes of map units at the ecoregion and subregion planning and analysis scales. (Hierarchy of ecoregions at a range of scales, R.G. Bailey, 1994).

Map Unit Descriptions

This text is organized following the national hierarchy structure. Provinces, the lowest hierarchical level at the ecoregion scale (Figure 2), are the basis for chapters. Each chapter consists of the Sections that the Province comprises. Each Section is described by the predominant environmental and biological features used in its delineation, along with other pertinent or characteristic factors. The abbreviated format of this national text necessitated that only a limited amount of information be presented. This information provides the user with a brief description of environmental features that characterize Sections for broad planning and assessment and are useful for comparing landscape characteristics among Sections. Section map unit descriptions were prepared by compilers in each Region following a standard format consisting of 11 elements.

Geomorphology. Geomorphology is the classification, description, nature, origin, and development of present landforms. This element describes the predominant geomorphic processes active in the Section that resulted in formation of the characteristic landforms. The geomorphic province and general landform features may also be described. The range of elevation in ft above mean sea level (m in parentheses) is described. Local relief prevalent in a radius of several miles may be presented.

Lithology and Stratigraphy. Lithology is the description of rocks on the basis of such physical characteristics as manner of origin, composition, and texture. Stratigraphy is the arrangement of rocks as classified by geographic position and chronological order. Classifications by the U.S. Geological Survey were used to provide a consistent national basis for this element (King 1976, King and Beikman 1976, 1978) provide more information on lithology and stratigraphy of the conterminous States; Wahrhaftig (1965) is a source of information for Alaska. Soil Taxa. Soils were characterized by phases of orders, suborders, or great groups that typify the map unit. Soil moisture and temperature regimes are included to help characterize some map units. The soil taxonomy developed by the USDA Soil Conservation Service (Soil Survey Staff 1992) was the basis of information in this element.

Potential Natural Vegetation. This element presents the potential natural vegetative communities, defined by K\"uchler (1964), that typify the map unit. Other more specific information on natural vegetation may also be presented, such as potential natural communities, historic vegetation, or existing communities. Sources of information vary by Region.

Fauna. Characteristic mammals, birds, reptiles, and amphibians of the map unit are named. Some historic, common, and characteristic species are usually listed. Threatened species are provided for some Sections.

Climate. Prevailing climate is characterized in terms of mean annual precipitation in inches (mm in parentheses) and mean annual temperature in degrees Fahrenheit (oC in parentheses). Seasonality of precipitation and relative amount that occurs as snow may also be presented. The growing season is presented as a measure of the length of time during which plant growth may occur, if soil moisture is adequate, and is defined as the mean annual range of days between the last spring and first fall minimum temperatures of 32 oF (0 oC). Additional information is presented by the USDA Soil Conservation Service (1981).

Surface Water Characteristics. Relative occurrence and distinguishing characteristics of rivers, streams, lakes, and wetlands are presented. Some major rivers may be identified.

Disturbance Regimes. This element lists the natural factors and forces that significantly influence ecosystem dynamics within a planning period.

Land Use. This element identifies the predominant changes to natural vegetative communities caused by human uses of land and water resources.

Cultural Ecology. Examples demonstrate how the historical relationship between humans and the natural environment has resulted in modified landscapes.

Compiled by the Forest Service administrative unit primarily responsible for summarizing information pertaining to the Section.

In some instances time was not available to obtain information for one or more elements in the Section description. These elements will be developed more fully when this text of map unit descriptions is revised. The description of some Sections is supplemented with a photograph that illustrates typical landforms, predominant vegetation, and, occasionally, fauna. Information in these Section map unit descriptions is presented in consistent national format. Map unit descriptions were edited for style, but not for technical accuracy. Compilers are responsible for information presented in each Section map unit description. \null \endtwocol

Figure 2 --Ecological units delineated at the Province level (Ecoregions of the United States, R.G. Bailey, 1994) are the basis for organization of this document. Each chapter consists of a Province and its components---Sections.

Five appendices present information to supplement the Section map unit descriptions. Appendix A includes references specifically cited in the Introduction and Glossary and lists selected references that provide general information about elements of map unit descriptions for some Provinces and Sections. Appendix B provides the area of each Section and Province. Appendix C is a selected glossary of terms used in map unit descriptions, and Appendix D lists the common and scientific names of selected flora and fauna. Appendix E lists addresses of Regional contacts to whom comments and suggestions regarding specific map unit descriptions should be addressed.

Many potential uses exist for the descriptions of ecosystems presented in this text. Perhaps the most important use is to provide a means for comparison and contrast of environmental conditions among Sections as a basis for region-wide assessment and monitoring programs. Material in this text will provide a common basis for communication and coordination among public agencies and groups at the international, national, state, and local levels of planning and evaluation. Researchers, land managers, and other users of research findings will have a common basis for suggesting limits of applicability of results from experimental studies. Another potential use of information in this document will be to provide a uniform basis for planning areas of coordinated work, especially among a wide range of resource disciplines. When used with the accompanying map, and perhaps paired with the companion text that describes ecoregions (Bailey In Press), information in this document can be used to illustrate the nested relationship of ecosystems, ranging from global to local levels. A single resource classification, such as a soils or existing vegetation map, may not satisfy all the needs of all users, but an ecological classification will come very close.