Subsection divisions show differences that are mostly due to surficial geologic systems, such as areas dominated by glacial moraines as opposed to lakebeds or outwash plains, as reflected by patterns in potential natural vegetation. Provinces were mapped by Bailey (1989a, 1989b). Sections and Subsections, adapted from work by Albert et al. (1996), were mapped by interagency teams.  The National Hierarchical Framework of Ecological Units 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 environmental and biological factors that directly affect or indirectly express energy, moisture, disturbance, and nutrient gradients which regulate the structure and function of ecosystems. These factors include climate, geology, geomorphology, water, soils, and potential natural vegetation. Using these factors, the National Hierarchy of Ecological Units organizes and defines biophysical environments, or the environmental components of ecosystems, into an orderly set of spatial scales based on measurable features and processes. The National Hierarchy takes the infinite variety of ecosystems and places them into a limited number of discrete, practical units that are mappable, repeatable, and distinguished from one another by differences in various structural or functional characteristics.

Albert, D.A.; Denton S.R.; Barnes, B.V. 1986. Regional landscape ecosystems of Michigan. School of Natural Resources, University of Michigan, Ann Arbor. 32 p.

Bailey, R.G. 1980. Descriptions of the ecoregions of the United States. Washington DC: U.S. Department of Agriculture, Forest Service. Miscellaneous Publication 1391. 77 p.

Bailey, R.G. 1989a. Ecoregions of the continents (map). Washington, DC: U.S. Department of Agriculture, Forest Service. Scale 1:30,000,000.

Bailey, R.G. 1989b. Explanatory supplement to the ecoregions map of the continents. Environmental Conservation 15(4):307-309.

Cleland, David T., Peter E. Avers, Henry McNab, Mark E. Jensen, Robert G. Bailey, Thomas King, and Walter E. Russell. 1997. National Hierarchy of Ecological Units. Yale University Press, New Haven & London. In (Mark S. Boyce and Alan Haney editors) Ecosystem Management: Applications for Sustainable Forest and Wildlife Resources. pp 181-200