1. Bailey, R. G. 1976. Ecoregions of the United States (map). US Department of Agriculture, Forest Service, Ogden, UT.
2. Bailey, R. G. 1980. Description of the ecoregions of the United States. US Department of Agriculture, Miscellaneous Publication 1391, Washington, DC. 77 pp.
3. Biasutti, R. 1962. Il paesaggio terrestre, 2nd ed. Unione Tipografico, Torino, Italy. 586 pp.
4. Borchert, J. F. 1950. The climate of the central North American grassland.Ann. Assoc. Am. Geogr. 40:1–39.
5. Burger, D. 1976. The concept of ecosystem region in forest site classification. Pages 1–6in Proceedings XVI IUFRO World Congress, Oslo, Norway.