The Forests of the United States (D. Smith). The Northeastern Region (R. Seymour). The Lake States Region (J. Johnson). The Central Region (G. Parker & C. Merritt). The Southern Appalachian Hardwood Region (D. Smith). The Southern Bottomland Hardwood Region and Brown Loam Bluffs Subregion (J. Hodges). The Southern Pine Region (L. Walker). The Middle and Southern Rocky Mountain Region (J. Long). The Northern Rocky Mountain Region (D. Adams). The California Region (J. Helms). The Pacific Northwest Region (S. Tesch). The Alaska Region (J. Zasada & E. Packee). Appendices. Index.
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John W. Barrett (1962) studied this question.