FOREST TYPES OF THE UNITED STATES

WILLIAM A. DAYTON

Because of the size of the United States, the diversity of its conditions, and the wealth of its vegetation (we have about four times as many tree species as does Europe) and because of differences in terminology and of opinions on classification, climaxes, and such, it is not surprising that ideas about the forest types of the United States are still somewhat controversial.

Dr. James Graham Cooper (1830-1902) , Army surgeon, explorer, and naturalist, seems to have been the first to publish a vegetative-type map of North America. It appeared in 1859 in his paper On the Distribution of the Forests and Trees of North America, with Notes on its Physical Geography. Overleaf is reproduced the United States part of Dr. Cooper's map; the original letters for his regions are retained, but hachures have been added to make their differentiation clearer to the eye. It will be observed that four of Dr. Cooper's regions are in the Eastern seaboard, three are in the Appalachians, six are in the Plains States, five are in the Rocky Mountain areas, three are in the Intermountain area, and two on the Pacific coast. Most of them are forested areas, at least in part.

Dr. Cooper was a link between ancient students of the subject and the modern investigators, who have added a great deal to our ken of botany.

Theophrastus of Eresus (372-287 B. C.) by the will of Aristotle became heir to the great philosopher's celebrated library, guardian of his children, and his successor as head of the Lyceum at Athens. Theophrastus has been called "primus verorum botanicorum" the first real botanist. He was perhaps the first to emphasize the relation of trees and other plants to their environment, and may rightly be regarded as the father of the concepts of ecological and vegetative types.

Nearly two centuries ago, Linnaeus,in his Philosophic Botanica, had a chapter on plant distribution correlated with the geographic regions, climate, soils, and the other factors of habitat.

Henry Solon Graves, who published Practical Forestry in the Adirondacks in 1899, is generally credited with the introduction of the term "forest type" in this country. The late Dr. Frederic E. Clements, a distinguished ecologist and author of Plant Formations and Forest Types, published in 1909, calls Professor Graves' types "plant (or forest) formations." He separates formations into associations, associations into societies, societies into communities (with two or more principal or secondary species), and communities into families (defined as groups of co-specific individuals).

The Ecological Society of America tentatively suggested this definition of "forest types" in 1934: "A forest stand essentially similar throughout its extent as regards composition and development under essentially similar conditions, i. e., essentially similar throughout as regards floristic composition, physiognomy, and ecological structure."

Ten years later the Committee on Forestry Terminology of the Society of American Foresters defined forest type thus : "A descriptive term used to group stands of similar character as regards composition and development due to certain ecological factors, by which they may be differentiated from other groups of stands. The term suggests repetition of the same character under similar conditions. A type is temporary if its character is due to passing influences such as logging or fire; permanent if no appreciable change is expected and the character is due to ecological factors alone; climax if it is the ultimate stage of a succession of temporary types. A cover type is a forest type now occupying the ground, no implication being conveyed as to whether it is temporary or permanent."

Adapted from Dr. James G. Cooper's Distribution of the Forests and Trees of North America (1859). The letters indicate: C, Lacustrine Province (Canadian Region); D, E, F, G, Appalachian Province (Alleghany, Ohio, Tennessean, Carolinian Regions, respectively); H, Mississippian Region; I, Floridian Region (part of West Indian Province); J, K, L, M, N, Campestrian Province: (1) Prairies, J (Texan Region), K (Illinois Region), L (Saskatchewan Region), (2) Arid Plains, M (Dacotah Region), N (Comanche Region); Q, Mexican Province (Chihuahian Region); R, S, T, U, V, Rocky Mountain Province (Arizonian, Wasatch, Padoucan, Utah, and Shoshone Regions, respectively); W, X, Y, 7, Nevadian Province (Californian, Oregonian, Kootanic, and Yukon Regions, respectively).

Raphael Zon, in Principles Involved in Determining Forest Types, published in 1906, emphasizes the importance of forest types in silvical studies of individual species, and sets forth a philosophy basic to determining forest types. The main considerations are physical conditions of climate, soil, and the like; man and his operations; accidents, such as fire and wind. He says that "one of the most important characteristics of a forest type is its stability, its resistance to invasion by other forms," and adds that the reproduction of the forest must always be considered.

Arthur W. Sampson (The Stability of Aspen as a Type, 1916) believes that aspen is a temporary type, replaced, slowly but surely, by conifers.

Carlos G. Bates, in Forest Types in the Central Rocky Mountains as Affected by Climate and Soil, 1924, states that, in a general way, the forest zones of that region correspond with air-temperature zones. He adds that a review of the facts leaves little doubt that the tree species of the central Rocky Mountains are controlled in their distribution almost wholly by the degree of insulation of the site, with the resultant temperatures, and by the closely related surface conditions of moisture.

A distinguished Finnish forester and ecologist, Aimo K. Cajander, places forest typification on a combined ecological and biological basis (The Theory of Forest Types, English translation revised by Mr. M. L. Anderson, 1926). He recognizes two kinds, in principle, of forest classification, according to quality and site. He says: "The features of a plant association are generally determined by those species which are present in the greatest abundance and frequency. Those species, however, which are present at a lesser rate of abundance, but are, nevertheless, always or nearly always present, are also, of course, equally characteristic of the association. Finally those species, which, though they may be more or less rare, are met with, however, almost exclusively in the association in question, are also characteristic of that association. On the other hand, of course, the absence of certain plant species is also a very important feature in the delineation of a plant association, although the definite establishment of absence is more difficult."

Arthur Freiherr von Kruedener, who published Waldtypen Klassifikation und ihre volkswirtschaftlich Bedeutung in 1927, based scientific classification of forest types on three factors: Climate, soil, and plant associates: "Was wir unter Waldtypen verstehen, sowie von den Faktoren Klima, Boden-Untergrund und Pflanzengemeinschaft, welche drei in ihrer Verbindung uns erst den Begriff des Waldtyps geben."

Gustaf A. Pearson, in Forest Types in the Southwest as Determined by Climate and Soil, 1931, distinguishes seven broad zones with four forest types: Woodland, ponderosa ("western yellow") pine, Douglas-fir, and Engelmann spruce. The soil differences, he says, appear to be due more to physical than to chemical differences, the more porous soils being best suited to tree growth, the upper altitudinal range determined by ability to withstand low temperatures and the lower altitudinal range to drought endurance. The soil, except locally, rarely acts as a limiting factor. He does not regard light as a limiting factor in the range of trees, but it may affect the composition of stands.

Marinus Westveld (Type Definitions Based on Statistics of Stand Composition, 1934) gives type definitions of the red spruce-yellow birch (with yellow birch subtype) as well as the red spruce-sugar maple-beech (with sugar maple subtype) types. In the red spruce-yellow birch type, the conifers usually make up more than 40 percent of the stand, with spruce and the balsam fir in about equal numbers, the yellow birch composing between 25 and 50 percent, and sugar maple seldom more than 5 percent of the total stand. In the red spruce-sugar maple-beech type, the conifers make up 25 to 45 percent of the stand, spruce usually being more abundant than fir. Sugar maples generally make up more than 10 percent of the stand and, combined with beech, usually considerably exceed the yellow birch in number.

Again, in the field of forest classification systems and their terminology, there is a large literature and differences in viewpoint.