DISTRIBUTION OF VEGETATION IN UNITED STATES. 43 



and above that elevation of an admixture of yellow pine, white fir, 

 and sugar pine. 



In the Rocky Mountains of Colorado the principal areas of forest 

 are dominated either by yellow pine or lodgepole pine, or more rarely 

 by an admixture of them. The higher elevations are characterized 

 by Douglas fir {Pseudotsuga m.ucronata), Engelmann spruce (Picea 

 engelmanni) , and by the fox-tail and limber pines {Pinus aristata and 

 P. flexilis) and the Parry fir {Picea parry ana) and alpine fir {Ahies 

 lasiocarpa) . 



The forest in the Black Hills of South Dakota and in the desert 

 mountains of Arizona and New Mexico is chiefly formed by yellow 

 pine. 



The Mesophytic Evergreen Forest of northern Minnesota is a com- 

 posite of white pine {Pinus strobus) on the deeper soils; of Norway 

 pine {Pinus resinosa) and jack pine {Pinus divaricata) on the lighter 

 soils; with tamarack {Larix laricina), black spruce {Picea mariana), 

 and white spruce {Picea canadensis) in wet soils; and arborvitse {Thuja 

 occidentalis) in the bogs. The deciduous broad-leaved trees are more 

 conspicuous here than in any portion of the western half of this forest, 

 bur oak {Quercus macrocarpa), basswood {Tilia americana), and sugar 

 maple {Acer saccharum) being the commonest species. 



In Maine the principal trees in the evergreen forest are spruce 

 {Picea nigra), balsam fir {Abies balsamea), white pine {Pinus strobus), 

 and hemlock {Tsuga canadensis). This group of trees is also charac- 

 teristic of the coniferous areas of the other New England States, of the 

 Catskill and Adirondack Mountains in New York, and of the moun- 

 tains of Pennsylvania. The small coniferous areas on the mountains 

 of North Carolina are chiefly composed of black spruce {Picea mariana) 

 and the Eraser fir {Abies fraseri) . 



Western Xerophytic Evergreen Forest. — The Xerophytic Evergreen 

 Forest is a dwarf and open form of "woodland" or "semi-forest" that 

 characterizes the edges of the Mesophytic Evergreen Forest through- 

 out the southern half of the western portion of that forest. The 

 Xerophytic Forest seldom covers extensive areas, except in northern 

 Arizona, and in all localities it becomes more open at the lower edges, 

 where it meets the Desert or the Desert-Grassland Transition, and more 

 closed at the upper edge, where it merges with the Mesophytic Ever- 

 green Forest. 



The Xerophytic Forest is similar to the desert in that its dominant 

 plants are widely spaced, leaving much unoccupied ground. It is, 

 again, similar to the desert and unlike the other forest areas in the 

 small stature of its trees, which never exceed 50 feet and frequently 

 attain less than 25 feet in height. The two types of tree w^hich dominate 

 the Xerophytic Forest are the nut pine {Pinus edulis, P. cembroides, 

 P. parry ana) and the juniper {Juniperus utahensis, J. calif ornica, J. 



