— 
Affinities: Western North America. 321 
3. An arid and woodless belt which occupies by far the greater part of the 
region. 
Botanically the alpine regions of the temperate zone in the northern 
hemisphere are southward prolongations of arctic vegetation, almost pure in 
the boreal parts, but more and more mixed with special types in lower lati- 
tudes, these special types being part of the flora which is characteristic of each 
continent in those latitudes. Leaving out of view a considerable number of 
temperate species which here and there become alpestrine, or persist in dwarfed 
orms within some truly alpine regions, the alpine flora of the United States 
does not comprise a large number of species. 
The bulk of the forest of the Rocky Mountains‘) consists of coniferous 
trees. Most of these are common to the region of the Pacific Coast, but none 
of the trees are found in the Alleghany Mountains or around the Great Lakes. 
The white spruce, Picea alba (canadensis), has been reported from northern 
Montana and Alberta, but probably erroneously; Picea columbiana, which 
resembles it in habit And the glabrous branches, has been mistaken for it. 
Picea columbiana is otherwise much more Klohely related to P, Engelmannt. 
Two prostrate shrubby species of Juniperus, $. sibirica and $. prostrata range 
across the continent. Both are sub-arctic and sub-alpine. Besides these none 
of the Rocky Mountain conifers are found in the east, although they are often 
represented there by closely related species”). (See ante Chapter 3, $ 4.) 
Among the deciduous trees and shrubs, the number of species common to 
the Rocky Mountains and the East is much larger. Many of these common 
species are boreal, but some are truly transcontinental or nearly so. Of the 
Salicaceae about three-fifths of the Rocky Mountain species are endemic, or 
western. The rest are nearly all boreal. The only ones that cannot come 
under the categories, perhaps are Salıx Bebbiana, S. candida and S. cordata. 
The eastern Populus monilifera (= P. deltoides) is represented in the Rocky 
Mountains by a western variety. Of the Betulaceae, Betula papyrifera, B. 
glandulosa, Alnus viridis (= A. alnobetula) and perhaps Alnus incana are foun 
both in the Rocky Mountains and in the east. The other four Rocky Mountain 
species are western. Of Fagaceae, Corylus rvostrata is common to the Rocky 
Mountains and the east. Besides by this species, the family is represented in 
the region by a few endemic low species of oaks of the white oak group. 
Ulmaceae is represented by three or four species of Celtis, of which C. 
occidentalis is found in the east. Ulmus americana is found here and in the 
valleys of the Great Plains, but scarcely reaches the Rocky Mountains. All 
the woody species of Hydrangeaceae, Grossulariaceae, Rosaceae, Rhamnaceae, 
Drupaceae, Papilionaceae, Rutaceae, Aceraceae, Celastraceae and Anacardia- 
1) Gray, Asa and Hooker, J. D.: The Vegetation of the Rocky Mountain Region and a 
Comparison with that of other Parts of the World. Survey VI, No. ı. 1880. Bulletin U. S. 
Geological and Geographical. Survey VI, No. ı. 1880. 
2) RvDBErG, P. A.: Composition of the Rocky Mountain Flora. Science new ser. XH: 871. 
Dee. 7, 1900. 
Harshberger, Survey N.-America. 21 
