42 THE VEGETATION OF THE UNITED STATES. 



are to be thus explained, as, for example, the contrast of the open 

 yellow-pine forest with the very dense lodgepole-pine forest. In the 

 mountains of the Western States there are also considerable differ- 

 ences in the coniferous forests due to altitude, and these differences are 

 visible in the floristic composition as well as in the physiognomy. 



The leading tree in the composition of all the western forests of this 

 character is the western yellow pine (Pinus ponderosa), which often 

 forms extensive pure stands at middle altitudes, is replaced by other 

 species at the highest elevations, and is sometimes the tree of lowest 

 range, as in the northern Rocky Mountains, or is mingled with the 

 oaks, junipers, and nut pines of the Western Xerophytic Evergreen 

 Forest at its lower range. The tree which most frequently replaces 

 the yellow pine in the domination of this forest is the lodgepole pine 

 (Pinus murrayana), which is extremely abundant in pure stands in 

 Idaho and Montana, occupies a belt of dominance from 3,000 to 5,000 

 feet in portions of eastern Washington, is almost the sole forest tree 

 in the Big Horn Mountains of Wyoming, grows with the Douglas fir 

 {Pseudotsuga mucronata) on the western side of the Coast Range, and 

 is locally dominant in the Rocky Mountains of Colorado. 



The most hygrophilous portions of the Mesophytic Evergreen 

 Forest are the coastward slopes of the mountains of Oregon and the 

 western slopes of the Rocky Mountains in Montana and Idaho. In 

 the former of these regions the lowest belt of forest is formed by yellow 

 pine, Douglas fir, and lodgepole pine; above 5,000 feet the Douglas 

 fir is the dominant tree, growing with yellow pine, sugar pine [Pinus 

 lamhertiana), white fir (Abies concolor), lodgepole pine, and noble fir 

 (Abies nobilis) ; while the higher forested elevations are dominated by 

 the alpine hemlock (Tsuga pattonii), together with lodgepole pine, 

 noble fir, white-bark pine (Pinus albicaulis), western white pine 

 (Pinus mo7iticola), and alpine fir (Abies lasiocarpa). 



The moister forests of the northern Rocky Mountains He between a 

 pure stand of yellow pine at lower elevations and an open stand of 

 alpine fir and white-bark pine above. The commonest trees of this 

 forest are western white pine and western larch (Larix occidentalis) . 

 The Douglas fir is frequent and the alpine fir (Abies lasiocarpa), the 

 lodgepole pine, and giant cedar (Thuja plicata) form a small percentage 

 of the arborescent vegetation. 



In the Sierra Nevada the lowest coastward fringe of forest is formed 

 by an open stand of the digger pine (Pinus sabiniana), and the main 

 body of the forest is formed of yellow pine, incense cedar (Libocedrus 

 decurrens), sugar pine (Pinus lambertiana) , Jeffrey pine (Pinus Jeffrey i) 

 and white fir (Abies concolor). At the highest timbered elevations 

 lodgepole pine, Jeffrey pine, and red fir (Abies magnifica) are the char- 

 acteristic trees. In the San Bernardino and San Jacinto Mountains 

 the forest below 6,000 feet is composed almost solely of yellow pine, 



