244 DISTRIBUTION OF PLANT COMMUNITIES 



sam fir (Abies halsamea) . This forest has been so depleted and modi- 

 fied by lumbering and fires that its true nature is difficult to recognize 

 in most places. 



The Rocky Mountain coniferous forests, which extend from 

 Montana and Idaho to New Mexico and Arizona on the mountains, 

 are of two main types. The higher parts of the mountains are occu- 

 pied by a spruce-fir forest in which the dominant trees are Engel- 

 mann spruce (Picea engelmannii) and alpine fir (Abies lasiocarpa). 

 This forest extends from the upper tree limit downward for 2000 

 to 3000 feet in most places. From central Colorado northward 

 there are extensive areas of lodgepole pine (P. murrayana) forest 

 in pure stand. These, it is believed, have been caused by past fires 

 and will, in a natural course of events, be gradually replaced by the 

 spruce-fir climax. Below the spruce-fir forest is the yellow pine 

 (P. 'ponderosa)-Dovig\?i^ fir (Pseudotsuga taxijolia) forest. The 

 yellow pine and Douglas fir are often accompanied by lodgepole 

 pine, limber pine (P. flexilis), and white-bark pine (P. albicaulis). 

 The yellow pine usually extends somewhat lower than the other 

 species and is often observed in pure stands. 



The forests of the Sierra Nevada and Cascade Ranges are similar 

 to those of the Rocky Mountains but possess larger numbers of 

 dominant species. The uppermost forest which corresponds to the 

 spruce-fir forest in the Rocky Mountains may be called the sierran 

 subalpine forest and is characterized by mountain hemlock (Tsiiga 

 mertensiana) , scrub pine (P. contorta), foxtail pine (P. balfouriana) , 

 western larch (Larix lyallii), and red fir (Abies magnifica). In addi- 

 tion to these the dominant species of the Rocky Mountain spruce-fir 

 forest occur as secondary species. The lower forest of these moun- 

 tains, which corresponds to the yellow pine-Douglas fir forest of the 

 Rocky Mountains, may be called the yellow pine-sugar pine forest. 

 It is characterized by the sugar pine (P. lamhertiana) , yellow pine 

 (P. ponderosa) , Douglas fir (Pseudotsuga taxifolia), white fir (Abies 

 concolor), and incense cedar (Librocedrus decurrens). The big tree 

 (Sequoia gigantia) is found only in this forest but is localized in 

 limited areas in the central Sierras. This forest is much more char- 

 acteristic of the Sierras than of the Cascades. On the east slope of 

 the Cascades it is replaced by a larch-pine forest characterized by 

 the larch (Larix occidentalis) and western white pine (P. monticola). 



The west coastal coniferous forest occupies the western parts of 

 Washington and Oregon from the western slopes of the Cascades to 

 the coast and extends into California along the coast as far as San 



