THE PETRAN MONTANE FOREST. 207 



constant. In the absence of the latter, the pine consociation meets chaparral 

 in the coast regions and along the slopes of the central Rockies. On the desert 

 slopes of the Great Basin it is often in touch with sagebrush, especially where 

 represented by the subclimax lodgepole pine. It may also come in direct 

 contact with the mixed prairie from Colorado northward, where it passes into 

 extensive savannahs, characteristic of the isolated ranges and uplands of the 

 Black Hills and adjacent regions. 



Associations. — The general occurrence of Pinus ponderosa, Pseudotsuga 

 mucronata, and Abies concolor through the montane climax was thought at 

 first to indicate the presence of a single association. A scrutiny of the list of 

 codominants reveals a fairly clear differentiation into a Rocky Mountain and 

 a Sierra-Cascade community. These have three codominants in common, 

 namely, Pinus contorta, P. flexilis, and P. albicaulis. The former differs much 

 in habit and habitat between the two associations, while Pinus flexilis is more 

 important in the Rocky Mountains and P. albicaulis in the Coastal region. 

 Of the remaining 13 codominants, 5 are restricted wholly to the Rocky Moun- 

 tain community and 8 to the Sierran. Tins differentiation is also emphasized 

 by the variation in habit and size of Pinus ponderosa and Pseudotsuga in the 

 two regions. This is so pronounced in the case of the pine that the common 

 form of the Rocky Mountains has generally been treated as a variety or even 

 as a species, while foresters have regarded the Douglas fir of the Pacific coast 

 as a distinct variety. A similar differentiation is reflected in the societies of 

 the forest. More than 75 per cent of the generic subdominants are the same 

 for both associations, while they have less than 25 per cent of common species. 

 Finally, the division of the montane climax has its causal justification in the 

 striking climatic differences between the Rocky Mountain and the Pacific 

 regions. 



The task of finding concise descriptive names for the two associations has 

 not been simple, owing to the all but universal presence of the major domi- 

 nants. After much consideration, it seems best to refer to the eastern com- 

 munity as the pine-fir association, and to the western as the pine association. 

 When it is desired to emphasize their geographical relation, the Rocky Moun- 

 tain association is termed Petran, and the Sierra-Cascade one, Sierran. 



THE PETRAN MONTANE FOREST. 



PINUS-PSEUDOTSUGA ASSOCIATION. 

 Extent.— The montane forest of the Rocky Mountain region extends from 

 central Alberta to the Guadalupe and Chisos Mountains of western Texas, 

 and southward from the mountains of New Mexico and Arizona to Sinaloa 

 and Durango. At the north its area is relatively narrow and it yields to the 

 transition association of the Coast forest in the Selkirk Mountains of British 

 Columbia and in the Kootenai and Coeur d'Alene ranges of northwestern 

 Montana. It is broadest near the center where it ranges from the Black Hills 

 of South Dakota and the Pine Ridge and* Wild Cat Mountains of Nebraska to 

 the eastern slopes of the Sierras in Nevada. It apparently finds its south- 

 western limit in the Charleston Mountains of Nevada and its southern in the 

 Sierra Madre of Durango and Sinaloa. It is the characteristic forest of the 

 mountain ranges of this vast region and is the most extensive of all the forest 

 associations of the West (plate 48) . 



