224 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



and Alberta, and in the Yukon, it lies in contact with the boreal forest. The 

 upper contact is everywhere with the alpine climax when this is present. In 

 many places the forest becomes so dwarfed and open as to form what is essen- 

 tially an alpine savannah. 



Associations. The subalpine climax resembles the montane one in its dif- 

 ferentiation. This is obviously due to the practically complete separation of 

 the Petran and Sierran axes, except in the north. As a result, the formation 

 has developed a distinct association along each axis where they are widely 

 separated and exhibits a transition area in the north, where they are contigu- 

 ous. Since the transition results from the mingling of two associations of the 

 same formation, it is undesirable to give a distinct value to it, as was done 

 with the broad ecotone between the Coast and montane climaxes. 



There are three chief reasons for recognizing two associations. The first is 

 that Picea engelmanni and Abies lasiocarpa are the two major dominants in the 

 Rocky Mountains, while they have several codominants from Oregon and 

 Idaho to Alaska and are lacking in California. The second reason is that 

 Pinus flexilis and P. aristata are typical of the Petran axis, but the former is 

 replaced in the Northwest and the Sierras by P. aUncauUs, and the latter by 

 P. balfouriana in California. The third lies in the fact that Tsuga mertensiana, 

 Larix lyallii, and Abies magnifica are confined to the western association. 

 Furthermore, the societies of the two associations are composed for the most 

 part of different species, though the genera are largely identical. 



The two associations may be designated as eastern and western simply, as 

 Petran and Sierran, or by using the names of typical dominants, as the spruce- 

 balsam and pine-hemlock associations. It seems preferable to use the terms 

 Petran and Sierran as a rule, since the division is similar to that of the montane 

 forest. In both cases the word Sierran is used to include the mountain axis 

 from California to British Columbia. 



THE PETRAN SUBALPINE FOREST. 

 PICEA-ABIES ASSOCIATION. 



Extent. The northern limit of the spruce-balsam forest seems to be in the 

 inland ranges of southern Yukon, but the contiguity of the two associations 

 and the boreal forest is such that it is impossible to distinguish their proper 

 limits with our present knowledge. This association is well-developed in the 

 Rocky Mountains of British Columbia and Alberta and extends eastward 

 toward the Lesser Slave Lake. It occurs throughout the main ranges of the 

 Petran axis from Montana to northern New Mexico and Arizona. It is 

 found in reduced form on the Charleston Mountains of southern Nevada 

 and the Panamint Range of southeastern California. It should probably be 

 assigned to the Blue Mountains of Washington and Oregon also, though Tsuga 

 mertensiana occurs on one peak. This illustrates the difficulty in drawing a 

 Umit between the two associations in the Northwest, and at present it must 

 suffice to assign the spruce-balsam community to the ranges of central Idaho 

 and southwestern Montana (plate 53). 



In altitude, the community ranges from 3,000 to 7,000 feet in the north 

 to 8,000 to 12,000 feet in Colorado and New Mexico. 



