THE PETRAN SUBALPINE FOREST. 225 



DOMINANTS. 



plcea engelmanni. plntjs aristata. plnus flexilis albicaulis. 



Abies lasiocarpa. Pinus flexilis. Pinus contorta. 



Picea engelmanni and Abies lasiocarpa are the two major dominants through 

 practically the entire area of the association, except for the ranges of the 

 Great Basin. Pinus contorta has a similar extensive range, but it drops out 

 in southern Colorado. Pinus flexilis is much less important, though it has the 

 widest range of all, extending from Alberta to New Mexico, Arizona, and 

 southeastern California. Pinus albicaulis belongs chiefly to the Sierran associ- 

 ation, but is found in the Rocky Mountains from Alberta to northwestern 

 Wyoming. Pinus aristata is essentially southern in distribution, occurring 

 from northern Colorado to northern New Mexico and Arizona and westward 

 to the Panamint and Inyo Ranges of southeastern California. 



Groupings. — The basic grouping throughout is that of Picea and Abies 

 This is varied in the north chiefly by the inclusion of Pinus contorta. Pinus 

 flexilis and P. albicaulis may occur in the community here also, and even 

 Larix lyallii enters it in Alberta. In northern Colorado the usual grouping 

 is Picea, Abies, Pinus contorta, and P. flexilis, while in central Colorado and 

 southward the lodgepole pine drops out and Pinus aristata appears. In the 

 Pike's- Peak region both Pinus contorta and Abies lasiocarpa are absent and 

 the forest consists of Picea engelmanni for the most part, while Pinus aristata 

 and P. flexilis become associated with it toward timber-line. On the desert 

 ranges of the Southwest, Pinus flexilis and P. aristata alone remain to represent 

 the subalpine forest. Extensive pure stands are frequent for Picea, Abies, 

 and Pinus contorta, while the mixed forest of Picea and Abies often covers 

 great areas without any other dominant except the subclimax Populus tremu- 

 loides. The last is an important tree throughout the subalpine zone, covering 

 burned areas everywhere, in the absence of the lodgepole pine especially. 

 It also resembles the latter in occurring in both zones. 



Factor and serai relations. — The precipitation in the central part of the area 

 ranges from 22 to 40 inches a year, of which the snowfall is 8 to 14 feet. On 

 interior ranges the rainfall may be somewhat less. The evaporation is much 

 less than in the montane zone, the reduction often exceeding 25 to 50 per cent. 

 At the lower limit the growing season is 3 to 4 months long, at the upper barely 

 2 months. The mean temperatures are 5 to 10 degrees lower than in the 

 montane forest, and near timber-line frost occurs frequently or regularly during 

 the summer. 



Picea engelmanni is the most mesophytic of the dominants, often growing 

 at the edges of streams and in bogs. It is followed more or less closely by 

 Abies lasiocarpa, while all the pines are much more xeroid. Pinus contorta 

 is the most mesophytic of these, while the remaining species are more or less 

 similar, P. flexilis usually growing in the driest situations. As to light rela- 

 tions, Picea is the most tolerant, though Abies often equals it. The pines are 

 all much less tolerant and do not differ markedly from each other in this 

 respect. Pinus contorta is the most tolerant, and P. flexilis and P. aristata 

 the least, though all must be regarded as intolerant. 



The water and light relations furnish a clear explanation of the successional 

 sequence (Clements, 1910: 54). The main body of the forest is composed of 



