208 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



DOMINANTS. 



plnus ponderosa. plcea pungens. plnus strobiformis. 



pseudotsuga mucronata. plnus flexilis. plnus chihuahuana. 



Abies concolor. Pinus flexilis albicaulis. Pinus arizonica. 



PlNUS CONTORTA. 



As already indicated, the first 3 species are to be regarded as the major 

 dominants of the association by reason of their abundance and wide occur- 

 rence. The lodgepole pine (Pinus contorta) ranks next in importance. It is 

 typically the subclimax dominant of the burn subsere in both the montane and 

 subalpine forests. However, it is more or less exclusive over such large areas 

 in the northern Rocky Mountains, and is so relatively permanent owing to 

 repeated fires that it must be considered with the climax. Picea pungens is 

 limited to the central Petran regions, and usually occurs in restricted stands 

 along the lower edge of the zone. More rarely, it forms a mixed forest with 

 yellow pine and Douglas fir, as in the Pike's Peak region, and in the Blue and 

 White Mountains of Arizona (Greenamyre, 1913). Pinus flexilis and P. 

 albicaulis are trees of wide range altitudinally, and hence are found in both the 

 montane and subalpine climaxes. They are more abundant and relatively 

 more important at upper elevations near timber-line, and hence are regarded 

 as belonging primarily to the subalpine forest. Pinus flexilis occurs through- 

 out the association, while P. albicaulis ranges from the northern edge to Yellow- 

 stone Park. Pinus strobiformis is a related pine which occurs only in south- 

 eastern Arizona and adjacent New Mexico, and thence southward into Sonora 

 and Chihuahua. Pinus chihuahuana and P. arizonica are close relatives of 

 P. ponderosa. They occur with the latter or represent it in southern Arizona 

 or New Mexico at elevations of 6,000 to 8,000 feet, and extend southward into 

 the Sierra Madre of Mexico. 



Groupings. — The four most important dominants, Pinus ponderosa, P. 

 contorta, Pseudotsuga, and Abies, regularly occur in pure stands as well as in 

 mixed communities. This is especially true of the two pines. In the case of 

 the lodgepole pine, this is a consequence of its ability to occupy burned areas 

 completely, while with the yellow pine it results from its extension far beyond 

 the mass of the association. With these two very important exceptions, the 

 montane forest is largely a mixture or consists of small alternes of the different 

 species. The minor dominants usually occur intimately mixed with the 

 major ones, though they too may form pure communities of small size. 



In general, Pinus ponderosa, Pseudotsuga, and Abies occur together through- 

 out the mass of the association. To the northwest, Abies becomes secondary 

 or is lacking, and the forest consists primarily of yellow pine and Douglas fir, 

 or of lodgepole pine. Since these are related successionally, one often contains 

 relicts of the other. In the Wasatch Mountains, Pinus ponderosa is mostly 

 absent and the forest consists of Pseudotsuga and Abies, while in the desert 

 ranges, farther west, Pseudotsuga is usually the missing one of the three. 

 Picea pungens is practically limited to the central area of the association, 

 represented by Colorado, Utah, northern Arizona, and New Mexico. It is 

 often in open woodland along streams, but it may be an important member of 

 the lower portion of the montane zone, mixed with Douglas fir and yellow 

 pine, or more rarely with Abies concolor. Pinus flexilis occurs with yellow 

 pine or Douglas fir, or with both on xerophytic ridges and slopes at lower 

 levels. P. albicaulis has a similar habit, but is much less common in this 

 association. As already indicated, Pinus strobiformis, P. chihuahuana, and 



