CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 265 



rado, while P flexilis is more common northward to where P. albi- 

 caidis takes over in the northern Rockies. 



Fire in the Douglas fir climax results in the establishment of 

 lodgepole pine or aspen stands, which bear the same relationships 

 here as in the subalpine zone. 



Ponderosa Pine Climax.— Below the Douglas fir, is a belt in 

 which Pinus ponderosa or a close relative forms a relatively open 

 climax forest that becomes savannah-like with decreasing altitude. 

 The widely spaced trees form little shade so that the ground cover 

 is made up of grasses, among which numerous species of Festuca. 

 Agropyron, Poa, and Muhlenbergia are important. Between the 

 zone of Douglas fir and the drier, lower altitudes with pure stands 

 of ponderosa pine is a fairly broad transition where the two trees 

 may share dominance. 



Although the climax is termed ponderosa pine, the species is 

 dominant only in the northern Rockies to the west of the con- 

 tinental divide. Elsewhere it is replaced by or in association with 

 closely related varieties whose ecological characteristics are sim- 

 ilar. Pinus ponderosa var. scopidorum is the important tree on the 

 east slope in the north and throughout the zone southward. In the 



Fig. 135. Subclimax stand of lodgepole pine in Montana.— U. S. Forest 

 Service. 



