18 Rydberg: Phytogeographical notes 



height and 3 to 6 feet in diameter with a greatly developed crown. 

 It is not found in the main Rockies, at least not within the United 

 States. It is not uncommon in the Selkirks and extends south 

 in the Bitter Roots as far as to the divide between the north and 

 middle forks of Clearwater River. 



In the Yellowstone Park and in the Tetons, the Subalpine Zone 

 begins at an altitude of 7,500 or 8,000 feet and the timberline is 

 at an altitude of about 10,000 feet. The trees are the same as 

 those of northern Montana, except that the Lyall larch is lacking 

 and the lodge pole pine is more common near the lower border. 



In the Big Horn Mountains both the Engelmann spruce and 

 the subalpine fir are scarce and in most cases the upper limit of 

 the lodge pole pine constitutes the timberline. 



In the Black Hills a Subalpine Zone can scarcely be spoken 

 of, as the highest mountains are less than 7,500 feet. The sub- 

 alpine species of the Rockies are wholly lacking. 



Montane Zone of the Southern Rockies 



The upper and lower limits of the Montane Zone in Colorado 

 are approximately at an altitude of 10,000 and 7,500 to 8,000 feet. 

 The former is about the upper limit of Finns scopidorum, the 

 latter near the lower limit of Pinus Murray ana, and Pseudotsuga 

 mucronata is rare below this altitude in Colorado. These three 

 trees together with Picea Engelmannii are the most important 

 timber trees of this zone. 



The southern slopes of the mountains and the tops of the 

 table lands are more or less xerophytic, and here the bull pine or 

 yellow pine, Pinus scopidorum, and the Douglas or red fir, 

 Pseudotsuga mucronata, are predominant. The northern slopes 

 and more protected valleys and flats are decidedly mesophytic. 

 The predominant tree on the northern slopes and in the valleys 

 is the Engelmann spruce. The lodge-pole pine, Pinus Murrayana, 

 is often found with the Engelmann spruce, but is often found on 

 the upper plateaus in pure stands or mixed with the more xero- 

 phytic species. 



The composition of the more xerophytic forest of the southern 

 slopes is as follows: 



The most important tree is perhaps the Bull,Pine, P. scopulo- 



