16 Rydberg: Phytogeographical notes 



Engelmannii or at lower altitudes sometimes with Pinus Murray- 

 ana. It is usually a small tree, very rarely reaching 70 or 80 feet 

 in height, with a trunk of 2 fee| in diameter, the average good-sized 

 tree being 45 or 50 feet high. It is found at an altitude of from 

 about 9,000 feet to the timberline. 



The limber pine, Pinus flexilis, belongs really to the Montane 

 Zone, where it has its best development, but extends into the 

 Subalpine Zone almost to the timberline. 



Subalpine Zone of the Northern Rockies 



As the Northern Rockies are of much larger extent than the 

 Southern and the conditions are much more variable, the descrip- 

 tion of the different zones of one locality will poorly fit for all 

 places. I shall therefore first select a fairly typical region near 

 the center of the district and then contrast with this the outlying 

 regions. The region selected is the main range of the Rockies in 

 northern Montana. The Subalpine Zone in this region extends 

 from an altitude of about 6,500 feet to the timberline. The most 

 important tree is, as in the Southern Rockies, the Engelmann 

 spruce (Picea Engelmannii) , but it becomes less predominant, and 

 the subalpine fir, Abies lasiocarpa, becomes more frequent. Both 

 are characteristic of the northern exposures. 



The Engelmann Spruce, just as in the Southern Rockies, is 

 not limited to the Alpine Zone, but runs down through the Mon- 

 tane Zone to a little below 5,000 feet, and is there confined mostly 

 to the northern slopes and valleys. In this region it sometimes 

 reaches a height of 125 feet, with a trunk 3 feet in diameter. 

 In some places especially on the west side of the Bitter Root 

 Mountains, it is more or less supplanted by the next. 



The Subalpine Fir, Abies lasiocarpa, grows between an altitude 

 of 6,000 feet and timberline, rarely lower down. Under favorable 

 conditions it reaches a height of 75 feet, with a trunk diameter of 

 2 feet. 



The White-barked Pine, or, as it is called in Montana, the nut 

 pine, Pinus albicaulis, takes the place of P. aristata on the southern 

 exposures, often growing scattered in grassy "parks." It re- 

 sembles the same somewhat in growth, although it is more closely 

 related to P. flexilis. It is a tree 20 to 40 feet high, with a trunk 



