Rydberg: Phytogeographical notes 17 



I to 4 feet thick. At the timberline it becomes low, crooked, and 

 gnarled. 



The Lyall's Larch, Larix Lyallii, is the most hardy tree of the 

 region. In moist valleys it reaches higher up than either the 

 spruce or the fir, and is more strictly a subalpine species than 

 either, growing mostly above 7,500 feet altitude. In the main 

 Rockies, it is not found south of latitude 48° 45', but runs further 

 south in the Bitter Root Mountains. It is mainly confined to 

 the western slope of the Rockies, and crosses the main divide 

 only north of Culver Glazier, latitude 48° 50'. It is a tree of 

 slow growth, becoming 25 to 40 feet high and having a diameter 

 of 2 to 4 feet. 



The Aspen plays here the same role as in the Southern Rockies, 

 but is perhaps not so common. 



In the lower portion of the Subalpine Zone, the lodge-pole 

 pine, limber pine and Douglas fir are found on the drier slopes. 

 In some places the former tree is the most common of all. The 

 limber pine is rare on the west side of the divide. 



In the Bitter Root Mountains the dividing line between the 

 Subalpine and Montane Zones is slightly lower. The principal 

 trees here are the same as in the main Rockies. The general 

 distribution of the subalpine fir is above 4,800 feet in the northern 

 part and 5,500 feet in the southern part, although it sometimes 

 descends as low as 3,000 feet. Here it is much more common 

 than Picea Engelmannii and is the characteristic tree of the 

 Subalpine Zone. The latter grows mostly at 4,000 to 7,000 feet 

 altitude. Lyall's larch is found along the main range of the 

 Bitter Root Mountains to the headwaters of the south fork of the 

 Clearwater River, latitude 45° 50'. Piniis alhicauUs grows 

 mostly above 6,000 feet altitude. Finns Murrayana ascends to 

 8,000 feet and Pseudotsiiga mucronata to 6,500 or 7,000 feet. 

 Pinus flexilis is wanting. The first of these three species, i. e., 

 P. Murrayana, is the most common tree on the eastern slopes of 

 the Bitter Root Mountains and there makes up about ninety 

 per cent, of the forest. 



To the other subalpine trees are here added the Alpine Hem- 

 lock, Hesperopeuce Mertensiana. It grows at an altitude of from 

 5,500 to 8,000 or even 9,000 feet. It is a tree of 60 to 100 feet in 



