20 Rydberg: Phytogeographical notes 



In open places they rarely reach a height of 50 feet and a trunk 

 diameter of 2 feet or a little more. In Colorado the species grows 

 at an altitude of 8,000 to 10,500 feet, sometimes descending below 

 8,000 feet and rarely reaching an altitude of 11,000 feet. Its 

 range extends farther north than that of the Pseudotsuga, viz., up 

 to Alaska, but not so far south. 



The Limber Pine or white pine of Colorado, Pinus fiexilis, is 

 rather local, never plentiful, and as a rule mixed with other 

 trees, mostly with Pinus scopulorum and Pseudotsuga mucronata 

 at lower altitudes and with Pinus aristata at higher ones. It 

 ranges from an altitude of 7,500 feet nearly up to timberline. 

 It is therefore a tree common to the Subalpine and the Montane 

 Zones. In general appearance it is intermediate between the 

 foxtail pine and the bull pine and is often mistaken for either. It 

 ranges from Alberta to western Texas and southern California. 

 The red cedar, Sabina scopulorum, belongs rather to the Foot- 

 hill Zone, but ascends often to an altitude of 9,000 feet and grows 

 associated with the bull pine. 



The composition of the mesophytic forest of the northern slopes 

 consists of the following trees: 



The principal tree here, as in the Subalpine Zone, is the 

 Engelmann Spruce, which has been discussed already. 



The subalpine fir is found mostly in the transition region 

 between 9,500 and 10,000 feet, rarely as low as 9,000 feet altitude. 

 The Balsam Fir or Colorado white fir, Abieslconcolor, takes its 

 place at lower altitudes, viz., between 8,000 and 9,000 feet. It is a 

 tall symmetrical tree with short horizontal branches, sometimes 

 reaching a height of 250 feet. Its geographical distribution 

 extends over the Southern Rockies, Sierra Nevada and other 

 mountains of California, north to the Siskiyou Mountains of 

 Oregon, but it is not found in the Northern Rockies. 



The Douglas fir and the lodge-pole pine are also mixed occa- 

 sionally with the Engelmann spruce. 



Along the water-courses the composition of the forest is as 

 follows : 



The Colorado Blue Spruce, Picea Parryana, is growing along 

 the water courses in the region mostly occupied by the bull pine 

 and Douglas fir. It is therefore often found ^on the southern 



