22 Rydberg: Phytogeographical notes 



side of the continental divide. On the eastern side the species 

 are somewhat the same, but the proportion is different. 



Eastern slope 



The most common tree is the lodge-pole pine, growing on the 

 dryer ridges from 5,000 to 8,000 feet altitude. It grows often in 

 pure stands or mixed with Douglas fir and limber pine. 



The Rocky Mountain bull pine, Finns scopidorum, if found at 

 all, is limited to the foothills or the mountain sides bordering 

 thereon. 



In the valleys with northeastern or northern exposure, the 

 Engelmann spruce often is mixed with red fir and the Alberta 

 Spruce, Picea albertiana. The latter is not generally distinguished 

 from Picea Engelmannii or else mistaken for the eastern white 

 spruce, Picea canadensis, which is not found in Montana, and 

 further south for the blue spruce, P. Parryana. It much re- 

 sembles the Engelmann spruce, but the twigs are glabrous and 

 the cone-scales more rounded. The Engelmann spruce runs 

 down as low as 6,000 feet altitude. 



The deciduous trees are found mostly along the water courses. 

 The most important are Populus balsamifera and P. angustifolia, 

 Betida papyrifera and Betula fontinalis, A Inns tenuifolia and a 

 few willows. 



Western slope 



The southern exposure is, as in the Southern Rockies, more 

 xerophytic and this is mostly occupied by the lodge pole pine and 

 the Douglas fir, ranging at an altitude between 3,000 or 4,000 to 

 7,000 or 8,000 feet. Below 4,000 feet the forest consists mostly 

 of the Western Yellow Pine, Pinus ponderosa. This is closely 

 related to P. scopulorum but has longer leaves and cones. Many 

 regard the latter as a variety of the former. P. ponderosa is 

 practically limited to the Pacific slope and has been reported from 

 very few localities east of the divide. Contrary to its southeastern 

 relative, it is found on rich soil. It belongs to the Arid Transition 

 Zone of Washington and enters the Montane Zone only at its 

 upper limit. It is a tree often over 200 feet high and with a 

 trunk 5-8 feet in diameter. 



On the northern exposures and in the canyoijs, the Engelmann 



