Rydberg: Phytogeograpiiical notes 21 



slopes. It needs a good deal of moisture, but at the same time 

 prefers an open forest, and stands less shade than the Engelmann 

 spruce. This explains perhaps why it succeeds better as a planted 

 tree in the Eastern States than any other western species. It 

 ranges in altitudes from 6,500-9,000 feet or sometimes 10,000 

 feet, but descends into the foothills only in the valleys. It 

 grows to the height of 90 feet and a trunk diameter of 2)4. feet, 

 or becomes rarely 100 feet high and 4 feet thick. It is limited to 

 the Southern Rockies, and has been collected, as far as I know, 

 only at one locality in what I regard as belonging to the Northern 

 Rockies, viz., the Teton Mountains of western Wyoming. 



The Narrow-leaved Cottonwood, Popidiis angustifolia, 

 grows along the streams throughout .the foothills as well as the 

 Montane Zone, in other words at an altitude between 5,000 to 

 10,000 feet, or rarely as high as 10,500 feet. It becomes some- 

 times 50 feet high with a trunk nearly 2 feet in diameter, but 

 this size is extremely rare. Individuals with somewhat broader 

 leaves are sometimes mistaken for the balsam poplar, P. bal- 

 samifera. Popidus angustifolia is distributed from North Dakota 

 and Washington to New Mexico and California. The Balsam 

 Poplar, Populus bahamifera, is rare in Colorado and belongs to 

 the northern Rockies. It has about the same altitudinal distribu- 

 tion as the preceding and attains the same size or a little larger. 



The W^estern Black Birch, Betula fontinalis, and the Rocky 

 Mountain Alder, Alnus tenuifolia, grow in similar situations, but 

 never become trees of any large size. They are found throughout 

 the Rocky Mountain region, in Colorado growing at an altitude 

 betw^een 5,500 and 9,000 feet, the latter sometimes reaching 

 10,000 feet. 



Several species of willows are also found but most of these 

 are mere shrubs. 



The hawthorns (several species of Crataegus) and choke- 

 cherry, Prunus melanocarpa, enter the lower valleys but belong 

 really to the foothills or Great Plains. 



Montane Zone of the Northern Rockies 

 The Montane Zone of the Rockies in northern Montana con- 

 tains more species than in Colorado, especially on the western 



