24 Rydberg: Phytogeographical notes 



The deciduous trees belonging to this zone grow all along the 

 water courses. They are: the cottdnwoods, Populus trichocarpa, 

 the tallest deciduous trees of America, P. hastata, P. Besseyana, 

 P. balsamifera, P. angustifolia (rare on the west side), and P. 

 tremuloides (the three first are seldom found east of the divide) ; 

 further, such willows as Salix Lyallii, S. sessilifolia, S. Macken- 

 zieana, S. Scouleriana, and several others, scarcely becoming 

 trees; the birches, Betula papyrifera, B. fontinalis, B. Piperi, B. 

 subcordata, and B. occidentalis; alders, Alnus tenuifoUa; several 

 species of Crataegus and Amelanchier, etc. 



The Montane Zone in the Canadian Rockies is practically the 

 same, but both its upper and lower limits are found gradually 

 lower and lower down the mountains the further north one goes. 



In the Bitter Root Mountains, it resembles much that of the 

 western slope of the Rockies. To the conifers are added the 

 Pacific Yew, Taxtis brevifoUa, which often becomes only an in- 

 significant shrub. The extent of the Zone is between 4,000 and 

 5,800 feet altitude and the most important trees here on the 

 northern exposures are the mountain white pine, Pinus monticola, 

 which, however, extends in the valleys as far down as 2,000 feet 

 altitude, and Larix occidentalis. On the southern exposures, P. 

 ponderosa and Pseudotsuga mucronata are predominant. 



In the Yellowstone National Park and mountains north thereof, 

 about half of the coniferous flora consists of the lodge-pole pine. 

 Otherwise the conditions are the same as on the eastern slope in 

 northern Montana, that is with a small amount of Picea Engel- 

 mannii and P. albertiana on the northern exposures; Pinus fiexilis 

 and P. ponderosa on the southern exposures, but the latter two 

 only near the lower limit; some Pseudotsuga on both, and aspen 

 and Cottonwood along the water courses. On the level plateaus 

 of the Yellowstone National Park the forest consists of pure 

 stand of Pinus Murrayana. A small number of Pimis monticola 

 has been found on Davis Creek and Slough Creek in southern 

 Montana and Abies grandis has been reported from Yellowstone 

 Park. 



In the Big Horn Mountains the coniferous forest is repre- 

 sented by but four species: Pinus Murrayana, P. fiexilis, Picea 

 Engelmannii and Pseudotsuga mucronata, of ,whicji the first makes 

 up perhaps ninety per cent, of all. 



