CHAPTER V. 



FOREST ZONES AND FORMATIONS. 



THE forests of the northern Rocky Mountains are included 

 in the Hudsonian, Canadian and Transition zones of 

 Merriam (43). Sometimes the Hudsonian and Canadian 

 are called the supalpine and montane zones respectively (55). 

 The Transition is confined mainly to the foothills, valleys and 

 plains. The Upper Sonoran, if it extends into Montana at all, 

 is confined to the lowlands and benches along 1 the rivers in the 

 plains and bears little relation to the forests. The zones as 

 such, however, are more easily defined by given sets of condi- 

 tions than by the species which are supposed to represent them, 

 since few species are so clearly limited by temperature, either 

 in latitude or .altitude, as to be plainly identified with either 

 zone. This is especially true with reference to the middle zones 

 in this region, viz., the Transition, the Canadian and the Hud- 

 sonian. The upper Sonoran and the Arctic-Alpine, as repre- 

 senting the extremes of conditions, are the more clearly recog- 

 nized by some of their species which, being at the extremes of 

 their ranges, are mainly excluded from the other zones. Such 

 are the Yucca and the cacti of the former and the dwarf willows 

 of the latter, although even some of these may transgress their 

 allotted limits. 



Many plants which Gary (11) cites as marking the upper 

 Sonoran of Wyoming occur in neighboring parts of Montana. 

 Among these may be mentioned the following : 



*Populus occidentalis (P. Sargentii) Yucca glauca 



acuminata Opuntia polyacantha 



*Salix amygdaloides * Plant ago Purshii 



fluviatilis ^Artemisia trident at a 



Five of these, however, (*) are so widely distributed in 

 Montana, and in such varied conditions, as far to exceed the pos- 

 sible limits of the Upper Sonoran Zone. 



The Transition Zone is a belt of wide vertical range and 



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