GENERAL FOREST ASPECTS 79 



differences in altitude are so pronounced and where soil and 

 climatic conditions present wide variations. 



The facts of horizontal and vertical distribution are pre- 

 sented from various sources,* with data on the relative impor- 

 tance of the various species. By this means it is hoped to present 

 a conception of the forest as it appears in different parts of 

 this region. It is admitted that the data here assembled can 

 be interpreted only in the most general way. The figures on 

 percentages are to be taken merely as indicators of the approxi- 

 mate proportions of the species in the forest covering of the 

 area in question, and not as an accurate final result. The extent 

 of the area and the extent of the work involved must entail a 

 large probability of error. The figures presented below, how- 

 ever, point to safe conclusions as to the facts of general distribu- 

 tion, which is the purpose of this paper, and from this stand- 

 point it is doubtful whether any further refinement of the per- 

 centage data would be either necessary or profitable. 



A study of the forests of the northern Rockies brings into 

 view certain aspects of their composition and distribution related 

 to particular topographic and climatic conditions of the different 

 geographic units or sections which they occupy. For convenience, 

 therefore, as well as for conformity with the points of ecological 

 significance the area will be considered by sections determined 

 in accordance with certain physiographic features which either 

 individually or in groups seem to stand in some important rela- 

 tion to the flora. Fifteen such sections are recognized. The 

 first three lie in adjacent parts of Idaho, the forests of which, 

 in their composition, should be considered in connection with 

 those of Montana, as in fact being in large degree the immediate 

 source from which many of the species have found their way 

 eastward into the higher mountains and valleys. The sections 

 are numbered from the west and will be discussed in the same 

 order. They are named for some significant physiographic fea- 

 ture. See Figure 12. 



The first or Priest Lake Section occupies the panhandle of 

 Idaho and includes the drainage of the Priest River and the 

 Kootenai in Idaho. It extends from the international boundary 



* The writer wishes here to acknowledge with thanks the generous co- 

 operation of the officers of the Forest Service in District 1, who supplied 

 much of the information essential to this discussion. 



