82 FOREST DISTRIBt'TIOX 



spruce favors the western slope. On the other hand Pinus pon- 

 derosa, Larix occidentalis and Pseudotsuga taxifolia are far 

 more heavily represented on the eastern side than on the west- 

 ern, forming respectively 2, 6, and 13 9"^ per cent on the western 

 side, as against 25, 24 and 25% on the drier eastern slope. 



Another feature of the difference is in the vertical distri- 

 bution of the species. On the western side the lower limit of 

 distribution is from 700 to 1000 feet below that of the eastern 

 and the upper limits from 500 to 2000 feet lower. It is noticeable 

 also that certain species like the grand fir and western hemlock 

 are confined to a much narrower vertical range on the eastern 

 than the western slopes, falling within a zone of 1000 to 1500 

 feet in the former and of about 3000 feet in the latter case. 

 These facts can only.be interpreted in terms of the moisture 

 conditions and the lov%'er limits of temperature encountered at 

 the higher altitudes. Several species listed in the table were 

 not of sufficient importance to merit calculation as to their 

 quantitative relations in the forest. 



The third or Clearwater section lies on the western slope of 

 the Bitter Root Range and embraces the drainage of the upper 

 branches of the Clearwater, its North, ^Middle and South Forks. 

 The area reaches the summit of the Bitter Root divide on the 

 east and covers about 100 miles from north to south and from 

 40 to 60 east and west. The course of its principal streams is 

 mainh- westward to a junction with the Snake River. 



In its northern part the forest vegetation is more nearly 

 related to that of the western slope of the Coeur d 'Alenes. The 

 white pine, arbor vitae grand fir and Douglas spruce are tlie 

 dominant species. In its southern part these species apprar 

 very sparingly, and the dominant species are yellow and lodge- 

 pole pines and Douglas spruce. In this part the climatic con- 

 ditions are merging into those of the arid plains of the Snake 

 ^'alley. The marked variations in the percentage of different 

 species will thus be interpreted accordingly. 



