94 FOREST DISTRIBUTIOJr 



most of the country is prairie, but in the hiMs especially in the 

 ravines and canyons the few forest species are distributed at 

 all altitudes. On portions of this area the yellow pine {F. pon- 

 derosa) is reported as forming 959<: of the stand and Juniperus 

 scopulonim about 2^; . J. communis is also reported. 



In the 15th, or ^Missouri Section, no forests of anj" conse- 

 quence occur. The margins of the benches above the river 

 valley are occupied by a sparse growth of Piniis ponclerosa and 

 Juniperus scopulorunt and the isolated elevations here and there 

 are similarly wooded. 



Reviewing the facts on general distribution as above stated, 

 several conclusions are evident. In the first place the species 

 decrease in numbers from the west toward the east, until of the 

 fifteen or more gymnosperms of the Idaho forest only four or 

 five appear near the eastern border of ilontana. ^ith the 

 decrease in the number of species comes also a decrease in the 

 volume of the forest. The same cause which limits the forest 

 flora to the more resistant species likewise limits the volume 

 production, viz., the lack of sufficient moisture. Again it will 

 be noted that the altitudinal range of the species in most parts 

 of the region is very wide. The influence which tends to nar- 

 row the vertical range of distribution is again the influence of 

 scant rainfall and the other conditions which tend toward a 

 xeroph\-tic environment. Such influences not only narrow the 

 vertical range, but push the lower limits of forest distribution 

 further up the mountain slopes. 



