702' 



JOURNAL OF FORESTRY 



at Stations i,ooo vertical feet apart. It is important to find that what 

 we may designate as "the cHmatic vakie" of slope exposure is so nearly 

 the same for a large number of dominant trees and shrubs, and is also 

 the same in its influence on the size of the populations of yellow pine at 



Fig. 4. — Graphs showing the composition of the adult stands of Arizona yellow pine on six 

 areas of exposure and altitude as indicated, the diameters being grouped by lo cm. intervals. 



different altitudes. In so far as a correlation of vegetational features 

 and climatic conditions may be taken as a proof of a causal connection 

 between the two, it would appear that the ratio of soil moisture to evap- 

 oration is the climatic complex which controls this set of phenomena. 

 In order to make a more significant comparison between the popula- 



