TORREYA 



July, 1914. 

 Vol. 14 No. 7 



THE INFLUENCE OF PRECEDING SEASONS ON THE 

 GROWTH OF YELLOW PINE 



By J. E. KiRKwooD 



Many of the mountains of western Montana have no trees 

 on their western and southern slopes, except at altitudes above 

 4,000 or 5,000 feet. Some support sparse open stands of yellow 

 pine and Douglas fir; in such places the ground cover usually 

 consists of grasses and a few shrubs. The barren aspect of these 

 slopes is due to the desiccating influence of wind and sun, to the 

 full force of which they are exposed throughout the growing 

 season. The annual precipitation in the vicinity of Missoula is 

 15.84 inches, which represents the mean of over twenty years of 

 observation. May and June are the months of heaviest rainfall, 

 furnishing 4.43 inches of the above mean. By the same reckoning 

 July and August together furnish 1.95 inches of the annual 

 precipitation, and during these months the soil becomes dry to 

 a depth so great that only deeply rooted perennials are able to 

 survive, and these with growth suspended, or at least very much 

 retarded. 



Situations on such slopes, locally more favorable, are occupied 

 by incipient forest growth which increases toward the greater 

 altitudes, where, owing to the storage of snow and to other 

 factors, conditions are more favorable for the growth of trees. 

 In this region the yellow pine and Douglas fir are the species most 

 resistent to drouth, encroaching gradually upon the prairie, and 

 eventually occupying it fully or giving way to other species of 

 forest trees. In the margin of the yellow pine type, where the 

 prairie and the forest blend, the conditions for forest growth are 



[No. 6, Vol. 14, of ToRREV'A, comprising pp. 97-114, was issued 8 June 1914.I 



115 



