PHYSIOGRAPHIC FACTORS 187 



mid atmosphere, low transpiration rates, and a plentiful supply of 

 water. 



In addition to local topographic effects are those of a regional 

 nature associated with mountains. The increase in precipitation 

 and decrease in temperature with increasing altitude result in 

 vegetational zonation. Within these zones, the local effects of to- 

 pography again become apparent so that zones lie at higher alti- 

 tudes on a south than on a north slope and the species of a particu- 

 lar zone will be found extending downward in ravines and upward 

 on ridges. 



A mountain may affect conditions for growth at some distance 

 from itself. Some mountains are centers over which rain clouds 

 form and from which they often move to provide moisture for 

 surrounding lowlands. At the same time, streams starting in moun- 

 tains and fed by precipitation there, flow down to valleys below. 

 Other mountains act as barriers when they lie at right angles to the 

 prevailing winds, for all the moisture may fall upon the mountain 

 and none be left for the area beyond. This explains the lack of 

 moisture in the Great Basin. The prevailing winds coming from the 

 Pacific lose their moisture over the Coast Ranges and the Sierra 

 Nevada. 



Finally, it is probable that mountains act as barriers to the nat- 

 ural migration of some species that are unable to compete with the 

 flora upon the mountain or to withstand the successive changes of 

 environment associated with increasing altitude. 



GENERAL REFERENCES 



L. D. Baver. Soil Physics. 



K.D. GLINKA. The Great Soil Groups of the World and Their Development. 



H. JENNY. Factors of Soil Formation. 



C. E. KELLOGG. Development and Significance of the Great Soil Groups of 



the United States. 

 R J. KRAMER. Soil Moisture in Relation to Plant Growth. 

 C. F. MARBUT. Soils of the United States, in Atlas of American Agriculture. 

 U. S. Dept. Agr. Soils and Men. 



