Figure 5-1. Bare areas are formed in this region of pro- 

 nounced changes by erosion, deposition, melting of glaciers, 

 and other factors. Note dense forest on lower slopes. Tracy 

 Arm, Alaska. (U. S. Forest Service.) 



are surpassed, other species take over and competition between 

 species tends to produce sharper boundaries in communities than 

 occur in habitat conditions. ^^^ The transition from one type of 

 community to another may be gradual, especially where the 

 gradient changes slightly with distance, but the transition may 

 be sharp where discontinuities'^^ in the environment occur, such 

 as considerable differences in the substratum which favor the 

 formation of discrete vegetation units. 



Types of Gradients and Their Relations to Vegetation 



In a given region a number of gradients are found, and within 

 each of them secondary gradients occur; for example, local 

 gradients are present on hillsides within the gradual gradient of 



138 • Dyxmansics of Coxitmuxiities 



