useful means to assign the proper status to successional and 

 climax communities. 



Environmental gradients are present in all areas. The patterns 

 of long-enduring environmental gradients occur with respect to 

 many factors such as the presence, areal extent, depth, and dura- 

 tion of snow-cover, exposure to wind and to insolation, soil 

 moisture, drainage, nature of substratum, occurrence and degree 

 of soil movement, freezing action, and presence of salt. The 

 presence and nature of gradients can be determined readily in 

 some places, with difficulty in others. The patterns of different 

 communities can be recognized in relation to the environmental 

 gradients, and are formed especially by populations of the more 

 sensitive species. In order to have only one type of community 

 over a very large area it would be necessary to have a nongra- 

 dient environment over the entire area, which cannot be attained 

 since geomorphic processes are operating continuously, although 

 often very slowly. Consequently the types of gradients will usually 

 remain essentially the same, for example the shift of snow patches, 

 or the increase in their size. It is conceivable that an environ- 

 mental condition at one or the other end of the gradient might be 

 eliminated. However, if the gradients present at the time have 

 reached the steady-state, then for a significant period the pattern 

 of gradients and their associated climax communities will remain 

 essentially the same, even though the actual disposition may 

 change to some indeterminate degree. 



The communities on the long-enduring gradients have a high 

 degree of stability and organization. The principal species in 

 each kind are well adjusted in their reactions to one another, such 

 as shading and accumulation of organic matter. Similar commu- 

 nities and similar patterns of communities are present in separated 

 areas in which environmental conditions are similar. So long as 

 environmental conditions remain substantially the same, the 

 communities appear to undergo fluctuations about a mean, rather 

 than directional change. These long-enduring communities have 

 the attributes of climax communities and fit well into a climax 

 mosaic pattern along environmental gradients. 



The presence of phasic cycles may be the cause, or one of the 



164 • Dyxmaxnics of CoMnxnimnities 



