FACTORS WHICH MODIFY THE TYPICAL SUCCESSION 217 



133. The Climax Stage of Succession.— AVhen the habitat be- 

 comes as mesic as is possible under the existing cHmatic and ])hysio- 

 graphic conditions tlie dominant species of the cHmax association 

 appear and in time gain control of the environment. The climax 

 association is the mature, adult organism of which the pioneer and 

 intermediate communities are developmental stages. The climatic 

 climax is the same throughout a climatic region whether it develops 

 through a xerarch or a hydrarch succession. The course of develop- 

 ment of a sere and its ultimate climax can usually be predicted, 

 therefore, with perfect definiteness if the climax of the region is 

 known. 



The dominant species of the climax association exercise such a 

 complete control over the environmental factors such as light, space, 

 food materials, etc., that they prevent the entrance into the com- 

 munity of many other species of plants. For this reason, while the 

 number of individuals in the climax community may be as great as 

 in the intermediate stages, the number of species is ordinarily much 

 smaller than in some of the earlier stages. The adjustment between 

 the dominant species of a climax association and the environment 

 approaches so near to perfection that the community is relatively 

 stable and is likely to persist as long as the climatic and physio- 

 graphic conditions remain unchanged. 



134. Factors Which Modify the Typical Succession.— There are 

 various topographic, physiographic, and biotic factors which modify 

 the typical or ideal succession. Most often these factors affect 

 only the rate; either accelerating or retarding the succession. In 

 some cases, however, the succession is reversed temporarily so that 

 its direction is away from rather than toward, mesism, a phenom- 

 enon that is comparable with the cutting back of a small tree to a 

 shrub stage. Such a shrub if left undisturbed under ordinary con- 

 ditions will develop again into a tree and in a comparable way the 

 plant community which has been cut back by one means or another 

 will, as a rule, develop again toward the mesic climax. 



A topographic factor that influences the rate of develojiment is 

 the degree of slope. As a rule, the more nearly horizontal and level 

 an area is the more rapid can the succession take place. With an 

 increasing angle of slope the succession is more and more retarded 

 and if the slope approaches too near the vertical no development can 

 take place at all. The direction of slope is also of some importance 

 in this respect. In a dry, hot region the vegetation of a north-facing 



