and eventually grow up through the shrub layer. After a period 

 of several years the large trees cast sufficient shade to inhibit 

 further establishment of shrub seedlings and to allow the estab- 

 lishment of seedlings of additional forest trees. 



Ultimately the plant community reaches a stage where it does 

 not undergo a directional change in species composition, but has 

 achieved a stability that is reflected by the fact that the youngest 

 plants in the area are the same species as the oldest ones. The 

 change in the plant communities that may occur on a given site 

 over a period of time is called succession. Succession may occur 

 after gross ecological disturbances, or may result from more sub- 

 tle changes such as the gradual filling in of a pond with the con- 

 sequent replacement of an aquatic community by a terrestrial 

 one, or from the occupation of a bare rock surface by lichens, 

 mosses, and ultimately other plants. 



In areas which are subjected to continual burning, a climax 

 plant community may never develop, but the site may be occu- 

 pied by a subclimax community. For example, Douglas Fir is not 

 a climax tree in many areas west of the Cascades in the Pacific 

 Northwest, but forests of this species are maintained over long 

 periods of time in this region because logging activities make 

 conditions suitable for estabUshment of Douglas Fir seedlings but 

 not of seedlings of the coniferous species that are components of 

 the climax vegetation. Likewise, repeated fires in many areas of 

 the western United States prevent the development of climax 

 plant communities. Plant communities that are not climax, there- 

 fore, cannot be preserved by maintaining a policy of "hands off." 

 For example, in many (but not all) areas of CaHfornia, Chaparral 

 is maintained by periodic fires. Conservationists must appreciate 

 the fact that some western plant species and plant communities 

 are more likely to disappear if they are undisturbed than if they 

 are allowed to burn occasionally. 



Another kind of climax that may develop is the disclimax. In 

 areas of the Central Valley that were occupied by native perennial 

 grasslands, the native species have disappeared and have been 

 replaced by introduced annual grasses which are maintained be- 

 cause of grazing. In this case, an unnatural plant community has 

 been induced which is not a component of the natural succession- 



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