218 



PLANT SUCCESSION 



slope will usually develop more rapidly than on a south-facing slope 

 while in a cool, moist climate just the reverse is true. 



An extremely important physiographic factor is erosion. This may 

 either hasten or retard development or it may destroy vegetation 

 and so put the sere back to a younger stage. Deposit is more likely 

 to hasten than to retard a succession, though it may do either 

 (Fig. 97). 





-^ •■/- K*^^^^ 





5,J&^^ 



t -^^ 



-^ 



^ . 



«^^.' 



Fig. 97. — A shingle beach. Plant succession is very greatly retarded in 

 such a place because of the unstable nature of the substratum and frequent 

 flooding. 



Parasitic plants or animals may retard a succession by destroying 

 certain kinds of plants or they may cut the sere back to an earlier 

 stage, especially if they attack the dominant plants of the com- 

 munity. Grazing animals often produce similar effects. ]\Ian has, 

 of course, caused very great modifications in successions over 

 extensive areas, these changes being too numerous and far-reaching 

 to be discussed here. Fire, which may be either a biotic or a climatic 

 factor, is largely destructive though it may be only retarding in its 



effect. 



REFERENCES 



Barton, L. V.: Germination of Some Desert Seeds, Contr. Boyce Thompson 



Inst., 8, 7-11, 1936. 

 Clements, Frederic E.: Plant Succession, Carnegie Inst. Washington Publ., 



242, 512, 1916. 



