CHAPTER XVIII 

 PLANT SUCCESSION 



Plant succession is a universal process by which all plant associa- 

 tions develop from beginnings on bare areas to maturity. As a 

 general rule succession is from xerism toward mesism or from 

 hydrism toward mesism. This is necessarily true because the 

 mature, or climatic climax, vegetation is the most mesic that the 

 climate can permanently support. There are some exceptions to 

 the general rule, however. For example, in a region where the 

 climax is a xeric type of vegetation, a sere that starts in water, as at 

 the edge of an ox-bow lake or other inland depression, may develop 

 progressively from a hydric condition to the xeric climax but in so 

 doing it would necessarily pass through a mesic stage so that the last 

 part of the sere would be from mesism toward xerism. We may say, 

 therefore, that developmental plant succession always proceeds 

 toward the climax and usually, though not necessarily, toward 

 mesism. 



]\Iore often, when succession is away from, rather than toward, 

 mesism it is also away from the climax and is destructive rather than 

 developmental in nature. For this reason it is necessary to dis- 

 tinguish between development and succession, or rather between 

 developmental succession and succession that is not developmental. 

 Succession that is not developmental is in general due to some change 

 in the habitat that makes it less mesic. Such a change may take 

 place rapidly or it may take place very slowly. In any case it is 

 destructive in its effect since it cuts back the association to a younger 

 stage. It is comparable to cutting back a tree, such as a willow, and 

 so making a shrub of it. If such a willow tree, after it is cut back, 

 is left undisturbed it will proceed at once to grow up again into a 

 tree. Likewise, when a plant association is cut back to a younger 

 stage by some means and then the cause ceases to act, developmental 

 succession begins at once and proceeds toward the climax. The 

 cutting back process, or succession away from the climax, has been 

 called retrogression by some ecologists. 



129. Initial Causes of Succession.— A sere always starts on a bare 

 area. Therefore, the causes of bare areas are the initial causes of 

 succession. Bare areas may be produced in a variety of ways. For 



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