INVASION AND SUCCESSION 



287 



area shows an initial stage and another an intermediate one, 

 the development of the former reveals the history of the inter- 

 mediate stage, while that of the latter indicates the stages passed 

 through to reach the ultimate one. Hence from a number of 

 areas it is possible to work out in a few years the probable se- 

 quence of stages in a succession that requires several decades 



Fig. 117. Decade method ot counting the annual rings of a tree or shrub 

 to determine the age ot a successional stage or formation. Check pins 

 are inserted in every tenth ring. 



or more for its development. Such a method is not final, but 

 it is the only feasible one for most successions. 



309. The relict method. When one stage yields to another, 

 some species persist for a longer or shorter period. Some last 

 through the next stage, while a tew may persist through several 

 successive stages. When the stages to which such relicts belong 



