206 



PLANT SUCCESSION 



always the same in chemical composition but it varies in the ma- 

 terials that are in solution in it and also in depth. 



130. Continuing Causes of Succession.— After a sere is once 

 started there are various factors that cause it to continue. A change 



HMM|' 



' -^r 







y- 





Fig. 90.— a mountain side on which the forests have been largely destroyed 



by fire. 



Fig. 91. — A forest of spruce and pine showing the destructive activity of man 



acting as a biotic factor. 



of climate in a region changes the climax vegetation and so brings 

 about a succession of plant communities. Climatic changes are 

 usually so slow, however, that, while they are valuable in the inter- 



