Clements • The Nature and Role of Plant Succession 



33 



remains. To be a sand binder, a plant 

 must not only be well-anchored and 

 hold sand, but it must also be able to 

 catch the load borne by the wind and 

 even more important, to keep its 

 "head" above the sand as the latter 

 heaps up about it. 



The early invaders are lowly annuals 

 of small requirements, which gradually 

 stabilize small areas for the entrance 

 of an ascending series of perennials, 

 either herbs or grasses. In the prairies, 

 grasses of progressively higher demands 

 replace each other in forming a perma- 

 nent cover, while in forest regions the 

 grasses yield ultimately to shrubs and 

 trees. 



The reconstruction of the adult 

 community is a simpler and still more 

 rapid process where fire or clearing has 

 destroyed the climax. The soil usually 

 is neither removed nor impaired, and 

 in the case of fire is often enriched by 

 the minerals liberated. 



Mosses and liverworts appear al- 

 most at once, and during the first full 

 season a complete cover of annual 

 herbs and grasses may be formed. Many 

 perennials and shrubs survive the fire 

 and their root sprouts soon appear in 

 large number, gradually overtopping 

 the herbs, reducing the light and taking 

 the lion's share of the water in the soil. 

 The herbs are conquered by bushes 

 and low shrubs; these are succeeded by 

 taller shrubs, and trees then begin to 

 straggle into the copses, or take more 

 or less complete control by means of 

 sprouts. After a few decades a young 

 climax forest is again in possession. 



A somewhat similar course is fol- 

 lowed in cultivated fields that are al- 

 lowed to "go back," the term itself in- 

 dicating some popular appreciation of 

 the process of succession. Annual weeds 

 dominate for a few years, and the usual 

 communities of perennials, grasses, and 

 shrubs gain successively a short period 

 of mastery, and return to forest or 



prairie often requiring but two or three 

 decades. 



FORCES CONCERNED IN SUCCESSION 



Succession depends for its oppor- 

 tunity upon the production of bare or 

 denuded areas, but the driving force 

 back of it is climate, each succeeding 

 community becoming less controlled 

 by soil or terrain and more by climatic 

 factors until the adult stage or climax 

 is attained. 



The actual growth of the commu- 

 nity is regulated by certain processes or 

 functions by means of which soil and 

 climate produce their effects. The ini- 

 tial processes are aggregation and mi- 

 gration, by which individuals are 

 brought together to form communities. 

 These react upon the soil and then 

 upon the local climate to render condi- 

 tions at first more favorable to them- 

 selves and later to the invaders that are 

 to replace them, the actual conquest 

 being brought about by the outcome of 

 the competition for water, light, and 

 minerals especially. 



Within each community there is 

 likewise a certain amount of coopera- 

 tion, as seen in the reaction that pro- 

 duces shade, increases the organic mat- 

 ter in the soil, minimizes the effect of 

 wind, or augments the moisture of the 

 air. The plants and animals of the 

 communit}^ also exhibit many essential 

 interactions, in some of which the 

 mutual benefit is striking, as in the 

 pollination of flowers by insects and 

 hummingbirds. When man enters the 

 situation, such relations become much 

 more varied and important, especially 

 in the hunting, pastoral, and purely 

 agricultural stages of human society. 



SUCCESSION OF RACES AND CULTURES 



It is obvious that human commu- 

 nities are subject to the control of cli- 

 mate and soil— to what have often been 



