PIONEER STAGES OF SUCCESSION 21 



o 



tree with respect to the sun but the shade under a similar tree in a 

 forest is continuous because of the shade produced by neighljoring 

 trees. Thus, while the reactions of the comnuuiity are the resultant 

 of the reactions of the individual plants, it is usually much more 

 than the mere sum of these reactions. 



The result of all these various reactions is that the vegetation is 

 kept in a dynamic state and succession proceeds toward the climax. 

 Vegetation is never entirely stable, but, since the climax association 

 is the most mesic community that the climate will support, it can- 

 not become any more mesic and, as compared to the earlier stages of 

 the sere, it is relatively stable and will persist indefinitely unless 

 there is a change of climate or the community is destroyed by one 

 means or another. 



Successions that are brought about by the reactions of the plants 

 themselves are sometimes called autogenic successions while those 

 caused by climatic or physiographic factors are called allogenic 

 successions. It is probable that a succession is never entirely auto- 

 genic or entirely allogenic but it is frequently possible to recognize 

 that a given case is largely the one or the other. 



131. Pioneer Stages of Succession.— The environmental condi- 

 tions on a bare area are extremely difficult for most plants. There 

 is no protection from the light and heat of the sun during the day 

 and nothing to check radiation at night. Also there is no protection 

 from wind, and, therefore, evaporation is very rapid. ^Moisture 

 conditions are extremely variable. After a rain there may be pools 

 of water on rock or soil surfaces but these may dry up quickly and 

 the substratum become very hot and dry. There is usually a com- 

 plete lack of humus and many of the essential mineral salts may be 

 lacking, or present in an unavailable form. The character of the 

 surface is often such as to make it very difficult for plants to gain a 

 foot-hold, either because of its hardness or its instability. 



Due to these extremely severe conditions the pioneer plants in all 

 xerarch successions are very xeric. On bare rock the first plants are 

 lichens. Lichens are divided into three groups based on their growth- 

 forms. These are crustose, foliose, and fruticose lichens. The 

 crustose lichens (Fig. 93) are so-called because they form thin crusts 

 on the surface of the substratum. They are the most xeric of all and 

 are the first to appear on a rock surface. The foliose lichens (Fig. 94) 

 are more or less leaf-like. They are as a rule somewhat less xeric 

 and they follow the crustose forms along with some xeric mosses. 



