210 PLANT SUCCESSION 



classes are represented, indicating that seedlings survive only when 

 there is an unusually favorable year. 



If a plant survives the seedling stage it is very like to grow to 

 maturity. The size that it finally attains and the rate at which it 

 grows depend upon various environmental factors, such as water, 

 light, and temperature, as well as upon inherited characteristics. 



The final process in ecesis is reproduction. Obviously annual 

 plants could not endure more than a single season unless they were 

 able to reproduce and perennial plants, while they might persist 

 for several or many years, could not be said to be wholly successful 

 in establishing themselves in a new area unless they could complete 

 their life cycles by carrying on sexual reproduction. Therefore, 

 ecesis can be said to be fully complete only when reproduction has 

 been successfully accomplished. 



Competition, the second process involved in the biotic causes of 

 succession, is a universal characteristic of plant communities. It 

 occurs whenever two or more plants make demands in excess of the 

 suppl}% increasing as the population increases until the community 

 approaches the climax in maturity and then decreasing. It can exist, 

 however, only between plants that are more or less equal in their 

 demands. There is no competition in this sense between a parasite 

 and its host, nor between a tree and an herb, but there may be 

 competition between two trees or two herbs or between a tree seed- 

 ling and an herb. Those species which, compete in any way are 

 called competitive species while those which can live in the same com- 

 munity without competing are called complementary species. In 

 the younger stages of the succession competition is largely confined 

 to the soil where the roots compete for water or nutrients or both. 

 As the population increases, however, competition may be as acute 

 above the surface of the soil as below it. The competition that takes 

 place in the air is largely for light. 



In a forest there is an apparent root competition for moisture be- 

 tween the trees and the herbaceous vegetation and tree seedlings. 

 Where the forest canopy is very dense there is often practically 

 no undergrowth and this was formerly attributed to the inability 

 of plants to grow in the dense shade. It has been found, however, 

 in some places, that by digging a trench around a small area in such 

 a forest in such a way as to cut oft' all roots entering the area an 

 abundant undergrowth may develop, thus proving that lack of soil 

 moisture is more important than lack of light in such a case. This, 



