CONTINUING CAUSES OF SUCCESSION 211 



however, is somewhat different than competition between plants 

 that make similar demands upon the environment. The trees, as 

 the dominant plants, are in control and the undergrowth never has 

 a chance to compete with them on anything like equal terms. 

 Tolerance, or the ability of plants to grow under a forest canopy, 

 is a complex phenomenon which is dependent upon several factors, 

 the most important of w^hich are probably the light and moisture 

 relations. It is not, however, so much a question of competition 

 as of enduring the conditions imposed by the dominant trees. 



Competition may take place between individuals of the same 

 species or between those of different species. In either case it is of 

 great importance both in the development of native vegetation and 

 in crop production. When a large number of seedlings are growing 

 together on a small area there is often intense competition for light. 

 The difference of only a small fraction of an inch in height between 

 two seedlings may determine which will succeed and which will fail 

 to grow to maturity. The first leaves of the taller seedling will 

 overshadow those of the shorter one and this will immediately give 

 the one a decided advantage over the other in the amount of photo- 

 synthetic activity and, therefore, in the rate of growth, and, as other 

 leaves are formed, this advantage will increase. If the seedlings 

 belong to the same species or to the same growth form it is likely 

 that the one that started at a disadvantage will soon die but if one is 

 a tree seedling and one an herb seedling both may eventually reach 

 maturity since the tree seedling will soon become dominant and the 

 herb seedling subordinate and competition between the two will 

 cease. 



Competition is an extremely important consideration in the grow- 

 ing of crop plants since if too many plants are grown on a given area 

 the actual yield will be reduced because of too much competition 

 while if too few plants are present the yield will again be reduced 

 because the available water, light, and nutrients will not be used to 

 greatest advantage. The most important reason for the cultivation 

 of growing crop plants is to prevent the development of weeds which 

 would compete with the crop plants. 



Reaction, the third biotic cause of succession, refers to the effects 

 of the plants upon the habitat. These effects are quite variable in 

 nature. As soon as the first plants appear on an area the soil- 

 forming reactions begin. The weathering or disintegration of the 

 rock, if the substratum is composed of rock, is hastened by the excre- 



