212 PLANT SUCCESSION 



tions from the plants and by the growth of roots in cracks and 

 crevices. Wind-blown materials and water-borne detritus are caught 

 by the plants and added to the soil. Dead plant bodies or parts of 

 plants are also accumulated and become a part of the soil. The 

 structure of the soil is also changed by the plants, in part by the same 

 reactions that produce soil. The addition of humus, for example, 

 decreases the average size of the soil particles and usually has a 

 tendency to make the soil more mellow. Sometimes the addition 

 of humus results in making the soil more compact. The precipita- 

 tion of the humus substances by the soil salts may even produce a 

 "hard-pan" layer which is impervious to water and even to roots. 



The reactions which affect the water content of the soil are 

 extremely important. As a general rule, during the entire course of 

 the succession the soil-water content is either increasing or decreas- 

 ing. These effects are brought about indirectly through the build- 

 ing-up of soil, the increase in shade, etc. In the case of a xerarch 

 succession the soil-water content increases while in a hydrarch succes- 

 sion it decreases so that in either case development is toward mesism. 

 The nutrients that are in solution in the soil water are also modified 

 by the reactions of the plants. They may be increased through the 

 addition of humus by leaf fall or by other means or they may be 

 decreased through the plants being eaten by animals or being re- 

 moved from the land by man. The soil organisms, both parasites 

 and saprophytes, are usually increased both in number of indi- 

 viduals and in number of kinds as the plant community develops. 



The reactions above the surface of the soil usually increase the 

 amount of shade and the relative humidity of the air and decrease 

 the temperature and the wind velocity. The local climate is often 

 actually changed by the development of climax vegetation, especially 

 if the climax is a forest. This is brought about by the large amount 

 of water that is transpired by the plants. This increases the rainfall 

 and results in a somewhat more humid climate. The number and 

 kinds of aerial organisms are affected in ways quite comparable to 

 those of the soil organisms. 



The reactions of a plant community as a whole are more important, 

 in a way, than the reactions of individual plants because they are 

 more effective. The leaves that fall from an isolated tree are likely 

 to dry up and blow away but the leaves from a similar tree in a forest 

 remain to form a mulch and later humus. The shade under the 

 isolated tree is intermittent because of the changing position of the 



