[Chap. V 



LOCAL PLANT COMMUNITIES 



47 



lied away from them (Fig. 25). Soil that has been thousands of years in 

 forming mider the influence of chmate, larger plants, and micro- 

 organisms is being washed from the land to the sea in some places at the 

 rate of more than 70 tons per acre annually. The subsoil that is exposed 



fiiHynf-^rfifp^i 



Fig. 25. During the century and a quarter that the phuits in this cemetery in 

 Ohio have checked erosion, sheet erosion on the adjacent gently sloping farm land 

 has removed the upper three feet of soil. Photo by C. H. Jones. 



after a few years is a very poor habitat for most plants ( Fig. 26 ) . Where 

 the soil is completely covered with vegetation the amount of erosion 

 annuallv is so small that it can scarcely be measured; the run-off of 

 water after rains is also reduced to a minimum. One of the important 

 problems in any worth-while soil conservation program is to discover 

 better methods of speeding up the natural processes of revegetation 

 wherever practicable, and of preventing further unnecessary destruction 

 of established vegetation. 



A first-hand study of local plant communities that occur on lawns, 

 campuses, eroding slopes, vacant lots, abandoned farms, or in forest 

 remnants introduces one to many botanical facts and principles. It also 

 raises numerous questions about the social behavior of plants, which 

 in turn depends upon the effects of the environment on the physiologi- 

 cal processes within these plants. These processes and their relations to 

 environment and plant behavior are subjects of discussion in later chap- 

 ters. The study of plant communities may lead not only to their biologi- 



