46 TEXTBOOK OF BOTANY 



abundance, and poverty grass and weeds again increase. When clippings 

 are continuously removed from lawns a similar loss in mineral salts 

 results and the growth of bluegrass and clover declines. 



The succession of plant communities. One might refer to the demon- 

 stration cited above as an example of a poverty grass and weed com- 

 munity being succeeded by a bluegrass-clover community. The succes- 

 sion in this case, however, consists only of a change in the relative 

 abundance of species already present. Succession in which there is also 

 the added feature of species invading the community from neighboring 

 communities mav be observed locally by noting ( 1 ) the kinds of pioneer 

 plants that first occupy an eroding soil slope or some other bare soil area, 

 and (2) the kinds of plant communities that successively follow each 

 other on this same area through the years. One may shorten his period 

 of observation and obtain a similar story by noting the kinds of plant 

 communities that now occupy artificially made slopes of different known 

 ages. They may be found around any town where local construction 

 work involves the movement of large quantities of soil. 



The pioneer plants that occupy such bare areas must of course start 

 from seeds that have been carried to the area by some means, and they 

 consist of annuals, biennials, and perennials that can survive in the 

 extreme environmental conditions that exist there. Most seedlings perish 

 in such situations. The presence of the pioneer plants brings about 

 changes in the habitat. They check erosion, increase the shade, and thus 

 decrease the temperature of the soil surface on hot summer days. Cer- 

 tain kinds of plants that were unable to survive in the pioneer condi- 

 tions may then develop on the area and increase in number. Their 

 abundance may produce conditions in which many of the pioneer 

 species are unable to survive. One of the most striking changes that 

 occur on such areas is the gradual disappearance of most of the pioneer 

 annuals and biennials, and the final dominance of perennials. Perennials 

 dominate the vegetation of the world, except in pioneer habitats. 



This occupation of bare soil areas by a pioneer plant community, the 

 subsequent ehmination of the pioneers, and the succession of other plant 

 communities exemplify some of the fundamental processes involved in 

 the revegetation of areas on which the original communities were de- 

 stroyed by man, by fire, by wind, or by water. 



As a nation we have recently become much concerned about the rapid 

 run-off of water from our agricultural lands and the amount of soil car- 



