42 TEXTBOOK OF BOTANY 



unlike the original one. Some of the species of the first community may 

 be entirely eliminated and other species may migrate into it from neigh- 

 boring communities. Through such changes one plant community may 

 be gradually replaced bv another, a process that is referred to as the 

 succession of plant communities. 



The principles of dominance, changes in mixed populations, and suc- 

 cession referred to above are abundantly illustrated by local plant com- 

 munities in lawns, parks, vacant lots, eroding slopes, abandoned farms, 

 and forest remnants. All of these contain excellent field material that may 

 be studied first-hand. Lawns are especially valuable materials for study 

 because the\' mav be obser\'ed convenientlv and they probably attract 

 the interest and add to the pleasure of more people than most other 

 types of plant communities. Furthermore, they are similar to the com- 

 munities of cultivated grasses in pastures, golf links, parks, and college 

 campuses; consequentlv, many of the facts and principles discovered 

 about one of these communities may apply equally well to the others. 

 In this chapter we shall refer principallv to the more evident facts about 

 local plant communities that occur in the Central States. Explanations 

 of man\' of the observations will appear in later chapters. 



Changes in mixed populations of plants. The results of a very exact 

 study of changes in the composition of a mixed population of plants in a 

 pasture in New York during a fi\'e-vear period are shown by the diagram 

 in Fig. 24. 



The diagram shows the relative abundance of seven species of plants 

 during the year in which the seeds were sown, and for each of the next 

 five years. The change in composition of the plant population from year 

 to year is merely the result of a relati\'e increase in abundance of certain 

 species and a relative decrease of others. Red clover is entirely elimi- 

 nated at the end of the third year. Here then is an example of a change 

 in a mixed population of plants over a period of years that is due neither 

 to changes in the heredity of the plants nor to changes in climate or soil. 

 It is due to the characteristic differences in the growth of the several 

 species of plants and to their effects upon each other. One of the most 

 important changes is the gradual increase and dominance of bluegrass. 



One may ask why lawns, pastures, and parks everywhere do not in the 

 course of time become dominated by Kentucky bluegrass. An analysis 

 of the areas not dominated by bluegrass reveals that they are either very 

 wet, very dry, very acid, sandy, shaded, or deficient in certain inorganic 

 salts, especially of nitrogen, or that they are areas where the soil tem- 



