CHAPTER V 

 LOCAL PLANT COMMUNITIES 



Under natural conditions plants live in communities; no plant lives alone. 

 A potted fern at a window appears to be alone in an environment con- 

 sisting only of the physical factors of the soil, atmosphere, water, gravity, 

 and radiant energy from the sun. Its roots, however, are surrounded by 

 millions of microscopic plants and animals whose numerous activities 

 may influence the development of the fern. As we shall see later, some 

 of the activities of these minute organisms are beneficial to the larger 

 plant, others are detrimental. Some of them are known to cause diseases 

 of the roots or of the whole plant. 



Plant communities. A lawn is a familiar plant communitv in which 

 the development of each individual plant may be influenced by the 

 plants that surround it. The intermingling of grasses and clover with 

 dandelion, plantain, and several other weeds is an illustration of the fact 

 that plant communities in nature are mixed populations composed of 

 several kinds of plants. Only in pure cultures, carefully prepared in 

 laboratories, may one expect to see plant communities composed of but 

 a single species of plant. In the practical consideration of certain com- 

 munities one may choose to disregard the microorganisms and look 

 upon a field of corn, of wheat, or of any cultivated crop as a community 

 of only one species if all visible weeds have been removed. The forester 

 may even disregard all the plants in a forest communitv but the trees, 

 and speak of a pure stand of beech or of hemlock. Usually in any mixed 

 population of plants a few species are much more abundant or modify 

 the habitat more than others; they are referred to as the dominant species 

 of the community. Plant communities are named according to their 

 dominant species; for example, a bluegrass lawn, a pigweed com- 

 munity, a tall-bluestem prairie, an oak forest, a beech-maple forest, an 

 elm-ash-silver maple forest. 



Mutual effects of plants within a community. In lawns, pastures, or 

 golf courses in which Kentucky bluegrass is the most abundant and 

 dominant plant, the presence of the grass in some way interferes with 



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