An important feature of vegetation is change. Within a growing 

 season the aspect of a community changes from spring to autumn 

 because of differences in the requirements and ecological ampli- 

 tudes of the various constituent species, some growing rapidly 

 and flowering early, others developing more slowly. The begin- 

 ning of growth, flowering, and fruiting of the same kind of plant 

 comes at different times from year to year because the environ- 

 mental conditions, particularly temperature and precipitation, 

 vary. On account of drought, disease, and other adverse factors, 

 or old age, some plants may fail to produce seed, or even die, and 

 the space they occupied may be taken by other species the follow- 

 ing year. Even the plants themselves, such as a dense stand of 

 pines, modify the environment, especially by reducing the light 

 intensity so that they cannot reproduce, and consequently, in 

 time, they are replaced by deciduous trees. 



Some types of vegetation at first appear uniform over large 

 areas, but close examination reveals variation from place to 



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