754 TEXTBOOK OF BOTANY 



plants upon each other indirectly through their effects upon the environ- 

 ment results in progressive changes in the communities that occupy an 

 area. It is sometimes referred to as "competition" among species and 

 communities. 



The processes of vegetation change, however, are best studied in your 

 own locality and in the field. Here it is possible only to call attention 

 to their occurrence. Many of the recent projects in forest planting, soil 

 erosion control, irrigation, and building of dams are excellent sites for 

 such studies. 



Vegetation, then, consists of larger and smaller groups of plants each 

 having a characteristic structure and appearance. The occurrence of the 

 larger groups ( formations ) , such as the boreal forest, eastern deciduous 

 forest, and prairie, is rather definitely related to climate. The distribu- 

 tion of the smaller communities ( associations ) within a climatic forma- 

 tion is correlated with local modifications of climate and diversity of soil 

 conditions. Because of gradual changes in climate, physiographic modifi- 

 cation of land forms, changes in available water, and the modification 

 of the environment by the plants themselves, rather definite successions 

 of plant associations occur within each of the formations. 



Succession includes changes in composition, in organization, and in 

 the dominant and accessory species. If continued for long periods of 

 time, it leads ultimately to plant associations in which the component 

 species are in equilibrium with the local climate, with the soil, and with 

 each other. These are called climax associations and may cover larger or 

 smaller parts of a climatic formation. Man has frequently altered these 

 natural changes in vegetation. 



Deciduous forest (Figs. 381-384). The most widely distributed forest 

 communities of the eastern United States belong to the deciduous forest 

 formation. The most characteristic trees of these various communities 

 are a score of oaks, several species of hickory, elm, ash, maple, linden, 

 poplar, and birch; also the beech, tulip, sycamore, and, until recently, the 

 chestnut. In the primeval forest many of these trees attained heights of 

 125 to 175 feet and trunk diameters of 5 to 14 feet. Many of the largest 

 trees of which we have records occurred in the lower Ohio River bottoms 

 and in the moist coves and valleys of the Appalachian Mountains and 

 plateaus. Probably the most important single species was the white oak, 

 which excelled in both abundance and dominance in a great variety of 

 habitats, partly because of its height, longevity, and freedom from 

 disease. The region of the deciduous forest was glaciated in the northern 



