[Chap. LIU THE VEGETATION OF NORTH AMERICA 753 



Plant associations have a rather definite organization. This may be 

 illustrated by a forest such as the white pine association or the hemlock- 

 hardwood association. Both are named for their dominant species, but 

 both have accessory and subdominant species of trees, and in the under- 

 growth characteristic shrubs, herbs, and smaller plants. The dominant 

 trees that form the canopy, or top layer, of the association are exposed 

 to more sunlight, more wind, and a drier atmosphere than are the suc- 

 cessive lower layers of small trees, shrubs, herbs, and ground cover 

 beneath. Atmospheric moisture and carbon dioxide concentrations are 

 highest near the soil. After long periods of occupancy the accumulated 

 duff and underlying soils become strikingly different. These different 

 layers of plants in a forest and the relations of the plants in each layer to 

 those above and below constitute the ecological structure and organiza- 

 tion of an association. 



Succession. Land forms are continually undergoing change. Uplands 

 are eroded by water and wind, and lowlands and flood plains are built 

 up by deposition of the products of erosion. Lakes and ponds are filled 

 up gradually; hill and mountain slopes are modified by slumping. All 

 these physiographic processes result in an orderly series of changes in the 

 environments of the various plant associations of these different sites, 

 and consequently most associations are undergoing changes in com- 

 position. 



When new land areas are exposed as when lakes are drained, cut-over 

 forest areas are subjected to intense fires, or sand dunes and other wind- 

 blown deposits cover an older landscape, a comparatively rapid succes- 

 sion of plant associations follows one another until a final stabilized asso- 

 ciation forms a climax. These stages, beginning with weeds and passing 

 through pioneer shrubs and tree stages to the climax forests, have been 

 described for many parts of the country. Similar studies have been made 

 in the grasslands. 



Succession may be readilv studied wherever there is abandoned faim 

 land; the age of the community can often be definitely ascertained from 

 historic records. On manv of these sites the primary cause of succession 

 is the changing of the soil and atmospheric factors by the plants them- 

 selves: shading, increasing or decreasing the water supply, adding 

 humus, increasing the stability and porosity of the soil. Pioneer plants 

 may make the habitat suitable to other species, which in turn may alter 

 it in ways that result in the death of the pioneers. Still other species may 

 follow these in time and have a similar effect on them. This influence of 



