21G 



PLANT .SUCCESSION 



community becomes less and less open. When the soil becomes fully 

 occupied by plants the community is said to be closed. The num- 

 ber of species also increases until the climax is nearly reached and 

 then it decreases. The duration of the pioneer stages is relatively 

 short and the duration of the intermediate stages increases until the 

 climax stage is reached. This is relatively permanent. 



The increase in population as the sere progresses is possible be- 

 cause the reactions of the plants are constantly making the habitat 

 more mesic and less severe. The first plants following the pioneer 

 species are likely to be herbaceous perennials. After the herbaceous 

 plants have rather thoroughly occupied the soil, shrubs usually 



Fig. 96.— a young tree stage of a hydrarch succession. Willows and soft 



maples. 



begin to appear and there develop one or more definite shrub stages. 

 This is likely to be true whether the sere is xerarch or hydrarch, 

 though the kinds of shrubs will be different in the two cases. Trees 

 may begin to come in at about the same time as the shrubs or soon 

 after, and they increase in numbers until they become abundant 

 enough to constitute a tree stage. In a xerarch succession the first 

 trees may be conifers or they may be such xeric trees as the black 

 and the white oaks (Qitercns velutina and Quercus alba) and the 

 hickories {Canja ovata, etc.). In a hydrarch succession the first 

 trees are likely to be willows (Salix) and cottonwood {Populm 

 deltoides) (Fig. 96). These first trees are followed by increasingly 

 mesic species until the climax is finally reached. 



