PLANT ASSOCIATIONS 



443 



The length of time required for one type of association to suc- 

 ceed and supplant another in such an instance as that just 

 described, as well as the composition and nature of the plant 

 populations which follow one another in any given succession, will 

 of course vary in different cases, but the general facts regarding 

 succession will hold for all similar habitats. Another familiar 

 illustration of plant succession is often observed in. forests where 

 fires destroy the trees over wide tracts. The first invaders are 



FIG. 289. A hillside once forested, but now bare and eroded 

 Photograph by United States Forest Service 



.* 



here, as in most other instances, herbaceous plants, including the 

 fire weeds (Epilobium and Lrechtites), which are able to live and 

 thrive in the burned-over area. The herbaceous plants are in time 

 succeeded by aspens and conifers in some regions, or by forests of 

 deciduous hardwood trees in others. Similar processes may be 

 observed along the shores of most lakes and streams where new 

 land is formed by changing water levels or in cultivated fields 

 and gardens where land is plowed and allowed to lie fallow long 

 enough for the first occupants to be routed by later competitors. 



