214 THE STUDY OF PLANT COMMUNITIES ■ Chapter IX 



These two types of causes of succession are commonly in opera- 

 tion at the same time, and their effects cannot always be readily 

 separated. Since they both result in vegetational change, it seems 

 unnecessary to distinguish between their effects in a general con- 

 sideration of plant succession. 



KINDS OF SUCCESSION 



Primary succession is initiated on a bare area where no vegeta- 

 tion has grown before. It may be observed on glacial moraine ex- 

 posed by recession of the ice, a new island, an area of extreme 

 erosion, newly deposited volcanic ash or rock, or any similar habi- 

 tat newly exposed to colonization. Such habitats are apt to be 

 unsuitable to the growth of most plants, and, consequently, the 

 pioneers that do establish themselves must have adaptations per- 

 mitting survival under extreme conditions. Moisture relationships 

 usually control their ability to invade the new area. If the habitat 

 is extremely dry, it is described as xeric; if wet, hydric; and if 

 intermediate, mesic. The successional trends are similarly referred 

 to as being xerarch, hydrarch or mesarch succession. 



Whatever the condition of the initial habitat, reaction of vege- 

 tation tends to make it more favorable to plants and always results 

 in improved moisture conditions. Thus xeric habitats become 

 moister and hydric ones become drier as succession progresses. 

 Because of the diversity of habitats upon which succession may 

 begin, there are an almost equal number of possible pioneer com- 

 munities. Within a climatic area, however, the variety of commu- 

 nities decreases as succession progresses because the trend is to- 

 ward mesophytism from both hydric and xeric habitats. Thus 

 unrelated habitats may eventually support similar vegetation and 

 may even undergo identical late stages of succession. 



Secondary succession results when a normal succession is dis- 

 rupted by fire, cultivation, lumbering, wind throw, or any similar 

 disturbance that destroys the principal species of an established 

 community. To what extent the development of vegetation on the 

 secondary area resembles primary succession is determined by the 

 degree of disturbance. Although the first communities that de- 

 velop may not be typical of primary succession, the later stages 

 again are similar. When disturbance is extreme, as after severe fire, 



