place because the topographic and soil conditions change. Com- 

 munities may be confined to relatively small areas because of the 

 alterations in the environment, as is readily noted when one 

 ascends a mountain or proceeds from a marshy lake shore to high 

 land. The environmental conditions may change gradually in 

 space, with corresponding alteration in the plant cover, forming 

 a cline, or the change may be abrupt so that sharp transitions 

 or discontinuities are produced. 



It is evident that there are many kinds of change that occur 

 in vegetation both in time in one place and in space from one area 

 to another, and that they are related to alterations in environ- 

 mental factors. These changes may be classified in three main 

 groups. The first includes changes in time in a specific commu- 

 nity that are directional with respect to loss of some of the pres- 

 ent species and the invasion of new ones, with accompanying 

 increase in complexity, as in ecologic succession where the 

 directional change is from the less complex and less stable vege- 

 tation to the more complex and more stable, culminating in the 

 climax or steady-state, while retrogression is the reverse of this. 

 The second group includes nondirectional changes which occur 

 over a period of time within one community, and may be cyclic 

 or noncyclic replacement, and fluctuations; or the changes may 

 involve several communities in one area, forming another kind 

 of cyclic replacement. The third main group consists of changes 

 in space, i.e., from area to area, such as gradual or sharp transi- 

 tions in vegetation-types, among which are vertical and horizontal 

 transitions, clines, and discontinuities. The treatment in the fol- 

 lowing pages will be in accordance with these concepts. 



Another classification of time changes was proposed by laro- 

 shenko"^^ and described by Major. ^^^ i^ any natural vegetation 

 these may occur at five definable levels: (1) seasonal, as seen in 

 aspection; (2) annual or cyclic— related to variations in climatic 

 or biotic factors; (3) successional; (4) historical— migration or 

 extinction in relation to long-term climatic change; and (5) 

 genetic — the evolution of a new flora. 



One of the important and difficult tasks of the student of 

 vegetation is to determine as fully as possible the status of each 



136 • Dynamics of Coxnmunities 



