Bergman and Stallard: climax formations 353 



whether they have appeared as the result of the initiation of more or 

 less extensive secondary successions. However this may be, the 

 general sequence of events in the succession seems quite clear. 



II. Secondary Succession 



Initial Causes : Any cause that destroys the existing vegetation 

 of an area may initiate a secondary succession. Such causes may be 

 changes in the physiography of a region, climatic or edaphic changes, 

 or changes resulting from the action of biotic agents. None but 

 biotic agents apparently cause secondary successions in Minnesota, 

 except to the extent that forest fires, for example, may be shown to 

 have been started by lightning. 



Secondary successions may begin at any point after the pioneer 

 stage and before the final climax. The point at which the succes- 

 sion does begin is determined by the extent to which the water- 

 content is afl:"ected by the disturbance of the habitat. A secondary 

 succession may be a very local one, such as might be produced by a 

 windfall or by the formation of a small pond, or it may cover an 

 area of hundreds of square miles. In a large area the water-content 

 of the habitat is apt to be aflrecte'd more profoundly, which would 

 cause the secondary succession to begin at an earlier or lower stage. 

 Secondary successions may be classified as follows : 



1. Flood succession 



2. Burn or clearing succession 



1. THE FLOOD SUCCESSION 



As indicated by the name, this is a succession due to the flooding 

 of an area from any cause whatsoever. The two principal agents 

 in causing secondary succession by flooding in northern Alinnesota 

 are man and beavers. The flooding may be over a very restricted 

 area or over an extensive one, in either case causing a more or less 

 complete hydrarch succession. If over an extensive area, the depth 

 of water is apt to become sufficient to initiate a primary succession 

 beginning with CJiara and other plants of that associes as already 

 described (page 337). 



In making reservoirs of Gull, Leech and Winnebegoshish lakes, 

 the water-level has been raised several feet over extensive areas sur- 

 rounding these lakes and along rivers tributary to them. In the 

 same way the building of dams in rivers to obtain power for light- 



