336 MINNESOTA BOTANICAL STUDIES 



climax for each of the principal successions in each of the climax 

 associations will be discussed. 



PIXUS STROBUS-PI\'US RP:SIXOSA ASSOCIATION 

 I. Primary Succession 



Initial causes : Wherever water collects in sufficient quantities 

 to destroy completely any vegetation which may have existed pre- 

 viously in the area covered, a primary succession may begin. The 

 lakes of Minnesota are largely due to glaciation, by which the drain- 

 age systems that had developed during the time from the Cretaceous 

 period down to the glacial epoch were more or less completely 

 blotted out, causing the development of new drainage channels. 

 The deposition of glacial till on the retreat of the ice-sheet also 

 blocked many of the drainage channels, causing the lower areas to 

 become filled with water from the melting ice. Some of the lakes 

 in northeastern Minnesota probably existed essentially in their pres- 

 ent form before the glacial period, occupying the troughs of the 

 central folds that have occurred in the earth's surface. Such lakes 

 have rock-bound shores and are usually long, narrow, and deep. 

 Owing to the hardness of the rocks forming their shores, they prob- 

 ably were little affected by the passage of the continental glacier. 



The effects of climate as initial causes since the glacial period 

 are of less consequence. The flooding of areas by man or by beavers 

 would initiate a primary succession if the areas were flooded to 

 sufficient depths. There is no evidence, however, that primary suc- 

 cessions in ^Minnesota have been initiated by biotic agencies. 



1. HYDRARCH SUCCESSION 



This succession, which has its origin in water and culminates in 

 the Pinus strobus-P. resinosa climax association, shows a series of 

 stages which may be indicated as follows : 



(1) Chara-PIiilotria Associes 



(2) Castalia-Xynilyhaca Associes 



(3) Scirpus-Zizania Associes 



(4) Carcx Associes 



(5) Chamacdaphnc-Andromcda Associes 



(6) Larix-Picca Associes 



(7) Ahics-Bciula Associes 



(8) Finns Association 



