354 



MINNESOTA ROTANICAL STUDIES 



ing or for the operation of machinery has resulted in the initiation 

 or secondary succession along the rivers affected for a distance of 

 several miles above the dams. Alan has been the agent in causing 

 these successions. Beavers, by building dams across streams, cause 

 a flooding of areas along the streams for some distance above the 

 dams. The areas aft'ected by beaver dams, however, are usually 

 small, although such areas are found quite often. 



The topography of a region determines in a large measure the 

 extent to which an area may be aft'ected by flooding and conse- 

 quently determines the stages at which secondary succession will 

 begin. If the shore of a lake or banks of a stream are high and 

 steep, a rise of several feet in the water-level may produce no very 

 evident eft'ect. On the other hand, if a lake or river is bordered by 

 marsh or swamp, a rise of a foot or even of a few inches may effect 

 the vegetation to a marked extent. The shores of Leech and Win- 

 nebegoshish lakes are diverse in character, but for the most part 

 are sufficiently high and steep so that the raising of the water-level 

 by the conversion of the lakes into reservoirs has not aft'ected suc- 

 cession except in rather local areas. Leech Lake furnishes the best 

 examples of succession. 



Leech Lake, on the east side, between Leech Lake River and Boy 

 River and south about an equal distance, is bordered by a tamarack 

 swamp. Similar swamps occur at the north end of Sucker Bay, 

 on the west side of the same bay and at the north end of Steamboat 

 Bay and along Steamboat River. The swamps also form a zone 

 along streams tributary to Leech Lake at the places named. 



The rise of the water-level has resulted in the killing of the tam- 

 aracks in the areas above named, except along the margins. As a 

 result, the Larix-Picca associes has been replaced by a Carex-Cala- 

 magrostis associes, which is similar in all its characteristic features 

 to the Car ex associes as described under primary succession (page 

 340), except that Calamagrostis canadensis and C. hypcrhorea be- 

 come codominants with the species of Carex. 



In the deeper water bordering the zone of C arex-C alaniagrostis , 

 Zizania and Phragmites occur. Along with these, especially near 

 the mouth of Boy River, Eqnisetnm fluviatUe appears conspicuously 

 as a socies, often dominating quite extensive areas. Scirpus and 

 Typha were absent from nearly all of the areas observed. 



In deeper water just outside the 'Zizania-Phragmites zone. Cas- 

 talia, Nymphaea and Potauiogeton were sometimes found. Their 



