376 MINNESOTA DOTANICAL STUDIES 



Tlie removal of the dominant Larix or Larix-Picca layer when 

 not followed hy burning, does not seriously affect the successional 

 sequence since the water relations of the habitat remain essentially 

 unchanged. The most marked efifects are in the changed light rela- 

 tions and in increased exposure to evaporation. These changes may 

 result in the disap]^earance of some of the shade-loving plants. 



In extensive swamp areas where filling from the edge cannot 

 alter the general conditions of the swamp, the effect of clearing is 

 to permit the development of the bog shrubs which had been sup- 

 pressed by the tamarack and spruce, and the consequent reestablish- 

 mcnt of an associes of Andromeda and Chamaedapline when these 

 are present. If Andromeda and Chamaedaphne be lacking, as hap- 

 pens in some areas, or present only in small numbers, an associes 

 consisting of Betula pitmila and species of Salix makes its appear- 

 ance. Usually the clearing is not complete, a greater or lesser num- 

 ber of young tamaracks being left. These now develop and with 

 their appearance the original dominant layer of Larix soon becomes 

 established again. 



In small, shallow swamps and in a zone along the edge of larger 

 swamps where the soil is built up more as the result of filling in, an 

 associes of Corjuis stolonifera and species of Salix appears after the 

 removal of Larix and Picea. Farther northward, Alniis inca)ia ajv 

 pears as one of the consocies along with Conuts and Salix. Betula 

 pinnila often occurs with these also. 



This Cornus-Salix zone is later invaded by Po pubis and Betula 

 papyrifera, with occasional individuals of Abies and Fraxinus nigra. 

 The invasion of these results later in the establishment of the Popu- 

 lus-Betnla associes. With the occurrence of windthrows or the 

 death of individuals in this associes. the spaces occupied by them 

 are invaded by seedlings of Aeer, Tilia, Ulmus and others of the 

 climax forest, so that the climax comes to replace Populus and 

 Betula as previously described. 



The occurrence of windfalls are to be regarded as miniature 

 clearings. In a Larix-Picea swamp in the deciduous forest area 

 when a windfall occurs, its place is invaded at once by Larix and 

 Picea, or by Larix alone toward the south, which results in keeping 

 the area indefinitely in the same developmental stage. This applies 

 in deep swamps where Sphagnum is abundantly developed. 



In swamps that are more nearly filled up and where in conse- 



