362 MINNESOTA BOTANICAL STUDIES 



dry season when the Sphagmiiii surface is partly dry. At such 

 times burning may result in killing out the Sphagnum and in the ini- 

 tiation of a secondary succession beginning with C arcx-C alamagros- 

 tis. The sequence of stages after the ajipearance of Carex-Cala- 

 magrostls up to Larix-Picea is essentially that of primary succession. 

 In some instances Populus may invade directly after a fire so that a 

 dense growth of seedling poplars is the dominant vegetation. 



When a tamarack swamp is drained and afterward burned, the 

 succession begins usually with the C arex-C alamagrostis associes. 

 As a result of the combined draining and burning, all the character- 

 istic plants of the swamp disappear. The water-content of the 

 habitat is still high but much less than if Sphagnum were present. 

 The disappearance of Sphagnum, Larix and other swamp species 

 permits the invasion of Carex and Calamagrosfis, so that the Larix- 

 Picea stage comes to be replaced by the associes of Carex-Cala- 

 tnagrosfis as the first stage of secondary succession initiated by 

 draining and burning. If the area afifected be mowed or burned 

 over each year, it may be kept indefinitely in this stage and valuable 

 hay meadows result. If the swamp be kept drained but not mowed 

 nor burned over, a secondary succession culminating in climax pine 

 forest will ultimately result. 



In such a case the Carcx-Calamagrostis associes is invaded first 

 by Bet Ilia pnuiila, Salix pctiolaris, soon followed by Salix bebbii, 

 S. discolor, Cornits stolonifcra and Alnns incana. As a result of 

 the invasion of these, the plants of the Carex-Calamagrostis associes 

 are killed out. The swamp gradually fills up by the washing in of 

 soil around the edge, aided by the accumulation of humus from the 

 decay of dead vegetation. The process of filling reduces the water- 

 content of the soil and permits the invasion of Populus, Abies, 

 Betula and sometimes Fraxinus nigra. The invasion proceeds from 

 the edge, gradually encroaching on the swamp. After a time the 

 associes of Abies-Betula as described on page 344 becomes estab- 

 lished. This is later invaded by pines as already described in nor- 

 mal primary succession. The successional stages of secondary suc- 

 cession in drained swamps from the initial stage up to the climax 

 pines appear more rapidly than in normal primary succession, but 

 in stages common to both the composition of a common associes and 

 the secondary species present in that associes are essentially alike. 



The removal of the dominant Larix-Picea layer without subse- 



