356 MINNESOTA BOTANICAL STUDIES 



which tliey differ in no essential respect. The fact that the clearing 

 has heen brought about by the action of a biotic agency in one case 

 and by a natural force in another is of no significance. 



The extent of the area cleared or burned affects the succession 

 by modifying the rate of migration into the burn or clearing. This 

 is particularly true in case of destructive tires where seeds or prop- 

 agules on or near the surface are killed by the heat of the fire. Over 

 large burned or cleared areas, invasion would be greatest near the 

 edge because of the greater number of disseminules carried by wind 

 from the surrounding forest. Light disseminules carried by wind 

 could invade farther and in greater number than those with heavy 

 seeds or fruits. Populus and Bctitla could invade an area more 

 rapidly, for example, than Piniis. 



Since most of the area covered by the pine forest has been cut 

 over in lumbering and also since most of it has been burned over at 

 one time or another, in some places several times, the pine forest as 

 we find it now is largely developed by secondary succession. Thus 

 the consideration of secondary succession due to lumbering and 

 burning is of much importance. Except for areas in which the 

 climax pine forest has developed through hydrarch or xerarch suc- 

 cessions, the study of succession in other areas is wholly a study of 

 secondary succession. 



Since fire is more destructive to the vegetation of an area and 

 therefore will cause secondary succession to begin at an earlier or 

 lower stage, the initiation of secondary succession as a result of 

 burning will be treated first. The successive stages will be taken 

 in order, the cause of initiation at any particular stage being indi- 

 cated in the discussion of that stage. 



Very destructive fires may not only remove the existing vegeta- 

 tion of a region, but also destroy the humvis that may have accumu- 

 lated on the ground and with it all the seeds and propagules of every 

 kind so that succession must begin at a very early stage. In areas 

 of rock where only a shallow layer of humus exists, without the 

 presence of soil, a fire may cause the initiation of a primary succes- 

 sion beginning with crustose lichens. The lowest stage, however, 

 in which a secondary succession can begin is with the Cladonia- 

 Polyfrichtmi associes. 



(1) The Lichen Associes 



When fire sweeps over a region destroying the forest and the 



