118 SUCCESSION 



(a) Succession in burned areas. It will suffice 

 merely to point out that "burns" may arise naturally 

 through lightning, volcanic cinders, lava flows, etc. , but the 

 chances are so slight that these causes may be ignored. 

 The causes of fires are legion, and, as they have little or no 

 effect upon results, they need not be considered. Prom 

 their nature, fires are of little significance in open veget- 

 ation, deserts, polar barrens, alpine fields etc., since the 

 area of the burn can never be large. In closed formations, 

 the extent of fires is limited only by the area of the veget- 

 ation, and the effect of wind, rain and other forces. Forest 

 fires usually occur during the resting period, except in the 

 case of coniferous forests. In grassland, the living parts 

 are underground during autumn and winter, when prairie 

 fires commonly occur. As a consequence, the repeated 

 annual burning of meadow or prairie does not result in 

 denudation and subsequent succession. On the contrary, it 

 acts in part as a stabilising agent, in as much as it injures 

 the typical vegetation forms of grassland much less than it 

 does the woody invaders. All formations with perennial 

 parts above ground,, viz. thicket, open woodland, and forest, 

 are seriously injured by fire. A severe general fire de- 

 stroys the vegetation completely : a local fire destroys the 

 formation in restricted areas, while a slight or superficial 

 burn removes the undergrowth and hastens the disappear- 

 ance of the weaker trees. In the latter case, while the pri- 

 mary layer of the forest remains the same, succession takes 

 place in the herbaceous and shrubby layers. These succes- 

 sions are peculiar in that they are composed almost wholly 

 of the proper species of the forest, and that they are very 

 short, showing only a few poorly defined stages. A local 

 fire initiates a succession in which the pioneers are derived 

 largely from the original formation, particularly when the 

 latter encloses the burned area more or less completely. 

 The constitution of the intermediate and ultimate stages 

 will depend in a larger degree still upon the size and 

 position of the burn. When a particular formation is de- 

 stroyed wholly or in large part, the first stages of the new 



