SUCCESSION 251 



building, etc. He destroys vegetation by fires, lumbering, cultivation, and 

 drainage, and if he can not control climate, he at least modifies its natural 

 effects by irrigation and the conservation of moisture. The operations of 

 man extend from seacoasts and swampy lowlands through mesophytic 

 forests and prairies to the driest uplands and inlands. Since the adjacent 

 formations determine in large degree the course and constitution of a succes- 

 sion, it will be seen that the effects of any particular activity upon vegeta- 

 tion will differ greatly in different regions. For convenience, all classes of 

 successions arismg from the presence and activity of man will be considered 

 in this place, though, as indicated above, some might well be regarded as 

 producing primary successions, while others produce anomalous ones. 



307. 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, 

 rhey need not be considered. From their nature, fires are of little signifi- 

 cance in open vegetation, 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 vegetation, 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 stabilizing agent, inasmuch as it injures the typical vegeta- 

 tion forms of grassland much less than it does the woody invaders. All for- 

 mations with perennial parts above ground, viz., thicket, open woodland, 

 and forest, are seriously injured by fire. A severe general fire destroys the 

 vegetation completely ; a local fire destroys the formation in restricted areas ; 

 while a slight or superficial burn removes the undergrowth and hastens the 

 disappearance of the weaker trees. In the latter case, while the primary 

 layer of the forest remains the same, succession takes place in the herbaceou.^ 

 and shrubby layers. These successions are peculiar in that they are com- 

 posed 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 for- 

 mation, 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 destroyed wholly or in large part, the first stages 

 of the new vegetation are made up by invaders from the adjacent formations. 



