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 vegcta- 
tion will differ greatly in different regions. For convenience, all classes of 
successions arising 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, 
they 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 suhsequent 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 iocal 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 herbaceous 
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 sticcession in which the pioneers are derived largely from the original for- 
mation, particularly when the iatter 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, 
