ENVIRONMENTAL FACTORS 



309 



disturbance, return to anything like the original vegetation is 

 problematical. Appropriate measures following proper study are, 

 however, nowadays leading to more and more effective conservation 

 — for example of vegetation with the object of maintaining water 

 supplies, of ranges against erosion, and of forests against fire. 

 Elsewhere by irrigation or drainage, damming, building, persistent 

 mowing, road and railway construction, mining and all manner of 

 other enterprises, Man alters the water and other conditions over 



Fig. 87. — Devastating results of overgrazing. Rill, gully, and sheet erosion on 

 overgrazed slope in San Joaquin Valley, California. (Courtesy of U.S. Forest 



Service.) 



vast areas — as he sometimes does the biological balance by tne 

 introduction or extermination of various animals and weeds or other 

 plants. 



In many regions fire is an important factor which, because it is 

 nowadays usually caused and controlled by Man, seems best con- 

 sidered here. The effect is rather like grazing or cutting in that 

 material is removed and if only small areas are affected they generally 

 return to something like the former state — especially if the fire is 

 a surface one that does not kill the larger thick-barked trees. How- 

 ever, big and severe ' crown ' fires may destroy the humus and 

 disseminules as well as most or all plants, and result in a community 

 of colonists followed bv a distinct ' burn succession ' of vegetation. 



