CHAPTER IV. 



THE DESTRUCTIVE AGENTS OF FORESTS. 



When forests once become established over certain areas 

 they continue as a rule, under natural conditions, without much 

 change from year to year. There are, of course, spasmodic out- 

 breaks of insects and other animal enemies of trees, brought on 

 by some unusual occurrence, and there are wind storms and 

 hail storms which throw the trees and injure their foliage, and 

 there are occasional fires set by lightning ; but, on the whole, as 

 the forests are renewed, the old trees dying off and the young 

 ones springing up to take their places, there is but little apparent 

 change. The natural variation in the general character of for- 

 ests occurs not by years or decades but by centuries and ages. 



In an undisturbed state the forest and all its forms of life, 

 both animal and vegetable, are held in an almost perfect bal- 

 ance by natural laws. Almost every plant and animal which 

 reaches maturity comes up through an uncertain early life, 

 taking chances with thousands of its kind, and continues to live 

 in danger to the end. Each species is restricted within certain 

 bounds. When it overreaches them its special requirements for 

 food, heat, light and moisture act as a check. When it in- 

 creases abnormally it creates favorable conditions for the multi- 

 plication of its natural enemies. The enemies of one plant or 

 anim-al have, in turn, their o^vn enemies, and so on indefinitely. 

 There is, therefore, a continual balancing and adjusting of 

 conditions and species, which results from the operation of the 

 law above referred to. 



When men take hold and begin to clear away the trees, 

 reducing one species and providing for the increase of another, 

 the whole well-regulated system is thrown out of order. Plants 

 spread out of their places and become weeds ; harmless mammals 

 become pests; and injurious insects multiply and destroy crops. 



