554 



BIOLOGY AND HUMAN LIFE 



wood lots from every point of view. From these investigations 

 we learn, first, the importance of avoiding certain injuries to 

 the forests, and, second, the methods of increasing their value. 



At the present time the people of this country are using up trees 

 about four and a half times as fast as they are being grown. This means 

 that before very long we shall have destroyed all the usable trees and be 



b 



AA-Lil^-A 



h^kAX-k-k U 



a 



Fig. 227. Cutting trees to preserve forests 



The preservation of the forest does not mean simply to avoid cutting timber. By cut- 

 ting trees in zones at intervals of a number of years, and by thinning out the trees 

 where they are too crowded, it is possible to make a given area yield continuous 

 crops of wood. The zone a was cut first, then zone b, and so on. By the time the 

 last strip has been cut the trees on the first strip are well along, and thus a succes- 

 sion of cuttings may be continued indefinitely 



practically without a suitable wood supply. A scientific study of the 

 growth of trees in the forest shows that it is possible to get all the wood 

 we really need without destroying our forest, if only certain principles 

 are followed (Fig. 227). 



It is to be noted that the ordinary virgin forest is almost at a stand- 

 still so far as growth is concerned. While new growth is constantly 

 taking place, this is only enough to offset the death and destruction 

 among old trees. 



398. Increasing forest area. There are over 80.000.000 acres of 

 land left barren by "timber mining" and fires. Such areas may be re- 



