8 PRACTICAL FOBESTEY. 



Finally, since we have found the yearly increase per tree and the 

 number of trees per acre, it is easy to find the average yearly increase 

 per acre. It is unfortunate that this simple and easy process is not 

 always reliable, because it is hard to find either an average acre or 

 an average tree. 



The yield of a forest is the aniount of wood that is taken from it 

 in a given tinie. When a forest is put under conservative nianage- 

 ment, one of the most important steps is to decide how much timber 

 can safely be taken from it; in other words, to determine the yield. 

 There are three principal ways of doing so. 



The first, and the least used, is to fix the yield at a certain num- 

 ber of mature trees. By this plan the yield of a certain forest might 

 be 100 pines, 260 spruces, and 180 hemlocks, each of a given diam- 

 eter, every year. 



The second way is to fix the yield at a certain amount or volume 

 of wood. Thus, the yield of a large forest might be fixed at 25,000,000 

 feet board measure every ten years, and that of another smaller 

 one at 750 cords every year. 



The third way is to settle upon a certain number of acres to be 

 cut over yearly or once in a given number of years. By this method 

 the yield of a forest of 600 acres might be fixed at 6 acres of mature 

 timber a year, and that of another at 300 acres every twenty-five 

 years. The time between two successive cuttiags on the same area 

 must be long enough to allow the young trees left standing to mature. 

 That time is found by studying the rate of growth in diameter. 



This method of determining the yield by area is much the most 

 practicable of the three for the forests of the United States, and ia 

 general it is the simplest and most widely useful of all, because it 

 does away with the difficult task of determining the yearly iacrease 

 m wood. 



The objects ia handling forests are so various that sometimes no 

 single one of these methods is satisfactory, and then combinations 

 of them are of great use. Thus, by combining the method by volume 

 and the method by area the annual yield of a forest might be estab- 

 lished at 250 board feet per acre. This yield might be cut from the 

 forest every year, or it might be allowed to accumulate for twenty 

 years, and then 5,000 board feet per acre might be cut. 



SIIiVICTJLTUBAIi SYSTEMS. 



After the yield has been found it must be cut not only without 

 injury to the future value of the forest, but in such a way as to in- 

 crease its safety and usefulness. To this end certain ways of handling 

 forests, called silvicultural systems, have grown up. They are based 

 on the nature of the forest itself, and are chiefly imitations of what 

 men have seen happen in the forest without their help. 



358 



