16 A PRIMER OF FORESTRY. 



of the average tree nlay be found by dividing its cubic 

 contents by the years of its age. Finally, since we 

 have found the yearly increase per tree and the num- 

 ber of trees per acre, it is easy to find the average 

 3 r early 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 amount of wood that is 

 taken from it in a given time. When a forest is put 

 under conservative management, one of the mo*t 

 important steps is to decide how much timber can 

 safely be taken from it; in other words, to deter- 

 mine the yield. There are three principal ways of 

 doing so. 



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

 certain number 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 diameter, 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 

 ever}^ year. 



The third way is to settle upon a certain number of 

 acres to be cut over } 7 early 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 cuttings on the same 

 area must be long enough to allow the } T oung trees left 



