318 DETERMINATION OF THE YIELD. 



to give permanently an annually equal return of the normal 

 age and amount, must contain the normal growing stock 

 corresponding to the rotation and method of treatment. 

 Foresters speedily applied this principle to the regulation 

 of the yield of forests by saying that, in order to lead an 

 abnormal forest over into the normal state, it is necessary to 

 establish the normal growing stock, in other words, to re- 

 move a surplus or to save up any deficit, as the case might 

 be. The method developed upon this basis is called the 

 Austrian assessment method. Authors differ as to the details 

 of the original method, but a general survey of the literature 

 on the subject gives the following rule for determining the 

 yield: 



" If the normal growing stock is present in a forest, then 

 the actual, or real, increment must be utilized; if the real 

 growing stock is greater than the normal, more than the real 

 increment must be removed ; if the real growing stock is 

 smaller than the normal, less than the real increment must be 

 xitilized, until the deficiency has been made good." 



In carrying this excellent idea into effect, however, errors 

 were introduced, which are still upheld by some foresters of 

 the present day. 



The procedure is described as follows : 



(1) The increment is calculated as the mean annual incre- 



ment of a series of years. 



(2) The normal growing stock is placed equal to the final 



mean annual increment corresponding to the normal 

 rotation multiplied successively by the ages of all age- 

 gradations; the sum of all these products gives the 



value G n = Ix ^ (see p. 235). Here /represents the 

 A 



normal annual increment of all age gradations, which 

 is equal to the volume of the oldest age gradation. 



(3) The real growing stock is obtained by multiplying the 



real final mean annual increment by the present age of 

 each age gradation. For this purpose it is necessary 



