91 



normally-stocked high forest divided as explained above 

 into three age- groups, the volume of material in the crops 

 ot the second group is equal to three-fifths of the volume of 

 tJie crops in the first group.* 



Therefore, whenever in a selection-worked forest the 

 volume ot the material in the group of fuU-aged crops 

 exceeds by two-thirds the volume of the group of medium- 

 aged crops, it may be admitted that these two groups cor- 

 respond to the two similar age- groups of a normally. stocked 

 high forest ; and if it is arranged to exploit the group of 

 full-aged trees in one-third of the number of years corres- 

 ponding to the age of the exploitable tree, there should be 

 no decrease in the yield during the following period when 

 the trees which, at the outset, constituted the group of 

 medium-aged crops will be felled. Purthermore, all risk 

 of overfelliiig may be avoided by omitting to take into 

 account the increment of the full-aged crops during the 

 period prescribed for their exploitation. 



In practice it is convenient to base the classification on 

 some more easily ascertainable factor, such as the circum- 

 ference or diameter of the trees, and thus prescribe the 

 maximum size for trees in the groups of medium-aged and 

 young crops. Usually the operation can be still further 

 simplified by determining the size corresponding to the age 

 of exploitability, and by assuming that the full-aged and 

 medium-aged crops comprise respectively trees exceeding 

 two-thirds the size of the exploitable tree, and trees from 

 one to two-thirds of that size. The method is very easy to 

 apply in calculating the yield of selection-worked forests. 

 First, the age of exploitability and the circumference or 

 diameter corresponding to it should be determined. Next, 

 a valuation survey should be made, and the trees and their 

 volumes classed as follows : — 



I. — In the group of full-aged crops — when they are 

 more than two-thirds the size of the exploit- 

 able tree. 



*That this is very approximately correct may be see h from fig. 1 by actually counting 

 the rectangles representing the production, or in a similar figure by assuming the average 

 annual growth per acre to be uniform and equal from year to year. If a be the area, v the 

 average annual production per unit of area, and n the number of years in the exploitable 

 a^e, the volume of wood in the first group would he — 



t "'^ + "^ X % and in the second group * ^^ ^ h'^^ x a 



2 2 



„ Volume of wood in first group f -f" 1 s 



Hence ■_ -^ =-= 3-^ — s=-t; ; = ■!• 



Volume of wood in second group t ■^ \ 



