103 



by the volumetric method, which consists in calculating the 

 total volume of wood produced in an interval of time 

 equal to the exploitable age, and by dividing this total by the 

 number of years in that age. The method is still sometimes 

 employed. 



The manner in wtioli the calculation is effected may best be explained by an 

 example. Suppose a forest of 1,600 acres, which it is wished to exploit at 120 years, 

 divided into cnmpartmenls or cultural sub-divisions each containing a crop of an uni- 

 form age. The exploitable age would be divided into, say, four periods of 30 years 

 each, and the area, ■provisionally, into four corresponding blocks, the first containing 

 the crops above 90 years old, the second those of 60 to 90 years, the third those of 

 30 to 60 years, and the fourth those below 30 years. 

 Thus :- 



And so on for the other blocks. This preparatory classification having been effected, 

 the Tolnme of material in each compartment is calculated, and to this volume is 

 added the estimated future increment up to the time when the compartment will be 

 regenerated, that is to say, 15 years' growth (the mean of the 30 years daring which 

 all the compartments would be regenerated, some at the beginning, some at the 

 end of the period) for the compartments forming the first block, 45 years' growth; 

 for the compartments forming the seoond block, 75 for those forming the third 

 block, and 105 for those in the fourth block thus : — 



l>t block, aotnal volnme 1,386,000 c. ft. estimated growth 15 years, 210,000 c, ft. total 1,696,000 c. ft. 



Total . 6,070,000 0. ft. 



Consequently, the total volume to be felled during the first period of 30 years, after 

 which the calculations would be revised, is 6,070,000-4-4=1,517.600 cubic feet, and 

 each year 1,517,500-4-30= 50,583 cubic feet. The permanent plan wnnld then be drawn 

 up by allotting, to the first block, to be regenerated during the period of 30 years, a 

 sufficient number of the compartments containing the oldest crops to furnish, includ- 

 ing future increment, 1,517,500 cubic feet of material. As the compirtments tempo- 

 rarity included in the first block will furnish 1,596,000 cubic feet, certaiii crops 

 should now be allotted to the second block instead. There would then be felled each 

 year during the period, in the compartments forming the revised first block, 60,683 

 cubic feet of material. During the first period, thinnings would be carried out in 

 the compartments of the other blocks until the end of the 80 years, when the compart- 

 ments in the second block would be taken in band and dealt with in the same way. 



This method of calculating the yield is now, owing to the 



d ifficulty of accurately estimating for long periods~tlie future 



growth, seldom used except in certain parts of Germany 



where, however, numerous improvements have been 



