92 THE THEORY AND PRACTICE OF WORKING PLANS 



10. BY VOLUME.— BASED ON DIAMETER CLASSES. METHODE 

 DE 1883 (" FRENCH METHOD ").* 



(a) Description of Method. — Instead of constructing a stand 

 table, the total volume of each diameter class should be deter- 

 mined. Some figures should also be obtained showing the 

 number of trees of the diameter desired at the end of the rota- 

 tion (exploitable diameter) which there would be per acre in a 

 normally stocked stand, if no other age classes were present. 

 Since these figures must be taken in the field, sometimes before 

 the exploitable diameter has been decided upon, several diam- 

 eters should be taken. 



When the desired exploitable diameter has been decided 

 upon, determine from the growth figures the number of years 

 necessary to produce this diameter. This number of years, 

 lengthened by a few years to allow for a possible delay in repro- 

 duction, will be the rotation. 



Divide the trees shown by the estimates into three groups as 

 follows : 



First group, old trees. Those containing two-thirds of the 

 exploitable diameter and above; e.g., if the exploitable diameter 

 is 30 inches, this group would contain trees between 20 inches 

 and 30 inches. 



Second group, medium trees. Trees having a diameter fall- 

 ing between one-third and two-thirds of the exploitable diam- 

 eter; e.g., trees between 10 inches and 20 inches. 



Third group, young trees. Everything with a diameter 

 less than one-third of the exploitable diameter. 



The calculation of the cut is based on groups i and 2, and 

 is made in the following manner: 



Find the volume of each of the first two groups. Then if 

 the volume of the old trees is to that of the medium trees as 

 5 is to 3 the proportion of the two groups may be considered 



* Adapted from Barrington Moore's article " Methods of Regulating the 

 Cut on National Forests," in Vol. VII, No. i, " Proceedings of the Society of 

 American Foresters " 



