98 THE THEORY AND PRACTICE OF WORKING PLANS 



are elasticity and a degree of accuracy not attainable with 

 formulas. 



11. BY VOLUME.— BASED ON DIAMETER CLASSES. ODIAN 

 METHOD.* 



(a) Description of Method. — This method is based on the 

 principle that a certain number of trees reach a size suitable 

 for cutting every year or period of years. The aim of the method 

 is to cut just this number of trees. 



The data required are: 



1. A careful enumeration of the growing stock. For this 

 purpose five or six broad classes are made from seedlings up to 

 mature trees. 



2. Growth figures, particularly showing the number of years 

 required to pass through each class. 



3. Figures showing the percentage of mortality suffered by 

 each class as it passes into the next class above and into the 

 final or mature class. 



The rotation is generally the sum of the number of years 

 required to pass through each age class till the exploitable size 

 is reached, with generally a few years added on to make it con- 

 servative. The felling period is a convenient subdivision of the 

 rotation and should be at least the length of time required 

 to produce enough material to justify the next cut. 



The annual cut is calculated in the following manner: The 

 number of trees in each class is multiplied by the percentage 

 which will survive till maturity. The results are added and 

 then divided by the rotation plus one-half of the felling period. f 



In order to find the growing stock of Class I trees the average 



* 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." 



t Half of the felling period is added to the rotation to allow for the number 



of Class I trees (the largest class) which should always be on the ground, because 



Felling period 



there should always be a number of Class I trees equal to the X 



2 



average annual cut. 



