100 DETERMINATION OF THE INCREMENT. 



5. Metlwds of Constructing Yield Tables. 

 The following methods have been proposed : 



a. Annual or Periodical Measurement of the Growing Stock of one and 



the same Wood ; in the second case the Intermediate Values are 

 found by Interpolation. 



The method gives absolute certainty that all figures of the 

 yield table are derived from the same quality class, but as the 

 preparation of the table would take a century and more, the 

 method has only theoretical value. Moreover, accidents 

 may happen which would render the wood unfit for further 

 observation. 



b. Annual or Periodical Measurement of the Growing Stock of a limited 



number of Woods of different Ages. 



In order to save time, it has been proposed to select several 

 woods differing in age by a certain number of years, say 20, 

 and to obtain from the measurements of each, extending over 

 20 years, part of the yield table. To make sure that the 

 quality of the several woods is the same, it is necessary that 

 they should have the same volume at the same age ; in other 

 words, the wood now 40 years old should have had, when 20 

 years old, the same volume as the present 20-years-old wood 

 has ; again, the 60-y ears-old wood the same volume when it 

 was 40 years old, as the present 40-years-old wood, etc. Or, 

 to put the matter differently, the 20-years-old wood should 

 have, when it becomes 40 years old, the same volume as the 

 40-years-old wood has now, etc. In addition, the progress of 

 the increment should be steady throughout. 



Although it is difficult to select localities on these lines, 

 which are exactly of the same quality, or woods which will de- 

 velop in the same manner, there can be no doubt that 

 ultimately satisfactory yield tables can only be obtained by 

 observing and periodically measuring suitable woods for a 

 series of years. Hence the method is actually followed. For 

 each quality class and age gradation several sample plots are 



