96 DETERMINATION OF THE INCREMENT. 



the former amounts of these out of the present quantity is more 

 or less speculative. 



Under these circumstances one of the following two methods 

 may he followed : 



A. Determination of the Future Increment according to the 

 Mean Annual Increment of the Past. 



The present volume of the wood is ascertained and divided 

 by its age, the quotient giving the mean annual increment 

 calculated on the growing stock present at the time of measure- 

 ment. According to the age of the wood, it may he assumed 

 that the mean annual increment will be laid on for a number 

 of years to come, or a somewhat diminished or increased 

 increment. 



The method gives fair results, if the calculation is made for 

 the time when the mean annual increment culminates, and 

 even for older woods ; it is less accurate in the case of younger 

 woods. Moreover, it is only applicable for a limited number 

 of years, during which no thinnings are made, say 10 years. 



B. Determination of Increment by means of Yield Tables. 



I. OF YIELD TABLES GENERALLY. 



1. Definition of Yield Table. 



It has already been explained that the progress of height, 

 diameter, basal area, and volume increment can be repre- 

 sented by curves constructed on the principle that the succes- 

 sive ages are marked as abscissae, and that the corresponding 

 ordinates represent the height, diameter, basal area, or 

 volume. Such curves indicate the appropriate quantities for 

 any age up to a fixed limit, generally the highest rotation 

 likely to be adopted. Instead of employing curves, the data 

 which they represent are read off and arranged in tables, which 

 are called Yield or Increment Tables. 



