1U6 DETERMINATION OF THE INCREMENT 



This method rests upon the assumption, that the selected 

 yield table correctly represents the progressive increment of 

 the wood, of which the increment is to be ascertained. As 

 this is only approximately the case, the degree of accuracy of 

 the method depends — 



(a.) On the degree to which r„ approaches Va, 



(h.) On the difference of ages, that is to say, the difference 

 between a and a -\- n ; the smaller this is, the more 

 accurate will be the result. 



In folluNving the above method, it is essential to measure the 

 volume on a " normal " sample plot, because only then can 

 the true qualit}^ class be ascertained by means of the volume. 

 If no fully stocked sample plot is available, that nearest to it 

 should be selected, and the proportion between the actual and 

 normal stocking ascertained. The actual volume must then be 

 augmented in the same proportion, before it is used for the 

 determination of the quality class and the selection of the 

 yield table. At the same time, this procedure is subject to 

 errors, as it is not always easy to determine correctly the 

 proportion between the actual and normal stocking. 



Generally, the method is better adapted to woods, which 

 have passed middle age, than to younger woods, as in the 

 latter the factors of the locality have not in all cases found 

 full expression. In the case of very young woods, it is alto- 

 gether useless to measure the volume for the purpose of 

 selecting the proper yield table. For such woods, the quality 

 class must be determined by means of an older wood growing 

 in the vicinity on a locality of similar quality ; the same 

 procedure is followed in the case of blanks. If no such older 

 wood is available, the soil and climate must be examined and 

 the best possible estimate of the quality made accordingly. 



As the measurement of the volume takes much time, and 

 as it is difficult to estimate the exact proportion between the 

 actual and normal stocking, it has been proposed to select the 

 proper yield table for a wood by means of one factor only of 

 the volume. It has already been explained, that of all, such 



