FUTURE INCREMENT 441 



It is difficult to calculate the past increment for a whole 

 crop for more than just a year or so by this means, as owing 

 to the removal of trees by thinning it is difficult to estimate 

 what are fair average sample trees of the crop that existed 

 prior to the thinning. And, also, the trees that are selected 

 will, owing to the increased growing space, have wider annual 

 rings than the true average for the past period. 



But this difficulty is not presented in reckoning the pre- 

 sent or the future increment, except that in the latter case 

 the annual rings will get narrower as the trees again require 

 to be thinned, or if the trees are already mature ; in these 

 cases, however, any change will be very gradual. 



In order to find the future increment the present contents 

 are deducted from the estimated future contents ; and in 

 order to find the future contents, it is necessary to know 

 what will be the increase in height, and in girth or diameter ; 

 and, as already indicated, this is taken as equal to the rate of 

 increase in height and diameter that has just recently taken 

 place. 



Thus, if measurements be taken at breast high, and if 



D = diameter in future (under bark) 



d = present diameter (under bark) 



H = height in future 



h = present height 



F/ = form factor 



The increment will equal : 



Now, while timber is still standing, the average in- 

 crease in height may be guessed ; but the increase in 

 diameter can only be found by finding the width of the recent 

 annual rings ; and this is best done by the use of a boring 

 tool, known as Pressler's borer. With this tool a hole is 

 bored about an inch or two inches into the stem, and a round 

 spill of wood is extracted, whilst the borer is still left in the 

 tree. This spill of wood will show the width of a certain 

 number of annual rings, and by this means the diameter 

 increase is ascertained. For purposes of greater accuracy, it 



