448 ESTIMATION OF INCREMENT 



increment on standing trees by the use of a Pressler borer, it 

 is evident that the method is complicated and uncertain. 

 The objections may be summarised by stating that : 



(1) The formulae are complicated, and can apply to one 

 year only. 



(2) The form factor is an uncertainty. 



(3) The increase in height growth is disregarded. 



(4) The actual use of the borer is harmful to the trees. 



(5) The width of the' annual ring, at the base of the tree, 

 is often not a fair average width. 



Therefore, it will almost invariably be the better plan 

 to fell a few average sample trees and investigate the 

 increase that takes place on the average diameter. 



The percentage increase on the total contents of felled 



timber can at once be found by the 4 formula ; the 



dxn 



diameter under bark being taken at half-way up the stem ; 

 and the width of the average annual ring can be found by 

 Pressler's borer, or, better still, by cross-cutting the stem at 

 this point. 



But, in order to be quite accurate, it is preferable to find 

 the total contents of the increment, and then to express this 

 as a percentage on the total contents, or on the contents of 

 that which is timber only, which latter must also be found. 



For example, supposing that a felled tree be 50 feet long, 

 and that the average diameter under bark be 7 inches, and 

 that the average width of the last few rings is inch each, 

 and that in the next year the height growth will be increased 

 by I foot : 



Then, by Schneider's formula, the percentage increase for 

 the current year equals : 



400 



= 9-5 per cent. 



This, however, as already indicated, is slightly too high a 

 percentage. 



So again, the increment and percentage may be found by 



