20 MENSURATION AND INCREMENT [CH. 



This formula gives the current annual percentage increment of 

 basal- area at breast-height (and also in cubic contents of the 



tree) as equal to 200 x -5 , while the diametral increment of any 

 stem is found in the proportion of 



p : 100 : : a : d, or p = 100 x -, 



a 



Comparing these two results it becomes evident that the 

 percentage increment in basal area of the stem, and in the cubic 

 contents of the tree, is always twice as great as the percentage 

 of increment in diameter. 



Schneider and Breymann's formula only give the current 

 annual percentage of increase during the year of investigation, 

 while Pressler's gives it for a short period of, say, ten years, 

 either past, present, or future. 



All three formulae rest on breast-height diameter of standing 

 timber, and assume that height and form-factor remain the same 

 for both periods. Therefore, to ensure accuracy, it is better, 

 when practicable, to fell a few average sample trees, and then 

 to cross-cut them at mid-height, measure the annual rings at 

 the mid-section, and then calculate the current percentage 

 increment by Schneider's formula. In cases of appreciable 

 height growth, the formulae for breast-high diameter should 

 be enhanced by a small percentage of up to 25 per cent. 



19. Increment of whole crops. 



We have already seen that the past increment of a wood 

 cannot accurately be deduced from the results of an investiga- 

 tion into the rate of growth of single trees, as many stems will 

 have disappeared or been removed in the thinnings. The past 

 increment of middle-aged and older woods will therefore be less 

 than that of the single average tree. 



This difficulty, however, does not present itself so much in the 

 case of the present and future increments, especially when we 

 only estimate such increment for a short term of years for practi- 

 cal purposes of management, and in such cases it is safe to assume 

 that for the next ten years the increment of the crop will be 

 about the same as during the last similar period. 



