450 ESTIMATION OF INCREMENT 



by measurements taken at half the length of the tree ; and, in 

 the making of working plans, some such calculations should 

 always be accurately made for crops approaching maturity. 



For purposes of greater accuracy, the increment, or the 

 percentage increment, may be obtained from data referable 

 to trees belonging to different diameter classes in the crop ; 

 instead of from data referable only to the average diameter 

 class. 



Attempts are often made to estimate the percentage that 

 is taking place in any crop by reference to average yield 

 tables. 



This is a method, however, that cannot be recommended. 

 It is almost impossible to obtain an accurate result, unless the 

 crop under consideration correspond almost exactly with the 

 data yielded in the tables. For instance, supposing that on 

 a soil of medium quality a smaller growing stock were found 

 than was indicated by the tables, for a soil of similar quality, 

 and that an attempt were made to assess the increment by 

 taking a proportionate amount of that indicated in the 

 tables, the result would be very inaccurate ; inasmuch as, 

 the actual crop, having been more heavily thinned than is 

 indicated in the tables, will, owing to the greater amount of 

 growing space allotted to the individual trees, increase at a 

 greater rate per cent, than the normal crop as found in the 

 tables. 



Where, however, the tables are used for this purpose, 

 care should be taken that the increment, for any period, be 

 not expressed as a percentage on any stock, which includes 

 an amount which should properly be removed as thinnings at 

 the commencement of the period and before the calculations 

 are made. 



Current Annual Increment. 



Another matter, which should receive a forester's attention, 

 and by which much valuable data can be obtained, is the 

 making of a comparison of the current annual increment per 

 acre, or the current average increment for short periods in 

 the life of a crop, with the average annual increment through- 

 out the entire life of the crop. 



