560 Forestry Quarterly 



and height-growth per cents; also comparisons of increment per 

 cent secured by stem analysis and by Pressler's approximation 

 formula. Schneider's (Koenig) formula comes in for consider- 

 able discussion ; the practical difficulty of determining the con- 

 stant which varies between 400 and 800 in that formula is 

 exhibited on examples. Breymann's approximation seems to 

 furnish results close to the precise formula, but the same diffi- 

 culty as in Schneider's exists when height and form increment 

 must be allowed for, the constants in the measurement of 250 

 trees varying between 160 and Q^Q. The author found that the 

 relative crown length, which Pressler considered so influential 

 on shaft form, exercises this influence only in younger trees. 



The author furnishes a table of constants for use in the formula 

 on pine, based on relation of height and diameter increment. 



If in single tree measurements only approximations can be 

 secured, this is still more so the case in measuring stand 

 increments. 



The cross-section or basal area is the most accurately ascertain- 

 able factor, and should therefore be used wherever possible to 

 gauge conditions or silvicultural management. The selection of 

 sample trees, a number rather than an average stem, must be 

 made with circumspection. The volume of the average stem 

 multiplied by the number of stems gives, to be sure, the total 

 present volume correctly, but a stem analysis of this tree cannot 

 be used for increment determinations of the stand, for it does 

 not represent the average tree at any other age. Only in older 

 stands, deductions from such trees forward (not backward) 

 may be admissible under certain circumstances. Examples show 

 the unreliability of using such average trees. 



An alleviation of the difficulty is afforded by forming stem 

 classes (diameter classes) and measuring the average stem of 

 each. For the increment of the stand the increment of the stout- 

 est stem class is determinative ; its average stem is to a less 

 extent shifting its position from period to period, for the stoutest 

 trees have at all times been in the upper class. In the lower stem 

 classes the variation of increment in the average stems is much 

 greater. While in a given case, the largest increment of several 

 sample trees of the I class exceeded the smallest 1.43 times and 

 in the II class 1.48 times; in the III class it did 2.6 times; in the 

 IV class, 3.14 times; in the V class, 3.29 times. 



