48 Forest Mensuration 



PARAGRAPH LXXVII. 



DIAMETER INCREMENT. 



The current diameter increment is obtained by counting and measuring 

 the rings on a disk through the tree. It is generally best to count from 

 the bark towards the center, along two radii standing perpendicular to 

 each other. 



The general laws of diameter growth are identical with those of height 

 growth relative to culmination, decrease and increase of absolute (Par- 

 agraph LXXV.) as well as of relative (Paragraph LXXVI.) increment. 



If we exclude the butt-piece below chest-height, the annual rings along 

 the tree bole measured at various elevations above ground show a grad- 

 ual increase of width with elevation, provided that the leaf canopy of 

 the forest is complete and uninterrupted e. g., the width of the ring 50 

 feet from the ground, formed in 1903, is greater than the width of the 

 ring formed 20 feet above ground in the same year. 



For trees standing in open crown-density, the width of the ring de- 

 creases with the elevation above the ground, especially within the crown 

 itself. 



A tree standing in a thin crown-density may show an even width of 

 ring all over the tree bole. 



For very old trees in closed stand it is sometimes found that the diam- 

 eter, say 40 feet -above ground, is larger than the diameter, say, 20 feet 

 above ground. 



The rings on a disk are not actually circles; they more closely ap- 

 proach the form of eccentric ellipses (see Paragraph XIII). 



PARAGRAPH LXXVIII. 



SECTIONAL AREA INCREMENT. 



The increment of the sectional area is obtained from the increment of 

 the diameters. Where greater exactness is required, and especially in 

 case of irregular rings, the planimeter or the weight of a piece of paper 

 having the form of the sectional area may be used for measuring to good 

 advantage (Paragraph XIII.). 



The increment of the sectional area at chest height depends on the 

 crown density overhead; further, on the quality of the soil. At chest 

 height the culmination of the current annual sectional area increment 

 takes place, in the case of dominant trees, fast growing species and com- 

 plete cover overhead, between the years 40 and 70. 



The culmination of the current annual sectional area increment occurs 

 always later than the culmination of the current height and diameter in- 

 crement. After culmination it remains uniform for a long time. 



The absolute increment of a sectional area higher up on the bole, com- 

 pared with the absolute increment at chest height, is found to be equal 

 to it in the case of dominant trees ; larger in the case of suppressed trees ; 

 and smaller in the case of isolated trees. 



