MEASUREMENT OF DIAMETER GROWTH ON SECTIONS 343 



tion fifty years old 

 showing eccentric 

 growth, position of 

 the two average 

 radii AB and AC 

 and rot on radius 

 AB. Decades of 

 growth are shown. 

 The growth must be 

 measured on radius 

 AC. 



An average radius is located. Its length must equal just one-half 

 of the average diameter inside bark (§25). To determine the average 

 diameter, calipers graduated to iVinch may be used (§ 189). In all 

 cross sections which are not perfect circles, the 

 lengths of the radii from the pith or center of 

 growth vary more widely than the diameters owing 

 to the fact that the pith is always located at one 

 side of the geometric center of the cross section. 

 Leaning trees grow largely on the under side and 

 this general law accounts for the position of the 

 pith. On an eccentric cross section there are but Fig ; _ tump se , c 

 two radii which are average in length and can be 

 measured for growth. It often happens that one 

 or both of these radii (Fig. 67) are interfered with 

 either by the undercut or by the presence of rot 

 or defects which prevent growth measurement. 

 If either one is clear, the section may be meas- 

 ured. Otherwise, if measurement is absolutely 

 necessary, a longer or shorter radius can be taken 

 and the measurements reduced by proportion to 

 the required length. 1 



Method of Counting Decades. The next step is to count the number 

 of annual rings and indicate with a pencil the points at which the decades 

 fall. Except in scientific investigations where each year's growth may 

 be separately measured to determine the influence of climate on annual 

 growth, the decade is ordinarily the smallest interval used in measure- 

 ment of diameter growth. For current periodic growth a five-year 

 period is sometimes used in order to get points for a curve in predicting 

 the growth (§ 279). 



Unless the total age of the stump falls on a decade, as thirty, or 

 forty years, there will be one fractional decade laid off, representing 

 from one to nine years, depending on this total age. The diameter 

 growth is always measured outward beginning with the pith or center 

 of growth. But in counting the annual rings to lay off these decades 

 of growth, two distinct methods of procedure are followed. In one, 

 the count begins at the center, laying off ten years from the pith, and 

 throwing the fractional decade to the outside as on the right side of 

 Fig. 68. By the other, the count begins at the cambium layer or 

 outer ring, and this throws the fractional decade to the center as on 

 the left side of the figure. 



Purpose of Counting Inward from Outer Ring to Center. The choice 



1 E.g., if the average radius is 9 inches, and a radius of 10 inches is measured, 

 each measurement must be reduced by the faotor -^ or .9 



