140 ELEMENTARY FORESTRY. 
diameter will give approximately the former diameter of the tree. 
From this diameter obtain the area at that time, and compute 
percentage of growth from the difference between that and the 
present area. 
For example: By cutting into the trunk of a tree, or by 
removing a core of wood with an accretion borer, and measur- 
ing the thickness of the annual rings for ten years, we find it to 
be .5 inch, and the present diameter of the tree inside bark is 
twenty inches. 
Increase in diameter for ten years................- 56251 inch 
Diameter of tree ten years ago..............+.-20—I=19 inches 
Present cross-sectional area with diameter 20 
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Area ten years ago, with diameter 19 inches...1.9689 square fect 
Increase in area for ten years............ .2128 square foot 
Per Cent Increase: s.ccenai ss angrea ees .2128 % 10010 & 1.96890=1% 
The Determination of the Rate of Mass Accretion of 
a Standing Tree with compound interest is a more difficult 
matter, but Pressler, an eminent German forester, calculated 
tables for average thrifty trees and for very thrifty trees, the use 
of which renders the work of computation very simple. The 
width of rings for the desired period is measured, and the diam- 
eter divided by twice the width of these rings. This gives rela- 
tive diameter, opposite which, in Pressler’s tables (see page 141) 
will be found a number which is to be divided by the number of 
years in the period. The result will be the per cent of accretion 
with compound interest. For example: A Cottonwood sixteen 
inches in diameter shows a growth of 2.2 inches on the radius 
for the last ten years. The diameter increase would then be 4.4 
inches, and by dividing the diameter by the diameter increase, 3.6 
is found to be the relative diameter. In Pressler’s tables, oppo- 
site 3.6 is found the number 81 in the column of average thrifty 
trees. Divide 81 by ten (the number of years), and obtain the 
rate of increase with compound interest, 8.1 per cent. 
