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 5X2=1 inch 



Diameter of tree ten years ago 20 1=19 inches 



Present cross-sectional area with diameter 20 



inches 2.1817 square feet 



Area ten years ago, with diameter 19 inches. ..1.9689 square feet 



Increase in area for ten years 2128 square foot 



Per cent increase 2128X100^-10X1.9689=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. 



