FOREST MENSURATION. 177 



proximately 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 

 measuring 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 .5X 2 = 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 .9889 square feet 



Increase in area for ten years 2128 square foot 



Per cent, increase 2128X 100 -f-10X 1 .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, calcu- 

 lated 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 diameter divided by twice the width 

 of these rings. This gives relative diameter, opposite 

 which, in Pressler's table (see page 178) 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 in- 

 crease would then be 4.4 inches, and by dividing the diam- 

 eter by the diameter increase, 3.6 is found to be the rela- 

 tive diameter. In Pressler's tables, opposite 3.6 is found 



