176 PRINCIPLES OF AMERICAN FORESTRY. 



desired period, measured on the average radius. The 

 current annual increase in diameter is taken as the average 

 of several years back, as five or ten years. It is deter- 

 mined by counting off the required number of rings from 

 the bark in and measuring their thickness. Twice that 

 thickness divided by the number of years in the period 

 will give the current annual diameter increase. 



The Height Accretion is Determined by counting and 

 measuring the annual cones which appear in a longitudinal 

 section, or by measuring the length of log between two 

 cross-sections which was grown in the time indicated by 

 the difference in the number of annual rings at the two 

 sections. 



For example: A log is fourteen feet long. The lower 

 end shows 178 annual rings and the upper end 150 annual 

 rings. The difference in the number of these rings is 28, 

 or twenty-eight years were required to grow the fourteen 

 feet in length between the two cuts. The number of 

 annual rings at any cross-section indicate the lifetime of 

 that portion of the tree above the section. 



Mass Accretion is the Increase in Volume of the Grow- 

 ing Tree. The volume increase of standing trees can only 

 be arrived at approximately, and is based on the measure- 

 ment of the volumes of trees of different ages; the differ- 

 ence will be the increase for the period. The increase 

 in volume is often calculated as simple interest, but where 

 the mass of the tree is considered as capital, interest is 

 computed as compound. 



The Rate of Mass Accretion of a Standing Tree May 

 be Determined in the following manner: In mature 

 trees the height growth per year is inconsiderable, and 

 may be disregarded for short periods of time. The pres- 

 ent and past volumes, then, vary as their respective 

 basal areas. Taking twice the width of the rings for the 

 period desired from the present diameter will give ap- 



