346 GROWTH OF TREES IN DIAMETER 



e.g., on a tree 21 years old, the decades are 1, 2-11, 12-21 years. On 

 a tree 29 years old the decades are 9, 10-19, 20-29 years. 



Purpose of Counting Outward from Center to Outer Ring. In tracing the growth 

 of trees in diameter, based on their age, to determine the average sizes reached at 

 each decade, the above averages might tend to conceal or flatten out any changes 

 characteristic of the juvenile period. ■ In this case a more clear-cut definition of 

 growth may be obtained if age is actually made the basis, and the same decades 

 averaged for each stump, e.g., 1-10, 11-20 years. 



For this purpose the count would be made outward from the pith, coinciding in 

 direction with the measurement of growth, throwing the fraction to the outside. 

 But this causes the fractional decades to fall in as many different columns as there 

 are trees of different ages by decades. In tree analyses it would result in measur- 

 ing different fractions at each upper section instead of the same rings. It does 

 not give current diameter growth for a stand. The age of the seedling, which is 

 usually a fractional decade, must still be added. For these reasons the first method 

 is considered standard. But for the purpose indicated, diameter growth based on 

 age, the last fractional decade on the outside although recorded could be dropped 

 in obtaining average growth of several trees; e.g., a 43-year stump can be computed 

 for its first four decades only. By this plan, the averaging is simplified. 



Method of Measurement. The measurement of diameter growth is 

 usually made with a steel rule graduated to inches and twentieths, or 

 .05 inch, which is the smallest graduation commonly employed. 

 When the radius has been laid off and each decade marked, the zero 

 of the rule is placed at the center and the distance read to each decade 

 point. The measurements are cumulative, that is, the rule remains 

 in the same position until the complete radius is read. This avoids 

 errors which are sure to occur in moving the zero from one decade 

 to another to separate the decade measurements. The form of record 

 is shown on p. 345. The accuracy of the reading should be checked 

 by noting that twice the total radius should equal the average diameter. 



267. The Determination of Average Diameter Growth from the 

 Original Data. The average diameter growth for the trees measured 

 may be obtained by arithmetical means, and by the aid of graphic 

 methods. 



Table LI shows the method of computing the average growth. 

 When the decades have been counted from the pith with the final 

 fraction rejected, each decade is full and the averages fall at 10, 20, 

 30 years, etc. This completes the table in the form desired. But 

 when the rings are counted from the outside, the first decade being 

 fractional, the growth is not shown for full decades, but for odd years 

 as 7, 17, 27 years, etc. 



To obtain the growth at the required decades, a curve of radius 

 growth based on age is plotted as shown in Fig. 69, each point being 

 plotted above its proper age. The radius scale is then doubled to 



