PROJECTION OF GROWTH BY DIAMETER CLASSES 361 



By plotting the values In column 2 on the basis of diameter, a curve 

 may be drawn to even out the irregularities shown. To apply such 

 a table in pretlicting growth for a period of 20 years, for 4-inch trees, 

 the grow^th of successive inch classes is used; e.g., the 4-inch tree takes 

 6.5 years to reach 5 inches, 5 years to reach 6 inches, and 5.3 jears to 

 reach 7 inches, or a total of 16.8 years. The next inch requires 6.6 

 years, 3.2 of which lie in the 20-year period, equivalent to about |-inch. 

 The tree will grow to be 7| inches in diameter in 20 years. In this 

 way the growth for each D.B.H. class can be predicted for am- given 

 period on the assumption that the basis of comparison is trustworthy. 

 This is the simplest method of growth prediction for trees in many- 

 aged forests. In obtaining the average number of years in the last 

 inch, all trees included in the table must be measured for the same 

 period, i.e., the basis must be |-inch of radius. If instead the last 

 20 years is measured, divided into half-inches of radius, and a fast- 

 growing tree used in the table as the equivalent of several smaller inch 

 classes, its influence on the average will be increased in like proportion 

 and too rapid an average rate obtained. 



Where trees are measured for a past decade or fixed period of years, 

 the results are expressed as growth in inches for the period. This rate 

 of growth may then be reduced to mean periodic growth (average 

 growth per year for the period). Dividing 1 inch by this annual 

 growth gives the number of years required to grow an inch 

 in diameter for each inch class. This method is equally reliable, and 

 most tables of current diameter growth have been derived in this 

 manner. 



The assumption underlying the basis of comparison, namely, that 

 the rate of diameter growth is a function of diameter, is most nearly 

 approximated in many-aged forests of tolerant species such as spruce 

 and for averages which include a wide range of ages and condi- 

 tions. 



279. Method Based on Projection of Growth by Diameter Classes. 

 For single stands or specific conditions, growth for trees of the same 

 diameter varies tremendously (§ 274 and § 275) and shows its greatest 

 diversity, first in even-aged stands, second, between open-grown and 

 shaded trees. For such problems, prediction based on past grow^th 

 of the present trees, rather than comparison, is a more reliable 

 method. 



For this purpose, past current growth is measured for the last 5- or 

 10-year period, or for two to four such periods, as required. If it is 

 assumed that future diameter growth will equal past growth, the growth 

 is tabulated as follows: 



