358 GROWTH OF TREES IN DIAMETER 



The application of such a growth study is to determine correctly 

 the average age of trees of given diameter classes and diameter groups 

 in a forest or stand when the basis of age for the stand cannot be directly 

 determined (§ 320). This presupposes that the stands are not even- 

 aged, but many-aged in character. In mixed many-aged stands or 

 groups, suppression usually plays a large r61e and again interferes with 

 this determination by requiring the substitution of the economic age 

 for the actual age (§263). But for the species such as the Southern 

 pines, which are fireproof to a certain extent, and the Western yellow 

 pine, for the same reason, the age groups may be intermingled and yet- 

 the dominant character of growth maintained. Under these circum- 

 stances, the direct determination of age based on diameter may be 

 used for determining the average age of diameter groups, especially 

 for the upper or dominant classes. 



277. Current Periodic Growth Based on Diameter Classes. The 

 Increment Borer. A more common application of growth based on 

 diameter classes is for the prediction of current periodic growth in specific 

 stands, for short periods, by predicting the growth of each tree in the 

 stand in diameter and correlating this data with volume growth. The 

 drawbacks to this method have been discussed in § 251. Dealing, 

 as it does, with the specific stand and actual number of trees, it 

 is directly applicable to stands of all degrees of density and to 

 the actual stocking found on the ground, and to this extent is 

 applicable directly to the existing forest without the necessity 

 for a yield table. Tables showing the growth in diameter which 

 may be expected of trees of given diameters may be applied directly 

 to stand tables showing the number of trees of these diameters on 

 the average acre. 



The current growth of trees of given diameter is measured either 

 on the stump or directly at B.H. Growth measurements taken on the 

 stump must be laid out on an average radius (§25). As the growth 

 in D.B.H. outside bark is frequently less than that on the stump inside 

 bark (§ 269) correct results would require the reduction of the radial 

 growth on the stump to its equivalent at D.B.H. This is not usually 

 done, first because for trees of the smaller diameters D.O.B. at B.H. 

 tends to coincide with D.I.B. on the stump; second, because the total 

 error thus incurred in measuring the growth based on age is proportion- 

 ately reduced in measuring current growth, although the percentage 

 of error remains the same. This may be considered too small to require 

 correction. When measured directly at B.H., it is important to secure 

 an average radius if possible. The only method by which this can be 

 done is to take two readings on opposite sides of the tree, and determine 

 the mean. 



