CLASSIFICATION OF TREES BY DIAMETER 151 



Species with thick bark will show a smaller volume for the same 

 diameters than those with thin bark, because of taking the diameter 

 on the bark surface and not on the wood. Individual trees with thick 

 bark will give correspondingly less volume than the average for the 

 diameter class shown in the table. Timber on exposed sites will be 

 over-estimated by tables based on diameter outside bark unless con- 

 structed locally for the same sites. Width of bark, therefore, is a cause 

 of variation in the attempted standardization of volume by diameter 

 classes, which is eliminated in the universal tables when these are based 

 on diameter inside bark, at either top of log, D.B.H., or stump. 



127. Classification of Trees by Diameter. Standard volume tables 

 are commonly based on D.B.H. outside bark. The actual diameter 

 of trees can be measured as closely as the nearest ^inch. The aver- 

 age of two measurements taken at right angles is considered the diam- 

 eter of the tree. 



For felled trees whose volume is to be measured in the construction 

 of volume tables, the diameters are recorded to the nearest actual 

 xVinch. But these volumes are classified later by 1-inch, or 2-inch 

 classes. One-inch classes have been adopted as standard for Eastern 

 species, while in the West, owing to the greater range of diameters 

 encountered, 2-inch classes are deemed sufficient. Each 1-inch class 

 includes all trees whose average D.B.H. is above .5 in the inch below, 

 and .5 and under in the given inch class; e.g., the 9-inch class includes 

 trees measuring 8.6 to 9.5 inches. In 2-inch classes, the even inch is 

 used. A 10-inch class would include trees measuring 9.1 to 11.0 

 inches. 



128. Classification of Trees by Height. Height is never used as 

 the sole basis of tree classes; diameter is the fundamental basis of 

 classification. But height exerts an enormous influence on the volumes 

 of trees of the same D.B.H., the extreme difference in volumes for dif- 

 ferent heights being more than 100 per cent. These differences in height 

 and volume for trees of the same diameters occur in stands of different 

 density, growing on different qualities of site, or at different altitudes. 

 They correspond with differences in the average taper per log, as dis- 

 tinguished in universal volume tables. 



It follows that the separation of trees of a given diameter class into 

 several height classes previous to averaging their volumes is another 

 way of distinguishing between trees of gradual and of rapid taper, 

 and that if enough of these height classes are made, the differences in 

 volume due to more or less rapid taper are distinguished even more 

 accurately than by introducing taper as a factor in the table. The 

 height, rather than any arbitrary amount of taper, is the real basis of 

 classification, and the actual average volume, rather than an assumed 



