348 GROWTH OF TREES IN DIAMETER 



illustrated in Fig. 69, in which the growth of five spruce trees is plotted, their rings 

 being counted from the outside inward. Each tree is plotted on the exact years on 

 which its measurements fall as determined by its total age. Where a large number 

 of trees are plotted, the points are not connected but form a band, on which the 

 curve of average growth is sketched by eye. This method is intended to save the 

 labor of calculating the averages arithmetically. 



Where trees of different ages are included in the average, the upper extremity of 

 the growth curve will represent a smaller number of trees, whose growth, if dominant, 

 will exceed the average rate, but if suppressed, will fall below it, causing the curve 

 to depart from a true growth curve, as illustrated in this Figure. 



268. Correction of Basis of Diameter Growth on Stump to Conform 

 to Total Age of Tree. The next step is to correlate this curve of growth 

 with the total age of the tree. The average age of seedlings must be 

 determined for the given average stump height (§257). The number 

 of years thus indicated is added to the scale by moving the zero the 

 required number of points to the left. This new zero causes a shift 

 in the age of each section to correspond. The curve now shows, not 

 the diameter of stump sections of various ages, but the diameter of 

 trees of various ages when measured at the height of the stump. 



269. Correlation of Stump Growth with D.B.H. of Tree. The third 

 step is to determine the D.B.H. for these same trees in order to correlate 

 this with age. What is desired is not the age of the section at B.H. 

 but the D.B.H. of the tree, whose total age and growth at stump are now 

 known. 



A tree of a given stump diameter, whose total age has been found, 

 has a set of upper diameters or tapers representing its form, as expressed 

 in a taper table (§ 167). Of these the most important is D.B.H. This 

 third step then consists simply of determining the average taper of the 

 butt, from stump height to B.H. so as to find the D.B.H. corresponding 

 to each inch stump-diameter class. 



Standard stump tapers show the D.I.B. (§135) of stumps at heights 

 of 1, 2, 3, 4, and 4| feet, corresponding to each D.B.H. class. But 

 to determine growth of trees at B.H. corresponding to growth on the 

 stump inside the bark, heights of stumps are usually averaged, and a 

 direct comparison is made of average D.B.H. outside bark with average 

 D.I.B. on the stump for all trees falling in the given stump-diameter 

 class. 



Stump tapers may be taken on the butt logs of felled trees in the 

 measurement of volumes (§ 168). The number of measurements so 

 obtained is often insufficient and may be supplemented by measuring 

 the diameter at stump height and width of bark to get D.I.B., on stand- 

 ing trees, together with D.B.H. Owing to the great variation in diam- 

 eters at the stump compared with D.B.H., a large number of stump 

 tapers are required to produce a curve free from irregularities, as illus- 



