380 GROWTH OF TREES IN VOLUME 



all sections which fall at the same height above the ground. The sum 

 of the age or period required for the average tree to reach this height, 

 plus the age or period represented by the growth of the section equals 

 the age of the tree regardless of the height of section. It is evident 

 then that the average curve of growth in diameter for any of these 

 sections can be plotted on a single sheet of cross section paper whose 

 horizontal scale represents the age of the tree and whose vertical scale 

 represents the diameter of any cross section. A cross section which 

 does not begin to grow in diameter for 17 years will diminish to zero 

 and the curve representing its growth will intersect the base or zero 

 diameter at 17 on the horizontal scale representing age of tree. 



In Fig. 70 (§ 269) a curve of stump diameter based on the age of the 

 tree was shown as intersecting this base at the age represented by the 

 seedling. On this same sheet a curve representing the D.B.H. and one 

 showing the diameter at the top of the first 16-foot log were indicated 

 with their points of intersection. On a single vertical line the points 

 shown were the diameters of a tree of a given age and indicated the 

 D.B.H., D.I.B. at stump and D.I.B. at top diameter of first log for 

 this age. But to get a curve showing these three dimensions for trees 

 of different ages in the illustration given, the points were not taken 

 from the growth of one tree, but by the measurement of several trees 

 differing in age, stump diameter and corresponding D.B.H. and upper 

 tapers. The connection of the points for these separate trees which 

 differ on the basis of age, gives the curves showing the increase in the 

 ■ upper diameters or tapers for trees of different ages. 



The method of plotting the upper diameters showing the growth 

 of an average tree at the different ages of its life is identical with this 

 previous method, with the exception that instead of these ages being 

 represented by the final, present or outer dimensions of separate trees, 

 they include the past, interior dimensions as well, by the measurement 

 of past growth. Even though the growth is an average of many trees, 

 the method still remains the same since each decade's growth is a com- 

 posite of the actual growth or internal dimensions of a number of trees. 

 The method of plotting the data is as follows: 



1. Prepare and plot a curve of average height based on age on a 

 separate sheet. 



2. Prepare on separate sheets, curves of average diameter growth 

 for all cross sections falling at each separate height, as for instance a 

 curve for sections falling at 8 feet, 16 feet, etc., including one for the 

 stump section. It is assumed that the height of seedlings based on 

 age has been determined and that D.B.H. has been correlated with 

 stump D.I.B. 



3. After determining the initial or zero year for each of the curves 



