354 GROWTH OF TREES IN DIAMETER 



275. Laws of Diameter Growth in Even-aged Stands, Based on 

 Age. The struggle of the individual trees for space produces different 

 results in even-aged and in many-aged stands, although the general 

 effect is a final reduction in numbers in either case. In the even-aged 

 stand the area occupied by an age class is definitely fixed. Expansion 

 of the crowns of individual trees can occur only by the prevention of 

 corresponding expansion of other crowns and by securing of additional 

 space through the actual death of the weaker trees. This process 

 results in a continuous differentiation of diameter classes in an even- 

 aged stand with advancing age. As the trees become fewer in number, 

 the difference in size of the survivors increases. These relations are 

 shown in Fig. 71, in which the number of diameter classes existing at 

 different ages in an even-aged stand is indicated. 



The growth in diameter of the trees which compose this even-aged 

 stand is shown in Fig. 72. The diminution in diameter growth due 

 to suppression of crowns affects successive trees of larger and larger 

 diameter. The average tree at a given decade is seen to fall into the 

 lower half of the stand in the succeeding decade and at some future 

 period will become suppressed and finally die. 



In Fig. 71 is shown the difference in basis and composition of the curves based 

 respectively on age and on diameter. The curve based on age in this figure is 

 composed of averages of all the diameter classes in successive even-aged stands, as 

 shown in the vertical columns. The curve based on diameter takes all trees of a 

 given diameter for each successive average, thus including trees from a number 

 of different age classes or stands as read horizontally in the diagram. This curve as 

 plotted in Fig. 71 is reversed, with the basis, diameter, plotted on the vertical scale. 

 The proper form of such a curve is shown in Fig. 73. The wide divergence possible 

 in the two bases, for dominant larger trees, is indicated in Fig. 71. 



It is evident that growth measurements of diameter based on age, which include 

 trees whose total age varies from 20 to 50 years, corresponding with the diameter 

 classes A to L in Fig. 72, will not be correct for any single tree in the stand D. The 

 portion of this curve representing the earlier decades is depressed or lowered by the 

 inclusion of the slower growing trees F to L which afterwards die. With the suc- 

 cessive dropping out of these trees from the average, the latter portion of the 

 curve shows a more rapid growth than that of the trees which compose it. 



To get the actual past growth of an average tree for a stand of a given age, C, it 

 is evident that only trees which have reached this age must be measured, A to E. 

 To secure average diameter growth for mature timber which in the future will be 

 grown to the given sizes and numbers per acre characteristic of this class of timber, it 

 is incorrect to include measurements of average trees for stands which have not yet 

 reached this age, F to L. By confining the selection of trees to timber of the desired 

 age and by taking the growth of all of the trees found on an area of sufficient size, 

 we obtain an average rate, showing the past growth of these trees, which is a true 

 growth curve, C. If it is desired to predict the rate of growth for the average tree of 

 a given age and character of mature stand, dominant trees must be selected from 

 younger stands rather than the average tree. The fewer of these trees, and the 

 greater their relative crown spread or dominance compared to the remaining stand, 



