434 THE DETERMINATION OF GROWTH PER CENT 



absolute quantity but a relation, the assumption is that this relation 

 once established for a given stand will apply to other stands of a similar 

 character but differing in area and total volume. Growth per cent 

 on sample plots could for instance be applied to determine the annual 

 growth on the stand within which they are located. 



In so far as it can be known that the relation between the volume 

 of the larger area and the growth on this area is the same as on the stand 

 sampled, the method is obviously correct. The error lies in applying 

 such growth per cent figures to stands or areas on which this relation 

 is not the same, because the average age, thrift, or other conditions, 

 differ from the sample area. The simplicity of assuming that growth 

 per cent for a sample tree, or for a sample plot, can be applied to large 

 areas has led to its use as a substitute for sound growth data in many 

 instances. No such short cut will actually measure the growth on a 

 forest comprising many stands of different ages, site quahties, and 

 densities of stocking. 



333. Use of Growth Per Cent in Forests Composed of All Age 

 Classes. Growth per cent is a direct expression of current growth in 

 its relation to past or total volume. Hence it varies with the current 

 growth curve. Current growth per cent is equal to mean annual 

 growth per cent in the year in which the mean annual growth culmi- 

 nates (§ 245). 



In a forest composed of stands of all ages, or in a stand composed 

 of trees of all ages, equally proportioned as to area or ultimate yield, 

 and under management, the current growth per cent for the whole 

 forest or the whole stand, when weighted by volume of each age or tree 

 class, will be equal to the mean annual growth per cent for every year, 

 since there is no change from year to year in either of the two factors, 

 total volume or increment, which determine it. 



For such a forest the average growth per cent can be found separately 

 for each diameter class. By weighting each per cent according to the 

 volume of the trees in this class for the stand, a composite per cent is 

 obtained which shows the present status of the forest, and is applicable 

 in predicting its growth. But accurately to determine this per cent, 

 the growth itself must first be found on the trees or plots measured. 

 If in determining this growth, the future factors are really considered, 

 the numbers reduced, and the rate of diameter growth and probable 

 suppression taken into account, the result is a quantitative statement 

 of growth for the next decade or two instead of for the past decade. 

 This prediction of growth, on a few acres or a small per cent of the stand, 

 can then be reduced to the form of a per cent of present volume, and 

 applied, in this form, to the remaining stand as a convenient means of 

 computing growth on the total area, 



