316 PRIN'CIPLES UNDERLYING THE STUDY OF GROWTH 



total growth for the short period. The length of these periods is com- 

 monly a decade, but may be from 5 to 40 years. The term current 

 annual growth is commonly used in place of the term periodic annual 

 growth, as indicating the average annual growth for a short period 

 instead of the separate growth for a single year, though this use of the 

 term is technically incorrect . 



Finally, the relation which the increase in volume or growth bears 

 to the volume of the tree or stand on which it is produced may be 

 expressed as growth per cent, and indicates the rate of increase with 

 relation to the wood capital required for its production. This growth 

 per cent may be computed for volume alone, for growth in quality of 

 wood, or for growth in the unit price of the product (§ 334). A growth 

 per cent figure is not an index of absolute increase in either volume, 

 quality or price, since it is merely the expression of a relation between 

 capital and increment existing at a given time. Growth per cent is 

 usually based upon a single year's growth, either current or average 

 for a period. One year's growth is seldom measured, since a decade, 

 or at a minimum, a five-year period is required to eliminate variable 

 factors affecting a single season's growth caused by climatic conditions. 

 Hence periodic annual growth is commonly substituted for current 

 annual growth as a basis for computing growth per c?nt. 



245. Relation between Current and Mean Annual Growth. Growth 

 may be studied either for an individual tree or for a stand, expressed in 

 terms of growth per acre. In either case, the current annual growth 

 in volume increases at first slowly and then more rapidly to a maximum, 

 after which it begins to decline and finally ceases with the death of the 

 tree or the beginning of actual decadence of the stand. The sum of the 

 current annual growths laid on for the entire period gives the total 

 growth. The total growth or volume divided by the age in years 

 gives the mean annual growth (Fig. 64). 



The mean annual growth is an average rate of growth representing 

 the total growth or yield at a given age, distributed or spread over this 

 period. The actual productiveness of the forest is in this way compared 

 with annual crops, which basis is otherwise obscured by the varying 

 rate or curve of growth in volume of the trees from decade to 

 decade. 



The mean annual growth at any given year is this average of past 

 production. Current growth for the year or decade tends to increase 

 constantly up to a given maximum. During this period the volume 

 added each year to the total volume of the stand is greater than the 

 average or mean annual growth up to that year. Hence this average 

 is raised and the curve of mean annual growth increases. But it can- 

 not increase at as rapid a rate as the current growth curve, since the 



