VALUE OF CURRENT GROWTH VERSUS YIELD TABLES 445 



while its actual average or mean annual growth, which takes in the true 

 period required, is much less. 



But in a many-aged stand, or on a forest composed of stands of all 

 ages, these elements counterbalance each other. As growth cannot 

 be measured on stands below merchantable age or size, it is not meas- 

 ured on the areas covered by such young stands, or on the portion 

 occupied by immature trees in mixed- stands. But as soon as these 

 stands or trees mature, the growth is measured all at once and greatly 

 exceeds the actual current rate on the areas measured or for the trees 

 in these age classes. Wlienever the age classes are distributed evenly, 

 the excess of current growth so caused is balanced for the area or forest 

 by the neglect of the current growth on the younger stands. It follows, 

 first, that in forests with well distributed age classes, the total current 

 annual growth actually laid on in stands of all ages should be about 

 equal to the current growth obtained by measuring only the merchant- 

 able stands, provided the maturing volumes of young timber are included 

 as current growth. For a single even-aged stand, or a forest devoid 

 of younger age classes, this premise does not hold good, and the current 

 growth for the period of early maturity will greatly exceed the real 

 rate for the area or total period. On such stands or forests this rate 

 will not be maintained, and the true yield must be found by dividing 

 by age, in the form of mean annual growth. 



342. Comparative Value of Current Growth versus Yield Tables 

 and Mean Annual Growth. The relative value and utility of the 

 methods of studying the increment on forests or large areas may be 

 summed up as follows: 



Increment or growth is always desired for areas of land rather than 

 individual trees. 



The rate of growth per year on an average acre is the object sought. 



Where forestry is a permanent land policy, the rate of growth desired 

 is that which represents the average for the life of a crop of timber, 

 and which can be maintained, in consequence, indefinitely. 



This rate can be found most accurately whenever growth can be 

 measured directly on the basis of area and total age, as in yield tables 

 for even-aged stands, and applied to the forest by the necessary reduc- 

 tion per cents. 



The current growth on stands or forests is best obtained from these 

 same yield tables. 



But where it is not possible or practicable to construct such yield 

 tables, current growth for short periods only can be measured directly 

 on merchantable trees, and applied in predicting growth of the stand 

 and forest. 



This method gains in accuracy over yield tables, by measuring 



