28 



The volume of an acre of the forest condition represented by the 35 

 acres equals 8,000 cubic feet and 32,000 feet B. M.; multiplied each 

 by 35 equals 280,000 cubic feet and 1,120,000 feet B. M. 



The volume of an acre of the forest condition represented by 25 acres 

 equals 10,000 cubic feet and 40,000 feet B. M. ; multiplying each by 

 25 gives 250,000 cubic feet and 1,000,000 feet B. M. Adding together 

 the volume of these three portions of the forest, we find the growing 

 stock of the forest equals 770,000 cubic feet cr 3,080,000 feet B. M. 



In selecting the sample area care should be taken that — 



(1) The species found in the forest be proportionally represented on 

 the sample area. 



(2) The density of crown cover of the sample area and the percent- 

 age of openings be the same as in the forest. 



(3) All the sizes of timber and the corresponding number of trees of 

 each size be found on the sample area in the same proportion as found 

 among the trees in the whole forest or the portion to which the 

 sample area refers. The selection of the sample area is a delicate 

 operation and therefore requires considerable skill on the part of the 

 estimator. The selected sample area should be staked off in the form 

 of a square, which may contain from one-fourth to 2 acres. 



HOW TO DETERMINE THE RATE OF GROWTH. 



As the knowledge of the contents of a forest growth is necessary in 

 order to determine its present value for purposes of sale or purchase, 

 so the knowledge of the rate at which its contents are changing, 

 increasing or decreasing is of the highest importance in determining 

 the profitableness of wood growth. The questions whether the annual 

 or periodic increase is sufficient to pay interest on the investment, and 

 whether it is proper to cut and utilize the wood crop or to allow it to 

 grow and accumulate longer, are answered by measuring its rate of 

 growth. 



Just as the contents of a forest or acre or stand is ascertained by 

 means of measuring one or more sample trees, the rate of growth of 

 the stand, acre, or forest for any period may be ascertained from these 

 sample trees. This calculation may concern itself either with the rate 

 at which the height accretion takes place, or the diameter accretion, or 

 the volume or mass accretion. It may also be made with reference to 

 a longer or shorter period. If the period is taken as one year we may 

 call it annual or yearly accretion; if for a number of years, for instance 

 a decade, we may call it periodic accretion. Again, the annual accre- 

 tion may be that of the one year for which we measure, the current 

 annual accretion; or else it may be the average of a number of years, 

 the average annual accretion, which is found by dividing the height, 

 diameter, or volume by the number of years it has taken to grow. For 

 instance, a tree 120 years old, containing 87 cubic feet, would show an 

 average annual accretion of ^i^=0.72 cubic feet, while its current 



