34 



divided by 5 gives 0.024 as the current area accretion for one year ; di- 

 viding this one-year growth by 2.64 and multiplying the result by 100, 

 we find that 6 is the per cent of area, and thus of volume accretion of 

 that particular tree of 22J inches in diameter at breast height. The 

 per cent of volume accretion, as has been seen, may be expressed by a 

 fraction, the numerator of which is the volume increase for one year, 

 and the denominator is the volume of the tree i^revious to that year. 



The difterence between the successive current accretions, though 

 increasing with age, are small in proportion to the increase of the 

 respective volumes, which are always enlarged by one year's growth. 

 In other words, the fraction or the per cent of accretion it represents 

 decreases steadily with age. 



Pressler gives a simple formula (-f^) which expresses the per cent of 

 accretion of the tree when it has reached its maximum stage of growth. 

 "A"' is the age of the tree when it reaches the stage of maximum 

 growth, i. e., when the current accretion becomes equal to the average 

 annual accretion. If the per cent of accretion of the tree obtained 

 from calculations is larger than -'^-. it shows that the average annual 

 accretion still inc eases; when it is less than -£-, the average annual 

 accretion is on the decrease. 



MASS ACCRETIOX WITH COMPOUXD IXTEREST. 



In determining the mass accretion with compound interest the gen- 

 eral formula of compound interest could be applied. To avoid calcula- 

 tions by logarithms Pressler gives a formula of his own, and a table of 

 figures based on it, the practical application of which is very simple : 

 Measure the diameter of the standing tree at breast height; extract by 

 Pressler's borer a cylinder of wood and measure off the width of the last 

 n years {n designates the number of years in the period for which the 

 calculation is to be made, generally five or ten years); then divide the 

 diameter by double the width of the last wrings, and the so called rela- 

 tive diameter is established. Finding then the relative diameter, thus 

 obtained, in the column of the relative diameters (Pressler's table, p. 40); 

 the corresponding number, given in the same line with the relative 

 diameter, should be divided by ?«, and the quotient will be the per cent 

 of accretion with compound interest. 



Example: Let us take the same tree for which the per cent of accre- 

 tion was determined with simple interest ; its present diameter at bi east 

 height is 22^^ inches: the width for the last five years is two-eighths of 

 an inch. Dividing 22i by double the width of the last 5 rings, we find 

 the relative diameter equals 45: (22^-^^=45). In Pressler's table we 

 find that 6.7 corresponds to the relative diameter of 45 Vrhen the tree is 

 of a very tlirifty growth. Dividing 6.7 by 5 we find the current annual 

 growth equals 1. > per cent with compound interest. 



In Pressler's table on page 40. for each relative diameter two figures 

 are given — one for an average thritty growing tree, the other for a very 



