PRINCIPLES OF FOREST ECONOMY. 301 



on which the annual growth takes place (the wood capital), just as in the. herd a certain number of 

 cows and bulls and heifers of various ages must be kept to secure a continuous supply of cattle, 

 and a tolerably uniform revenue on the investment. 



In order to be able to determine what this wood capital is to be and how much the yield or 

 revenue that can be expected the manager must have knowledge of the manner and rapidity with 

 which the, crop develops. 



It is not, necessary to go into details of the methods developed to ascertain the amount of 

 wood growing per acre at different ages, or how to determine the rate of g owth and the, quanti- 

 tative as well as qualitative accretion. It will, however, be needful to indicate briefly what in 

 general the results of such measurements would be in order to get an insight as to how these will 

 influence the methods of management. 



While individual trees of the same species may develop very differently aiul seemingly without 

 law, when we deal with larger numbers under forest conditions we may more readily discern that 

 a more or less precise law and rate of growth can be established for each species and condition. 

 Of course different soil and climatic conditions and the character of the site influence the rate of 

 development of forest growth, yet on all sites the relative rate at various periods remains more or 

 less constant. 



Thus for a given species and site we will be able to discern after a brief seeding stage a 

 juvenile stage, when trees develop in height growth at the expense of diameter growth; an 

 adolescent stage, when height growth decreases and diameter growth accelerates, and a mature 

 stage, when height growth practically ceases and diameter growth, although persisting, declines. 

 The growth in volume progresses differently because the very wide rings .or layers which are laid 

 on in early life, and which denote rapid diameter growth, cover only a small circumference, while 

 the much narrower ring of a later period laid oil over a much thicker stem represents a much 

 larger volume. 



Thus the rate of growth in white pine decreases in height and thickness practically from the 

 polewood stage forward, while the rate of growth in volume increases up to the sixtieth or eightieth 

 year, and then continues uniformly for a century or more before it declines. 



Or to illustrate in figures, a white pine seedling only 1 foot high and one-half inch in 

 diameter, with hardly an appreciable volume of stem, will have reached a height of 30 feet in 

 twenty years, (iO feet in forty years, 100 feet in one hundred years ; the width of the rings will have 

 averaged one-eighth to one-sixth inch during the first thirty years, while at one hundred years the 

 average will have come down to one twelfth inch; but the volume growth, which during the first 

 thirty years was but a fraction of a cubic foot, has after sixty years attained a rate of 1 to 2 cubic 

 feet per year, and is kept at that rate to a great age two hundred and fifty to three hundred 

 years. 



If we substitute the red or Norway pine we will find the progress quite different. It may start 

 out at about the same rate as the white pine, and at sixty years may also have attained a rate 

 of 2 cubic feet per year, but soon the rate begins to decline, and in the one hundred and twentieth 

 year with a volume of 80 cubic feet the average accretion is only two thirds cubic foot per year. 

 Its average growth for the one hundred and twenty years has now become equal to the current 

 rate of growth. 



The tree then passes its maximum capacity of wood production, for from this time on its 

 current growth falls behind its average, and from the standpoint of quantitative production the 

 tree should now be cut. 



But there is a growth in value which does not progress continuously and proportionately 

 with the growth in volume, and which is also an important factor in deciding when a tree is 

 to be cut. 



Generally in all lumber and timber markets the prices are classified, and sticks, boards, etc., 

 are priced according to size as well as freedom from defects and knots. For instance, poplar logs 

 under 12 inches may have no price at all, logs of 10 to 20 inches may bring $15, those of 20 to 29 

 inches may bring $20, and if over 30 inches $25 may be paid per 1,000 feet B. M. contained in the 

 log. Hence, although the quantitative development may have decreased in the log of 29 inches, 

 it may still pay to hold it over until the better-paying size is attained. 



