162 ECONOMICS OF FORESTRY. 



found on good sites between the twentieth and for- 

 tieth year, with over 160 cubic feet per acre, and 

 on poorer sites a decade later; while the slow- 

 growing beech shows its culmination between the 

 fiftieth and seventieth year, with 190 cubic feet 

 per acre. 



In general, the volume of a stand progresses 

 much more slowly than that of a single tree, and 

 much more regularly, since it expresses all the 

 variable conditions. It is a matter of simple 

 mathematical demonstration that the maximum 

 average accretion occurs when it is equal to the 

 current accretion, i.e. equal to the accretion of the 

 particular year. In other words, when the accre- 

 tion which has occurred through a series of years, 

 divided by the number of years, happens to be as 

 large as the accretion of the current year, the high- 

 est average production per acre and year has been 

 attained. This occurs mostly before the fiftieth 

 year with light-needing species and on good sites, 

 later on poor sites and with shade-enduring species, 

 but, to be sure, the value accretion, which depends 

 upon the amount of large-sized material, culminates 

 very much later. 



If a group of some hundred trees have grown 

 together in dense stand, they develop so regularly 

 and interdependently that the following relations 

 will prevail : the contents of the average tree will 

 be found to equal very nearly one-tenth of the vol- 

 ume of the three stoutest and the seven slimmest 



