142 North American Forests and Forestry 



periods of the life of a tree, is fairly constant among 

 trees of the same species grown under similar con- 

 ditions. It is possible, therefore, to ascertain by 

 taking the average of a very large number of trees 

 felled and examined, to calculate the probable in- 

 crease of wood which other trees, still growing, will 

 show at any given period of their future lives. Ta- 

 bles furnishing such information are known as yield 

 tables, and have been, in Europe, constructed with 

 great care, for all trees of commercial importance 

 in that country. As far as is known to the author, 

 as yet no such tables have been constructed and 

 published regarding American trees, except a set, 

 prepared by Messrs. Pinchot and Graves, for white 

 pine grown in the mountains of Pennsylvania, and 

 another for spruce in the Adirondacks. 



These are some of the considerations growing 

 out of the botanical nature of trees and required for 

 determining when a forest is ripe for the axe. Clear- 

 ly you must not fell the trees while they are still so 

 young as to furnish little and inferior lumber ; but 

 while you may know, in this way, the lower limit of 

 felling, you have not yet sufficient data to find the 

 upper limit. You cannot tell whether you would 

 most profitably cut your forest at a hundred or two 

 hundred years of age, for it is clear that, barring 

 accidents, the longer you wait the more lumber you 

 will harvest. The upper limit, however, is set by 

 financial calculations in the following manner : 



It costs something to keep your forest untouched, 

 and yielding very little revenue, year after year. 



