98 NO TES ON FORES TR Y. 



(80-100 years), we should probably find that the 

 proportion in which they would stand to the acre 

 at all periods would be somewhat as follows : 



At 1st period, 1- 20 years, 800 

 2d 20- 40 400 

 3d 40- 60 200 

 4th 60- 80 iOO 

 5th 80-100 50 



And in a forest worked by selection on a hundred 

 years' rotation, we should require an average of 

 approximately one-fifth of these numbers for each 

 period to the acre. 



If there are no blocks of trees of uniform ages 

 at hand to afford data for fixing the number 

 which an acre will carry, we arrive at it easily by 

 measuring the spread of the branches of a typical 

 tree of each period, and dividing the square of the 

 spread in feet into 43,560, the number of square 

 feet in an acre. For instance, if the spread of the 

 branches were 30 feet, then 43,560 -r- 30 2 = 48, the. 

 number of trees of that spread which would stand 

 on the acre without undue crowding. 



The average increment of such a forest is approxi- 

 mately ascertainable, and the timber to be felled 

 annually should be limited to such annual incre- 

 ment ; for if trees of the older periods are in excess, 

 and we keep cutting out all the mature timber, we 



