IMPROVEMENT OF THE FOREST 207 



would mean the protection of the valuable, trees against 

 the crowding of less valuable ones, and the stimulation of 

 growth in a stand which is stagnating from overcrowd- 

 ing, an investment in thinnings is warranted. 



The owner of a forest is interested in knowing 

 whether there is not a critical period before which thin- 

 nings should be made if they are to be of any real benefit. 

 There is such a critical period, which differs with differ- 

 ent species and with different conditions, and which 

 depends also on the object of management. The forest 

 should be thinned before the crowns of the dominant 

 trees are so far reduced that the trees cannot respond 

 to the improved conditions and accomplish the objects of 

 management. If, for example, the purpose is to produce 

 a maximum of large trees and a large volume of timber 

 measured in board feet, the thinnings must be begun 

 while the crown is relatively long. If the crown is per- 

 mitted to become very small and short, occupying only 

 10 or 15 per cent, of the stem, the trees are not capable 

 of much benefit from thinnings. In managing white 

 pine for the production of box boards, the aim should be 

 to begin the thinnings before the crowns of the dominant 

 trees are on an average reduced below 40 per cent, of the 

 total height of the trees. If for convenience the ratio be- 

 tween the crown length and that of the full stem is called 

 the crown-ratio, the critical period in white pine managed 

 for box boards is when further postponement of thinnings 

 would be likely to reduce the crown-ratio to less than 40 

 per cent. 



