54 BULLETIN 544, U. S. DEPARTMENT OF AGRICULTURE. 



would bo prohibitive. The remaining trees would be particularly 

 liable to windthrow, and the thinning would produce doubtful results 

 in securing an accelerated growth. With the attainment of their 

 principal height growth, the trees composing a crowded stand have 

 adjusted themselves through the abnormal reduction of their crown 

 surface to the restricted growing space, and the possibilities of stimu- 

 lating the growth in such a stand are not promising, unless the stand 

 is to be left intact for upwards of 30 years afterward. 



The principle to be followed in making thinnings depends upon the 

 object of management and the age when thinnings can be begun. If 

 it be desired to secure the maximum volume production, which cul- 

 minates, according to Table 14, between the thirty-fifth and seventieth 

 year, depending on the unit of volume and the site quality, thinnings 

 should be undertaken by the twenty-fifth or thirtieth year on the 

 best sites and be confined to the dominant and intermediate trees. 

 If, however, quality production is the object desired, the first thin- 

 nings may be delayed until the thirty-fifth or fortieth year, being 

 confined to the lower and intermediate crown classes until the 

 sixtieth or seventieth year. This would especially encourage the 

 development of form and quality. Thereafter the stand would be 

 thinned more heavily. The last third of the rotation, or approxi- 

 mately after the eightieth or the one hundredth year, the cuttings 

 would be in the form of accretion cuttings which would isolate a 

 certain number of the best trees for the encouragement of a more 

 rapid growth in volume. 



The object of thinnings for volume production should be to free 

 the tallest trees and gradually reduce their numbers without removing 

 the weakest, except where they have a dry top, since they assist 

 natural pruning, cover and protect the soil, and add to the strength 

 of a close canopy. In the early period, when the trees are only to a 

 limited extent merchantable, the fewest number of trees possible 

 should be removed. The greatest advantage, accordingly, can be 

 derived by cutting only those trees which occupy a position in the 

 upper crown level, but which are interfering with the development of 

 the stronger growing individuals. This will relieve the intense strug- 

 gle taking place among those trees which are finally to form the 

 mature stand and will allow them to develop with a minimum of 

 hindrance from their neighbors. To be most effective, thinnings 

 should be frequent and light in early life and heavier and less fre- 

 quent in later years. Such early thinnings, however, involve a con- 

 siderable expense, while the later ones may render the stand liable 

 to windthrow, so that it will often be necossaiy to compromise. 



In judging of the need for thinning, the relation of the crown length 

 to the total height of those trees which are to form the final cut should 



