METHODS OF MAKING THINNINGS 183 



increase of 40 to 60 per cent in the length of the rotation 

 should be expected. 



2. Reduction of the growth of the stand. The removal of 

 the thriftiest dominant trees leaves large openings which are 

 supposedly filled by rapid expansion of trees of inferior crown 

 classes which remain. Only where the species are extremely 

 shade enduring, the site good and the thinnings kept light 

 will this assumption hold good. In the average stand re- 

 peated use of Borggreve's method will reduce the growth and 

 eventually leave a body of trees, formerly in overtopped and 

 intermediate crown classes, which are suffering from exposure 

 to sun, wind and insects and fail to conserve the factors of 

 the site. 



3. There is even more difficulty experienced than in the 

 French method in felling the large trees and getting the mate- 

 rial from the area without injury to or hinderance from the 

 subordinate stand. 



Selection thinnings should not be started until good height 

 growth has been attained and natural pruning is far advanced. 

 The best results from such cuttings will be obtained when 

 they are not initiated until between the fiftieth and eightieth 

 years and are used with rotations of 125 to 200 years for the 

 production of high quality timber. Before the Borggreve 

 cuttings are begun, light thinnings as needed under the ordi- 

 nary method may be employed. The disadvantages are 

 largely overcome if a late initiation of the method is combined 

 with early thinnings from below and the employment of a 

 long rotation. 



Special Methods of Thinning. Occasionally stands in 

 great need of a thinning are found of such density as to make 

 the removal of individual trees in a thinning operation very 

 difficult and expensive. To overcome this special methods 

 can be employed. Young stands, 10 to 20 years old, of un- 



