IMPROVEMENT OF THE FOREST 223 



through their removal would be large. When possible, 

 the good trees are retained, either as reserves or as an 

 upper-story, or brought to merchantable size as an in- 

 complete stand. 



Pruning. 



The pruning of forest trees is confined chiefly to cut- 

 ting off the lower branches, in order to have as much of 

 the stem as possible clear of knots, and so raise the grade 

 of the lumber produced. As a rule, forest trees are not 

 pruned at all, but there is a natural cleaning of the stem, 

 called natural pruning, by the loss of the lower branches 

 as a result of the crowding of the trees. Often, however, 

 natural pruning fails to give satisfactory results. With 

 many species the branches remain on the tree long after 

 they die, and their stubs produce loose knots in the lum- 

 ber. The branches of white pine, even in crowded stands, 

 persist for many years after their death; but after a time 

 they drop off, and the wood subsequently made yields 

 lumber clear of knots. Very old white pine-trees, there- 

 fore, yield a good proportion of clear lumber if they have 

 grown in crowded stands. But if this species is managed 

 on a short rotation very little clear material is ordinarily 

 obtained, unless the trees are pruned. An unpruned 

 stand, cut at from 50 to 60 years of age, yields but little 

 clear lumber; whereas, if the trees had been pruned when 

 from 20 to 30 years old the wood produced during the 

 last half of the rotation would have been free from knots. 



Pruning is a relatively expensive operation. It in- 



