METHOD OF THINNINGS. 37 



forest at the height attained by the crowns of the 

 more promising trees. Advantage will of course be 

 taken to remove from among the suppressed trees 

 those which cannot survive until the next thinning, 

 but this merely constitutes a simple utilization of 

 produce and has nothing in common with the opera- 

 tion of thinning as a thinning, because it contributes 

 nothing towards the improved growth of the forest, 

 nor ought it to be undertaken at all unless the value 

 of the wood obtained thereby at least covers the cost 

 of exploitation. 



A thinning is said to be moderate when the crop 

 is opened out only to a moderate extent : it is severe 

 when the crop is opened out more widely, without 

 however going so far as to break the continuity of 

 the leaf-canopy. 



For whatever be the nature of the thinning, the 

 leaf canopy must always be preserved. The reasons 

 for this are simple, and depend on the age of the 

 forest. In the first place, when the crop has reached 

 the stage of low poles, the struggle for existence is 

 only just beginning to be injurious ; the full height 

 of bole is far from being attained, and if the crowns 

 are now isolated, the lengthening of the bole by 

 natural pruning is stopped; even when the leaf- 

 canopy closes up again overhead, the lower branches, 

 having had time to attain a strong development, 

 leave behind them on falling off large knots which 

 depreciate the value of the timber ; the trees having 

 as yet a small girth in comparison to their height are 

 liable to bend down and grow crooked ; the ground 

 is incompletely protected and becomes hard and 



