212 A MANUAL OF FOEESTKY. 



3. The Theory of Thinning. 



"Whatever the objects of management may be, the 

 theory of thinning may be summarised in the answers 

 to the following three questions : 



(1.) At what age of a wood should thinnings com- 

 mence ? 



(2.) To what trees should they extend in other words, 

 how heavy should the cuttings be ? 



(3.) After what intervals should they be repeated ? 



Definite answers to these questions can only be given 

 on the basis of accurate comparative statistics. The col- 

 lection of such data is now in active progress in Europe, 

 but some time must necessarily elapse before final con- 

 clusions can be drawn from them ; in the meantime, only 

 the results of general observations are available for 

 guidance. 



It has been explained above that during the struggle 

 for existence, four different classes of trees are produced, 

 namely, dominant, dominated, suppressed, and dead 

 trees ; and the question arises in how far each of these 

 should be interfered with at each thinning. It is 

 obvious that the dead trees must be removed at every 

 thinning, as they cannot influence the other three classes J 

 while their presence in the wood is a constant source of 

 danger from insects and fungi, and in some cases from fire. 

 Except in some special cases, the suppressed trees also 

 should be removed, but the extent of interference with 

 the dominated and dominant trees depends, apart from 

 species, chiefly on the character of the locality and the 

 objects of management. On fertile soils an interruption 

 of the cover overhead is of comparatively small impor- 

 tance, but on inferior and even middling soils this 



