196 Studies in Forestry [CHAP. ix. 



of the crop, are most favourably kept in view when the degree 

 to which thinning is carried out, at each time of repeating the 

 operation, is only moderate although on soils of better than 

 a merely average quality it may well be somewhat freer than on 

 those of a poorer nature. 



3. Partial Clearances in Tree-forest. 



Partial clearances are made in tree-forest when over 15 % 

 of the basal area is removed for the express purpose of affording 

 freer supplies of light and air, so as to stimulate the rate of 

 increment, and thus enhance the technical and the financial 

 value of the timber. 



This sylvicultural operation, which is merely a more vigorous 

 and emphasized expression of the theory of thinning, may be 

 carried out after once the timber crop has passed through the 

 most active period of growth in height ; for then the advantages, 

 offered by increment in girth and in full-woodedness towards 

 the top-end of the bole, are not, to anything like the same 

 degree as during any earlier stage of development, prejudiced 

 by such increment being perhaps stimulated at the expense 

 of the rate of growth in height. The removal of up to 15 % 

 of the total basal area, when the crop stands in full canopy, 

 is still a thinning; whereas the removal of 18 or 20 % is 

 already a partial clearance. But the latter operation is practi- 

 cally merely a continuation of the former ; only it is carried out 

 with a freer hand. 



Whilst, in thinnings, the conservation of the productive 

 capacity of the soil is one of the most important objects to 

 receive consideration, in a partial clearance its maintenance, by 

 means of the tree-crop alone, is seldom attainable except on 

 soils of the very best quality ; hence underplanting is usually 

 necessary, simultaneously with the partial clearance being 

 made *. Throughout the whole of the operations of tending 



1 A fuller consideration will be given to this subject in Chapter XI, 

 On Underplanting. 



