CHAP, x.] Stimulation of Increment 215 



But in the case of the shade-bearing genera, Beech, Spruce, 

 and Silver Fir, which (except in cases of accident) remain in 

 dense canopy throughout their whole period of growth, right 

 up to their normal maturity, the effects of the freer exposure 

 to light, air, and warmth procured for the individual stems by 

 heavy thinning, or by partial clearance when they approach 

 the prescribed time of fall, are often during the first year 

 marked by a two-fold to ten-fold increase in the breadth of the 

 annual zone of woody-fibrous tissue 1 . This stimulation of 

 increment can also in the case of these -trees be attained with 

 a far less degree of clearance ; hence, with proper care, there 

 need be small danger of the soil being in any way injuriously 

 affected owing to the partial and temporary interruption of the 

 canopy to a slight degree. This fact is easily explained by the 

 smaller absolute measure of light and air available for these 

 shade-bearing species when grown in canopy of normal density. 



Thus, whilst the laws regulating the increment in all species 

 of forest trees are substantially constant, the extent to which 

 the enhancement of increment may take place after partial 

 clearance has, so far as observations have yet been recorded, 

 been found to be practically in the inverse ratio to the require- 

 ments of any particular species with regard to light and 

 freedom of crown. 



The Causes of the Enhanced Increment explained. 



Various causes have been assigned to the effects produced 

 in enhanced increment after partial clearance 2 . Th. Hartig 

 considered it to be due rather to the utilization of reserves of 

 productive matter collected and stored up in the stem whilst it 

 stood in close canopy, than to any increased assimilation in 

 direct consequence of the increase in foliage that takes place 

 when the individual tree obtains a larger growing-space; 



1 Grasmann, Beitrag zur Lehre vom Lichtungszuwachs, 1890, p. 9. 



2 Idem p. 4. 



