LIGHT IN KKI^ATION TO TREE GROWTH. 



195 



three years alter the hrst planting ; otherwise it will be necessary 

 to give up entirely the filling of the blank places, or the older 

 competitors must be removed from small areas and these then 

 replanted. 



These facts show that the so-called " light increment," or in- 

 crease of growth after logging or thinning, is not due alone to 

 the greater access of light to the remaining trees. By thinning 

 a stand, not only are the light conditions changed, but the 

 competition of the roots is diminished, which leads to an 

 increase of moisture in the soil. The leaf litter is also more 

 readily decomposed, and the soil in this way becomes enriched 

 with nutritive substances, all of which result, of course, in an 

 acceleration of growth after thinning. 



4 Soil fertility. — Plenty of chemical, nourishment in and 

 favourable physical conditions of the soil increases tolerance. 

 At the same light intensity the assimilative energy of the green 

 leaf increases with increase of nourishment in the soil. Thus, 

 the assimilative energy of trees grown in a deficient light but 

 on good soil may be the same as that of trees grown in full 

 light but on poor soil ; or, in short, trees on good soils can stand 

 more shade than trees on poor soils. This has been clearly 

 demonstrated by Hartig (1897), who thought that the weight 

 of the young leaf-bearing shoots may, to some degree, serve as 

 an indication of the amount of foliage in a tree. At the same 

 time, the amount of wood produced in a tree for each pound , 

 of small leaf-bearing twigs serves as a criterion of the work 

 of the leaves. Thus, measurements made by him on oaks 

 in the Bavarian Spessart gave the average results shown in 

 Table III. 



Table III. — Atnouut of wood produced annually for each pound 

 of small twigs. 



These figures show that the leaves of young trees function 

 with greater energy than the leaves of older trees. The same 



