144 Studies in Forestry [CHAP. vn. 



formation of pure forests, and amounts, in other words, to the 

 statement that the capital value of the woodland soil, i. e. its 

 capacity for yielding interest or returns in timber under proper 

 treatment, must not be tampered with, risked, or diminished by 

 injudicious management. 



3. Throughout the whole period of rotation each species must 

 find the due amount of growing-space and of exposure to light, air, 

 and warmth suited to its requirements. This condition is not 

 at all confined to the development of the crown, but applies 

 equally to the extension of the root-system ; and it becomes of 

 all the more importance the nearer the individual species of 

 trees approach to their physical maturity. 



Proceeding from the general considerations that the possi- 

 bility of forming mixed crops of various species is de- 

 pendent 



i. On their individual capability of protecting or improving 

 the productive capacity of the soil, 



ii. On their relative demands with regard to light, and 



iii. On their relative rate of growth in height, 

 the following rules for the formation of mixed forests were 

 formulated by Heyer * : 



1 . The principal or ruling species must be capable of improving 

 the soil. That is to say, the matrix should consist of genera 

 and species like Beech, Hornbeam, Spruces, Silver Firs, 

 Douglas Fir, and Scots or Black Pines. 



2. Shade-bearing species may be grown in admixture with each 

 other when their rate of growth is about equal, or when the 

 slower-growing species is protected against the quicker-growing. 

 The protective measure adoptable in the latter case consists 

 of giving some start at first to the slower-growing species, of 

 allowing it to form the great majority of the crop, or of favour- 

 ing it at the time of natural reproduction, and later on by 



1 Op. cit., pp. 45-57. 



