12,2 Studies in Forestry [CHAP. vi. 



its main supplies of nutriment from a different layer of soil 

 although long scoffed at by a certain class of sylviculturists 

 (headed by Hundeshagen), who saw the advisability of admix- 

 ture explained solely by the relation of the various species 

 towards light and shade is based on a principle equally sound 

 from the theoretical and the practical points of view ; for it is 

 a fact that, in the majority of mixtures acknowledged to be good, 

 the root-systems of the different species vary considerably in 

 depth. 



That, in addition to greater density of growth, which has 

 already been referred to in the previous section, the productive 

 capacity of the soil is at the same time more thoroughly utilized 

 can be proved, if requisite, by actual crop measurements made 

 in Silesia in 1880. These showed that, on soil similar as 

 to conditions and quality, the average annual increment in 

 eighty-year-old crops was : 



Cubic ft. per acre. 



Pure forest of Scots Pine 18-3 



Pure forest of Spruce 19-9 



Mixed forest of Scots Pine, Spruce, and Silver Fir . 23-5 



But, beneficial though the influence of the overshadowing of 

 the soil by the leaf-canopy undoubtedly be, there are occasions 

 on which even it may perhaps be carried farther than is de- 

 sirable or advantageous ; as, for example, in dense woods of 

 pure Spruce or Silver Fir, where the close, thickly-foliaged 

 canopy overhead, and the deep sponge-like growth of moss 

 covering the soil, intercept and retain a very much larger pro- 

 portion of the atmospheric precipitations, both in winter and 

 in summer, than deciduous trees or other coniferous species 

 (Pines) with somewhat thinner growth. For whilst, of all the 

 precipitations in the course of the year, on the average about 

 25 % are intercepted by the foliage and branches, and 75 % 

 reach the soil in woodlands, these data vary considerably 

 according to the kind of tree forming the crop. The averages 



