268 Studies in Forestry [CHAP. xn. 



The nearer any soil approaches to dryness, the more neces- 

 sary, and at the same time the more difficult, it is to husband 

 the soil-moisture. It is a fact that very many species of trees, 

 like Oaks, Elms, Maple, Sycamore, and indeed all essentially 

 light-demanding trees, are more intolerant of shade on soils 

 from which they have difficulty in obtaining their requisite 

 supplies of water for transpiration, than on those which yield 

 them an abundance of moisture and food. Hence, on dry soils, 

 there is not only greater necessity for maintaining close canopy 

 overhead in the interest of the productivity of the soil, but also 

 at the same time a much stronger natural tendency on the 

 part of the trees to have thinly-foliaged crowns, and to strive 

 after an increase in growing-space. Under these latter circum- 

 stances the only proper sylvicultural treatment that can be 

 adopted is to cut out and dispose of the kinds of trees that 

 soon attain their maturity (such as Birch, Aspen, Willow, Ash, 

 Maple, Sycamore), thin out individual trees of the nobler 

 genera not likely to develop ultimately into valuable stems, 

 and then underplant the Oak, Larch, or Pine retained with 

 the species of underwood best suited to the given soil and 

 situation. By this means the soil is not only afforded a good 

 mechanical protection against the evaporating and drying effects 

 of sun and wind, but considerable masses of dead foliage are 

 thereby provided for humification, and for the improvement of 

 the soil generally (see last Chapter, page 243). It may here be 

 remarked that the Spruce is less adapted for forming the 

 underwood on dry soils than any of the other shade-bearing 

 species (Beech, Hornbeam ; Silver, Douglas, and Nordmann's 

 Firs), for its shallow roots are apt to interlace and form 

 a network throughout the upper layers of the soil, and to 

 intercept the aqueous precipitations, thus hindering the per- 

 colation of a fair proportion of the water down into the lower 

 soil, whence the deep-seated roots of the standard trees 

 draw by far the greatest portion of their supplies of food and 

 moisture. 



