SILVICS 5 



light may be gauged by the general density of the foliage of the 

 crown and the capacity of overshadowed twigs to retain life. 



Until very recently it was thought by foresters that the amount 

 of light to which a tree has access determines its growth more 

 than any other factor. Recent investigations of soil moisture 

 indicate that this is an even more important factor, so that 

 while it was customary formerly to attribute increased growth, 

 after thinning a forest, to more light, it now appears that it is 

 due as much to the increased supply of water available for the 

 roots. 



The physical qualities of soil especially as related to ability 

 to retain moisture, are more important than the chemical con- 

 stitutents, for almost all soils are chemically able to bear trees. 

 The root systems of various species vary greatly with regard 

 to shape and the depth to which they reach; but even shallow- 

 rooting kinds derive advantage when the soil over which they 

 grow is deep, owing to the greater fertility within easy reach 

 of their roots. Some species, like the oak, have a strong tap 

 root; others, such as beech and birch, develop strong side 

 roots, but no tap root; and still others, like spruce, have a 

 pronounced shallow-root system. Spruce and birch require 

 least depth of soil, oak most. Whether the soil is loose or 

 binding is a matter of great importance for tree growth. As 

 a rule, the broadleaf trees do better than the conifers on the 

 stiff er classes of land, although soils of average tenacity are on 

 the whole most suitable for all kinds of trees. The chief con- 

 stituents of soil are clay, lime, and sand; and as clay yields the 

 most valuable materials for plants, the qualities of soil are often 

 determinable to some extent by the quantity of clay in them. 

 Clay soils are hard and interfere with the movement of moisture; 

 sands are too porous; limes too easily heated; loamy soil, there- 

 fore, is usually best. 



The chief chemical constituents of wood are carbon, hydrogen, 

 oxygen, and nitrogen, but the ashes of wood also contain sulphur, 

 phosphorus, chlorine, silicon, potassium, sodium, calcium, mag- 

 nesium, and iron. The carbon is obtained solely from the car- 



