EFFECT OF SOIL ON FOREST VEGETATION. 35 



(2.) A suitable degree of porosity. 

 (3.) A suitable degree of moisture. 

 (4.) A suitable chemical composition. 



1. Siirjirieiif Depth. 



The depth is measured by the thickness of the layer of 

 soil, and of that portion of the subsoil which can be pene- 

 trated by the roots. In due proportion to depth are the space 

 available for the root system, the store of nourishing sub- 

 stances, the stability of the trees, and the state of moisture 

 in the soil. 



The root sj'stem differs considerably in the several species ; 

 some develop a tap root, which is maintained for a shorter or 

 longer period, such as oak, elm, Scotch pine, silver fir, maple, 

 sycamore, ash, lime, larch ; others have strong side roots, 

 which send down deep going rootlets, such as alder ; others 

 again go to a moderate depth, as beech, hornbeam, aspen and 

 birch ; finally some spread altogether near the surface, such 

 as spruce. The nature, composition and degree of moisture 

 of the soil modify the root system to some extent, which in 

 young trees frequently differs from that in older trees. 



On the whole, certain species thrive well only in deep soil, 

 while others can subsist in shallow soil, though they prefer 

 the former. The best indicator of the depth of soil is the 

 height growth. A sufficient depth produces full height 

 growth ; with deficiency of depth the height growth falls oft". 



Hess* classifies the trees as follows, in respect of their 

 demands for depth of soil : — 



Species wkicli are satisfied icitli shalloic soils : Spruce, 

 mountain pine, birch, aspen, mountain ash. 



Species ichich require moderate depth : Austrian pine, Wey- 

 mouth pine, beech, hornbeam, black poplar, tree willows, 

 alder, horse chestnut. 



* Encyklopadie imd Methodologiu der Foist wisseuschaft, by Dr. Kichard 

 Hess, 1«8S. 



D 2 



