P^FFECT OF FOKEST VEGETATION ON THE LOCALITY. 41 



Any classification like that given above must, however, be 

 received with some caution ; its object is merely to give a 

 general idea of the subject. In practice deviations occur 

 constantly, according to the local conditions under which the 

 trees grow. 



Section V. — Effect of Forest Vegetation on the 

 Locality. 



It has been shown in the previous sections that the 

 condition of the locality governs the growth of forest trees. 

 It is now necessary to consider the effect of forest vegetation 

 on the locality, more especially on two of its factors — 

 humus and moisture. This effect is chiefly produced by the 

 following two agencies : — 



(1.) The protection which growing woods afford to the soil 

 and adjoining layers of air. 



(2.) The humus which is formed by the fall of the leaves, 

 branches, twigs, flowers, fruits, etc., and by certain 

 plants growing under the shelter of the trees. 



In well stocked, or crowded, woods the crowns of the trees 

 form a thick leaf canopy, or complete cover overhead. If 

 the trees are all of the same age and height, the leaf canopy 

 is at a uniform height above the ground, that height being 

 at first small, but increasing with age. In woods of uneven 

 age the cover is of a somewhat different nature ; it consists 

 of groups of crowns at varying distances from the ground. 

 In either case the cover overhead protects the soil and 

 adjoining layers of air against sun and wind ; in even aged 

 woods more against sun, and in uneven aged woods more 

 against wind. 



Again, the trees shed their leaves, flowers, fruits, and even 

 branchlets, while mosses and other plants, which thrive 

 under the shelter of the leaf canopy, die ; thus a layer of 

 humus on the soil is formed. Finally the roots of the trees 

 penetrate into the soil and keep it together. 



