32 IMPORTANT TIMBER TREES 



experience shows that a departure from that mean almost 

 invariably occurs on the side of an absence of a sufficient 

 supply, and this brings us to a consideration of an import- 

 ant feature connected with the growth of timber trees. 



The Forest Floor. When the ground is covered with a 

 growth of trees, the twigs, leaves, branches, and dead trees 

 which fall and decay produce, in time, a covering of a spongy 

 character that not only is capable of itself retaining water 

 but prevents its rapid run-off, thus giving it time to perco- 

 late into the earth, which is always looser and more porous 

 on account of such covering. From there it is absorbed, in 

 part, by the roots of the trees and sent to the leaves, as 

 elsewhere shown, and the supply of water is longer retained 

 than when the naked mineral soil is exposed, the greater 

 part, however, entering the porous soil and supplying 

 springs and streams with a gentle flow, and thus in a 

 large measure preventing excessive floods on the one hand 

 and dried-up springs and stream-beds on the other. While 

 some of the water is evaporated from the surface, that pro- 

 cess goes on slowly where the ground is largely shielded 

 from the sun and wind. This moisture-holding, spongy mass 

 of decaying leaves and wood which covers the ground on 

 which the trees grow is known as the Forest Floor, and its 

 proper maintenance is of great importance. To produce 

 and keep it satisfactorily the trees in the forest must stand 

 close enough to shade the ground completely, and there 

 must be enough decaying leaves and wood to provide an 

 ample thickness. 



This decaying matter forms the well-known humus, that 

 most valuable constituent of the soil which converts the 

 disintegrated and decomposed rocks of the earth from com- 

 parative barrenness into fertility. Mingled with the mineral 

 earth, it forms the fertile soil. By its accumulation the 

 ground is constantly enriched, and the forests thus pay an 

 annual rental for the ground they occupy. In cultivated 

 fields there is no such accumulation, and instead of a con- 

 stant enrichment there is a continued drain upon its fer- 



