42 LOCALITY AND FORKST VKOETATION. 



The shelter from al)ove, tlic linmns on the soil, and the 

 roots of the trees together prodiu'e certain efTects, wliich may 

 be summarised as follows :— 



(1.) The temperature of tlic soil and the adjoinhi^ air is 

 lowered during the day and laised during the night ; 

 hence the extremes of temperature are moderated, 

 and the climate rendered more equable. 



(2.) The mean temperature of the soil and the adjoining 

 air is lowered. The reduction is greatest in summer, 

 next in spring, next in autumn, and it is slight in 

 winter ; it is also greater in the soil than in the 

 adjoining air. 



(8.) The relative humidity of the air is increased, the eva- 

 poration from the upper layer of the soil reduced, and 

 precipitation may be increased. 



(4.) A favourable degree of moisture in the upper layer of 

 the soil is maintained.* 



(5.) Noxious forest weeds are kept in check. 



(6.) A steady and suitable progress in tlie decomposition 

 of the humus is ensured, whereby the physical 

 qualities of the soil are improved, or at any rate 

 maintained. 



(7.) An additional supply of organic and nitrogenous 

 matter taken by the plants from the atmosphere and 

 brought into the soil by the falling leaves, flowers, 

 fruits, and twigs, decaying mosses and other plants, 

 is procured. 



(8.) The soil is protected against the mechanical action of 

 water and air currents. 



In order to produce these effects in a high degree, it is 

 necessary that the leaf canop}' should be dense, so that it 

 not only keeps out sun and air currents, but also yields a 

 heavy fall of leaves for the production of humus. Only 



* Iicceiit invest if^iit ions niiulc in Russia and in France seem to indicate, tliat 

 llio level of the i:roiind water in the sod is Inwcied by a itoj) of forest veirctation 

 in luc;ilitic< witli a small or inodei-ate rainfall. 



