EFFECT OF FORESTS ON MOISTURE. IS 



to 2-21 degrees. It has also been found that frost penetrates less 

 deep into forest soil than into soil in the open. 



(c) The Temperature of Trees. — A point of interest is that the 

 temperature of living trees differs from that of the surrounding air. 

 It is lower by about 2 degrees near the foot of the tree, and by 

 about 1^ degrees in the crown, the actual amount depending on 

 the species. 



The data given as regards temperature seem to justify the 

 following conclusions : — 



(1) The climate of forest countries is more equable than that 



of open countries. 



(2) The mean temperature of air and soil in forest countries is 



lower than that in open countries. 



(3) Vegetation awakens later in spring in well stocked forests 



than in the open country. 



(4) Plants growing under the shelter of a forest crop are less 



hable to suffer from late and early frosts, or from drought, 

 than those growing in the open. 



2. Effect of Forests on Moisture. 



Air can hold only a certain maximum quantity of vapour, 

 which varies with the temperature. When the maximum has 

 been reached and more vapour is introduced, a part becomes 

 condensed. The absolute vapour in the air is measured by its 

 tension upon a column of mercury, as in a barometer. The 

 proportion of the absolute to the maximum tension of vapour, at 

 a certain temperature and pressure, is called the relative humidity 

 of the air. Although the humidity of the air depends in the first 

 place on the general distribution of heat and air pressure over 

 the earth, the vegetation on it also affects it locally. That 

 effect may be felt in the degree of humidity of the air, the amount 

 of precipitation, the degree of evaporation, and in the feeding of 

 springs and rivers. 



(a) Humidity of the Air. — The observations so far available show 

 that forests have little or no effect upon the absolute humidity of 

 the air ; at any rate the effect is negligible. It is different with 

 the relative humidity. As the temperature of forest air is lower 

 than that of air in the open while the absolute humidity remains 

 the same, it follows that the relative humiditv of forest air must 



