16 FOREST INFLUENCES. 



cal processes on wliicli its growth depends. It is not yet clear as to 

 A\^hetlier the chemical changes that take place at the surface of the leaves 

 should give out any heat ; it is more likely that heat is absorbed, namely 

 rendered latent, especially in the formation of the seed; the process of 

 germination usually evolves this latent heat; the immense quantity of 

 water transpired and evaporated by the forests tends to keep the leaves 

 at the same temperature as that of the surface of water or moist soil. 



(6) Humidity of air. — The annual evaporation within the forests is 

 about one-half of that in the open field; not only is the evaporation 

 within a forest greatest in May and June, but the diiference between 

 this and the evaporation in the open field is also then a maximum, 

 which is the saving due to the presence of the woods. The average 

 annual evaporation within the Avoods is about 44 per cent of that in the 

 field. Fully half of the field evai^oration is. saved by the presence of 

 the forest. 



The quantity of moisture thrown into the air by transpiration from 

 the leaves in the forest is sometimes three times that from a horizontal 

 water surface of the same extent, and at other times it is less than that 

 of the water. The transpiration from leaves in full sunshine is deci- 

 dedly greater than from leaves in the diffused daylight or darkness. 

 The absolute amount of annual transpiration as observed in forests 

 of mature oaks and beeches in central Europe is about one- quarter of 

 the total annual precipitation. 



The percentage of rainfall, evaporated at the surface of the ground, 

 is about 40 per cent for the whole year in the open field and about 12 

 per cent for the forest, and is greater under deciduous than under 

 evergreen forests. 



The evaporation from a saturated bare soil in the forest is about the 

 same as that irom a water surface in the forest, other conditions being 

 the same. 



The presence of forest litter like that lying naturally in undisturbed 

 forests hinders the evaporation from the soil to a remarkable extent, 

 since it saves seven- eights of what would otherwise be lost. 



The total quantity of moisture returned into the atmosphere from a 

 forest by transpiration and evaporation from the trees and the soil is 

 about 75 per cent of the precipitation. For other forms of vegetation it 

 is about the same or sometimes larger, varying between 70 per cent 

 and 90 per cent; in this respect the forest is surpassed by the cereals 

 and grasses, while, on the other hand, the evaporation from a bare 

 soil is scarcely 30 per cent of the precipitation. 



The absolute humidity within a forest exceeds that of the glades 

 and the plains by a small quantity. The relative humidity in the 

 forest is also larger than in the glades or plains by 2 per cent to 4 per 

 cent. Forests of evergreens have from two to four times the influence 

 in increasing relative humidity than, do forests of deciduous trees. 



The gauges in European forest stations catch from 75 to 85 per cent 



