310 FoREbTRY INVESTIGATIONS U. h. DEPARTMENT OF AGRICULTURE. 



under and within the crowns, but makes little difference above them. The surface of the forest 

 crown appears meteorologically much, like the surface of the meadow or cornfield. It is as if the 

 soil surface has been raised to the height of the trees. (Pp. 09-72, Bui. 7.) 



(5) Air temperature in general,— From the preceding generalizations it appears that the forest 

 affects the temperature just as any collection of inorganic obstacles to sunshine and wind; but as 

 an organic being the forest may be also an independent source of heat. Careful observations of 

 the temperature within the trunk of the tree and of the leaves of the tree show that the tree 

 temperature is affected somewhat by the fact that the water rising brings up the temperature of 

 the roots, while the food material from the leaves brings their temperature down, and the tree 

 temperature, considered as the result of the complex adjustment, is not appreciably affected by 

 any heat that may be evolved by the chemical processes on which its growth depends. It is not 

 yet cleai as to whether 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. (Pp. 73-95, Bui. 7.) 



(0) 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 

 difference 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 woods is 

 about 44 per cent of that in the field. Fully half of the field evaporation is saved by the presence 

 of the torest. (P. 90, Bui. 7.) 



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 decidedly 

 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. (Pp. 77-80, Bui. 7.) 



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. (P. 98, Bui. 7.) 



The evaporation from a saturated bare soil in the forest is about the same as that from a water 

 surface in the forest, other conditions being the same. (P. 99, Bui. 7.) 



The presence of forest litter like that lying naturally m undistuibed forests hinders the 

 evaporation from the soil to a remarkable extent, since it saves seven eighths of what would 

 otherwise be lost. (P. 100, Bui. 7.) 



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 lespect 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. ( P. 101, Bui. 7.) 



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 1 per cent Forests of evergreens have from two to four times the influence in increasing 

 relative humidity than do forests of deciduous trees. (Pp. 102-105, Bui. 7.) 



The gauges in European forest stations catch from 75 to 85 per cent when placed under the 

 tiees, the balance representing that which passes through the foliage and drips to the ground or 

 runs down along the trunks of tiees, or else is intercepted and evaporated. The percentage 

 withheld by the trees, and which either evaporates from their surface or trickles along the trunk 

 to the ground, is somewhat greater in the leafy season, though the difference is not great. 

 Deciduous and evergreen trees show but slight differences in this respect. More rain is usually 

 caught by gauges at a given height above the forest crown than at the same height in open fields, 

 but it still remains doubtful whether the rainfall itself is really larger over the forests, since the 

 recorded catch of the rain gauge still requires a correction for the influence of the force of the 

 wind at the gauge. (Pp. 106-110, Bui. 7). 



In such cases, where over & large area deforestation and reforestation have seemingly gone 



