34 



for its requirements. *It is possible also that a portion of the moisture given out by the 

 leaves is restituted to the soil otherwise than as rain, and that, just as in a Wardian case, 

 a limited quantity of moisture is taken up again and again. The water that is exhaled 

 by foliage cannot all be lost by evaporation into the atmosphere, otherwise the resulting 

 fall in temperature, in forests, would be greater than that observed. Moisture may be 

 continually streaming down from the boughs to be absorbed in part by the hygroscopic 

 soil of forests, and it will be seen that the distribution of temperature and the air circula- 

 lation are such as to cause this effect.** 



According to Ebermayer, the mean temperature of the air in woodland is 1 to 

 17 Fahr, less than that outside, the difference being 30 in spring, 38 in summer, 

 11 in autumn, and insensible in winter. At the level of the crowns the mean tempera- 

 ture is about 05 less than that of the outside air, and 11 more than that of the air at 

 the foot of the trees ; in summer this last difference reaches 39. These mean differences 

 show that the cooling effect of a forest in the day time is greater than the warming effect 

 at night. In July at midday (for Europe) the forest air is 8 cooler than that outside, 

 and in August during the afternoon 71. At night it is warmer by 4 P 4 in July, and 37 

 in August. 



The distribution of temperature affects the air currents. In the day time, the cool air 

 of the forest flows outward and spreads over the open land ; as it becomes heated it 

 rises and flows back over the tree tops, where it is cooled by the leaves and gradually 

 sinks back to the surface of the forest. This circulation cools the adjoining land, prevents 

 to some degree its dessication, and carries the moisture exhaled by the leaves down to 

 the surface, where it is absorbed in part by the hygroscopic soil. 



At night, the conditions are reversed. The cold and dense air outside now streams 

 in along the surface, and rises as it gets heated, to be cooled again by contact with the 

 leaves, and finally flow back over the tree tops to the open. It would seem that the 

 forest soil will thus be deprived of the moisture which it had gained in the daytime. 

 But, at night, the differences in temperature being small, the circulation is slow in pro- 

 portion ; then the evaporation from the leaves becomes much reduced : and the cool and 

 moist air flowing in from outside, where it is often depositing dew, will rather add if any- 

 thing to the humidity of the forest soil. The air circulation at night will thus keep the 

 adjacent land somewhat warmer, and may sometimes serve to prevent frost. 



The reduction of temperature in forests is due to : (a) The great radiating power of 

 foliage, which keeps the surface cooler than bare or partially protected soil, (b) The 

 absorption by the leaves of radiation which becomes stored in the wood and can be 



* This would explain how it is that some trees seem to grow out of bare rock and derive their nourish- 

 ment from the small portions of soil accumulated in the crevices. 



** Eisler comes to the general conclusion that the final loss from evaporation is less in a forest than in 

 pasturage, and does not exceed 0'04 inch a day in a fir forest with moist soil and plenty of sunlight. 



