392 TRANSACTIONS OP THE 



EFFECT OF TREES AND VEGETATION ON CLIMATES. 



The following on climates, and how they are affected by trees and vege- 

 tation, is from the Encyclopedia Britannica, volume six, ninth edition, in 

 which it says: 



When the ground is covered with vegetation the whole of the sun's heat 

 falls on the vegetable covering, and as none of it falls directly on the soil, its 

 temperature does not rise so high as that of land with no vegetable covering. 

 The temperature of plants exposed to the sun does not rise so high as that 

 of soil, because a portion of the sun's heat is lost in evaporation, and the 

 heat cannot accumulate on the surface of the leaves as it does on the soil. 

 Hence, the essential difference between the climates of two countries, the 

 one well covered with vegetation, the other not, lies in this, that the heat 

 of the day is more equally distributed over the twenty-four hours in the 

 former case, and, therefore, less intense during the warmest part of the 

 day. 



But the effect of vegetation on the distribution of the temperature dur- 

 ing the day is most markedly shown in the case of forests. Trees, like 

 other bodies, are heated and cooled by radiation, but owing to their slow 

 conducting power the times of the daily maximum and minimum tempera- 

 ture do not occur till some hours after the same phases of the temperature 

 of the air. Again, the effects of radiation are in the case of the trees not 

 chiefly confined to a surface stratum of air a very few feet in thickness, 

 but, as already remarked, are to a very large extent, diffused through a 

 stratum of air, equaling, in thickness at least, the height of the trees. 

 Hence, the conserving influence of forests on climate, making the nights 

 warmer and the days cooler, imparting, in short, to the climates of districts 

 clad with trees, something of the character of insular climates. Evapora- 

 tion proceeds slowly from the damp soil usually found beneath trees, since 

 it is more or less screened from the sun. Since, however, the air under the 

 trees is little agitated or put in circulation by the wind, the vapor arising 

 from the soil is mostly left to accumulate among the trees, and hence it is 

 probable that forests diminish the evaporation, but increase the humidity, 

 of climates within their influence. The humidity of forests is further 

 increased by the circumstance that when rain falls, less of it passes 

 immediately along the surface into streams and rivers; a considerable 

 portion is at once taken up by the leaves of the trees, and percolates the 

 soil, owing to its greater friability in woods, to the roots of the trees, whence 

 it is drawn up to the leaves and there evaporated, thus adding to the 

 humidity of the atmosphere. 



Much has been done by Dr. Marsh and others in elucidation of the 

 influence on climate of forests and the denudation of trees, in so far as 

 that can be done by the varying depths of lakes and rivers, and other non- 

 instrumental observations. Little, comparatively, has been done anywhere 

 in the examination of the great practical question of the influence of forests 

 on climate, by means of carefully devised and conducted observations made 

 with thermometers, the evaporating dish, or the rain gauge. The most 

 extensive inquiry on the subject yet set on foot has been for some years 

 conducted in the forests of Bavaria, under the direction of Professor Eber- 

 meyer, and a like inquiry was begun in Germany in 1875 — the more 

 important results being, that during the day, particularly in the warm 



