318 M. BECQUEREL, ON THE CLIMitIC EFFECTS OF FORESTS. 



flnence has been established by Humboldt, aod the meteorologists, as 

 follows : 



They shelter the soil against the sun's rays; they maintain it in a 

 greater degree of humidity, and facilitate the decomposition of the leaves 

 and litter, which they change into humus ; and they act as a cooling cause, 

 by producing active aqueous transpiration from the leaves and by mul- 

 tiplying in the expansion of their branches the surfaces warmed by the 

 solar heat, and the surfaces cooled by nocturnal radiation. ^ In regard 

 to the action last mentioned, positive experiments show that the layer 

 of atmosphere in contact with a meadow, or a field covered with herbage 

 or vegetable leaves, becomes cooled, by nocturnal radiation, other things 

 being equal, several degrees — sometimes as many as G, 7, or 8 centigrade — 

 below the temperature of the atmosphere at some meters above, while 

 nothing of this kind takes place over a naked soil, which becomes warm 

 or cool according to the nature of its component parts. We will add, as 

 we have demonstrated, that the leaves as well as the trunk and branches 

 become warmed by solar heat, and retain into the night a portion of 

 this acquired heat. This effect should counterbalance the cooling from 

 nocturnal radiation. We have not, thus far, taken account of the fact 

 that the warming of the trees by the sun has a considerable effect upon 

 the temperature of the atmosphere outside of the woods, as well as within 

 them. 



To explain the thermal influence of the trees upon the temperature of 

 the air, it will be proper to unite with some old observations others that 

 we have made upon the temperature of the air at different heights, near 

 and at the surface of the trees. Messrs. Ilumboldt and Boupland, in 

 sleeping on the grass during the clear tropical nights on the plains of 

 Venezuela and the Lower Orinoco, experienced a damp cooluess, while 

 the atmosphere one or two meters above them had a temperature of 26 

 to 27 degrees of the centigrade scale. Within the equatorial and trop- 

 ical regions, where the nocturnal radiation operates with the greatest 

 force, by reason of the serene sky, the increase of temperature as we rise 

 above the soil becomes evident, as in the middle latitudes, but in much 

 higher degree. We therefore do not observe in the torrid zone any 

 change in vegetation from sea-level to a height of GOO meters, and from 

 that level to 1,200 meters we still retain the flora of the tropics. 



We may now explain why, in our latitudes, certain kinds of cnltiva- 



iThe effects of radiation of heat from foliage is thus explained by Humboldt: 

 The leaves of a tree, of course, are in position neither parallel to one another nor 

 horizontal. Tbey present difteront inclinations. But Leslie and Fourier have shown 

 that the effect of these inclinations on the quantity of heat emitted by radiation, or 

 the radiating power of a surface estimated in one direction, is equal to that which is 

 possessed by a surface perpendicular to that direction. At the commencement of cool- 

 ing caused by radiation, the leaves forming the highest crest of a tree are the first to 

 lose heat. The next lower layer of leaves, liaving their upper surfaces facing the under 

 surfaces of the higher leaves, will give out to the latter more heat than they receive 

 back; and the result of this difference of radiation causes the process of cooling to 

 spread until all the leaves of the tree are reduced in temperature. Therefoie the refrig- 

 erating power of the tree, by radiation alone, depends on the extent of surface of tl\e 

 leaves; so that whore the horizontal section of a tree may not contain more than 400 

 fiquare feet, the effect of its leaves acting in mass will be several thousand times 

 greater than 400 square feet of soil uncovered, or covered only with grass. — (Asie Cen- 

 irale, iii, p. 204.) 



Elsewhere in the same work Humboldt, in speaklngof the threefold action cf forests in 

 cooling the air. by directly shading from the sun, by evaporation from the leaves, and 

 by radiation of heat, remarks, that a knowledge of the extent of forests compared with 

 the surface of the country which is uncovered is a point of the greatest importance in 

 judging of its climatology. The want of forests increases both the heat and dryness of 

 the air; and this dryness, in diminishing the extent of vapor and the vigor of vegeta- 

 tion, reacts in the heat of the climate. — (76., iii, p. 199.) 



