448 The Influence of Forests on Climate. 



THE INFLUENCE OF FOEESTS ON CLIMATE. 



M. Becquerel lias recently brought this interesting question 

 before the French Academy. The first portion of his paper 

 gives details respecting the extent of forest land in France at 

 various periods ; the second discusses the climatic action of 

 forests, and in it he states that their influence will be found to 

 depend on their extent, the nature of the trees, whether having 

 deciduous or permanent leaves, on the power of evaporation 

 exerted by the leaves, and on their capacity of receiving or 

 radiating heat, and lastly on the character of the soil and 

 subsoil. 



The effect of trees in giving shelter against winds is obvious 

 aiid important. Their leaves are a powerful and incessant 

 cause of moisture in the air, the least reduction of temperature 

 occasioning a precipitation of the moisture evaporating from 

 them, and which on falling penetrates the soil directly, when 

 it is permeable, and through the intervention of the roots 

 when it is not. 



By means of the electric thermometer the temperature of 

 trees has been observed for many years, and it appears that 

 the trunk, branches, and leaves grow warm or cool in the air, 

 as is the case with inorganic bodies ; but in the north the 

 mean temperature of trees is a little higher than that of the 

 air. When tree trunks reach three or four centimetres in 

 diameter they do not acquire their maximum temperature till 

 after sunset. In summer this occurs about nine o'clock, while 

 the air is warmest about two or three, according to the season. 

 Variations in temperature take place very slowly in' trees, and 

 rapid changes of air temperature have no influence on them. 

 "When the leaves cool themselves by nocturnal radiation they 

 retake from the body of the tree the heat which they lose. 

 About six in the morning the temperature above a tree, and at 

 one metre below it, in soil situated N. or S., is equal. Hence 

 we may conceive how trees that have been warmed by solar 

 radiation may act on the temperature of the air, and not lower 

 it as much as has been supposed. 



With regard, to the influence of destroying the trees* of a 

 country on its mean temperature, M. Boussaiugault concluded 

 from his own and other observations in tropical America, that 

 abundance of forests and moisture tend to cool a climate, while 

 dryness and aridity favour heating of the soil. On the other 

 hand, Humboldt, discussing thermometrical observations made 



* Our neighbours have the convenient word " deboisement" for which we 

 want an equivalent. Untreeing might not be thought elegant, but would be in- 

 telligible. " Disforesting " would do, but it has a legal signification. 



