FORESTS AND THEIR CLIMATIC INFLUENCE. 405 



indepeudeut of couductibility, -wliicli strives against a reduction of tem- 

 perature below zero, and preserves them for a certain time from the dis- 

 astrous effects of excessive cold. The action varies ^yith the diameter 

 of the tree, and probably with the species to which it pertains. 



In northern regions the temperature of vegetables comi)ared with 

 that of the air is very remarkable. M. Bourgeaud, under the 5Sth de- 

 gree of latitude, at places where the temperature descends in winter 

 below the degree of congelation of mercury, that is to say to more than 



— -40°, has substantiated the following facts: First. In the Popidus hal- 

 sami/era and Abies alba, during eight months, from November, 1857, to 

 June, 1858, at 9 o'clock in the morning, the moment at which he sup- 

 X^osed the temperature to be very nearly a mean, the mean temperatures 

 of the air and the tree were the same ; which accords with the observa- 

 tions above spoken of, and is in conformity with the principle that the 

 temperature of plants unceasingly tends to form an equilibrium with 

 that of the ambient air, notwithstanding the eflicient causes which are 

 incessantly in action to increase or diminish it. Second. The monthly 

 temperatures presented little difterence in the tree and in the air, al- 

 though there were very great differences in the maxima and minima ; 

 in the month of January, for example, the maxima and minima were in 

 the air +0°, (43° F.,) and— 34o.C0, (—29° F.,) and in the poplar 



— 20.20, (280 F.,) and — 29o.70, (—21° F.) Third. During the eight 

 months of observation the mean temperature in the soil, at a depth of 

 0'".913 (3 feet) and 0™.G09, (2 feet,) was twice as high as in the air. 



The thaw takes place ordinarily in May, spring at once commences, 

 and soon after summer arrives ; the rapidity of vegetation is such that 

 the cereals sown in the month just mentioned are reaped toward the 

 end of July; blossoms appear on the poplar when the temperature of 

 the air is at +13°.4:7 (oG^ F.) and while the earth still freezes at a depth 

 of 0"\G09, (2 feet,) and 0'".913, (3 feet.) The leaves display themselves 

 in the first days of June, when the roots are buried in strata of earth 

 where the temperature is still at zero, (32° F. ;) like effects are produced 

 when the branches of the vine are introduced into a conservator}- while 

 the stalks and roots are in the ground outside ; the buds and even leaves 

 begin to be developed when it is freezing externally at 8^ and 10° below 

 zero, (18° and 14:° above zero F.) We have here a new proof of the 

 intluence of the temperature of the air upon that of trees, showing that, 

 even when the roots are in frozen earth, vegetation may jiroceed under 

 that influence. The Populus bahamifcra and Abies alba, as well as other 

 species, undergo exposure to a cold of — 40°, without injurj- to their 

 organization, but the roots of these trees are in strata of earth which 

 are not sensibly reached by the frost. A proof that there is here a cer- 

 tain resistance to cold is the fact that the greatest minima of temper- 

 ature, being — 34°.G0, ( — 29° F.) in the au^, were, in the poplar, only 



— 29°.70, ( — 21° F,) and that the temperature has been twice as high 

 in the tree as in the air. 



After having stated the relations which have been found to exist 

 between the temperature of the air and its variations, as compared with 

 those of vegetables, it remains for us to show what temperature of the 

 air has been realized above trees of large growth, such as a chestnut 

 21'".25 (70 feet) in height, at the summit of which had been placed one 

 of the solderings" of an electric thermometer in contact with the leaves. 

 Multiplied observations have demonstrated that the temperature of the 

 air above the chestnut tree depends chiefly on the calorific state of the 

 leaves and branches which warm or cool the ambient air more or less, 

 according as they have been exposed a longer or shorter time to solai 

 radiatiouj or to noctm^nal radiation. 



