320 M BECQUEREL, ON THE CLIMATIC EFFECTS OF FORESTS. 



We see, therefore, that a tree warms itself in the air, like any inert 

 body, and the more rapidly according as its body is of less volume and 

 its bark of greater absorbing power. It is also true that, having sur- 

 rounded the trunk of a plum-tree, to the height of 2 meters, with a cov- 

 ering of tinned iron, which has a great reflecting power, the temperature 

 of the air being generally the same as before, the difference between 

 the maximum and the minimum went down from 13°.07 to 5^.2 (C). We 

 see from this that the temperature of the plum-tree had become more 

 uniform. Upon takingoff this covering the difference between maximum 

 and minimum increased, and became as great as before. Coverings of 

 metals, or of straw, diminish the variations of temperature within the 

 tree, and render the changes of heat more regular, and we may conceive 

 that the nature and thickness of the bark ought to exercise a great in- 

 fluence upon the warming of trees. Some experiments made upon the 

 Opuntia, and other plants, tend to show that the leaves and small 

 branches speedily acquire the temperatures of the surrounding air. 



Upon comparing the mean temperature of the air with that of the in- 

 terior of a horse-chestnut, 0.5 meter (19.68 inches) in diameter, it was 

 found that the mean of the temperatures observed within the tree during 

 thirteen months exceeded by 0o.36 (C.) that of the air at its surface, and 

 by 00.83 of the air on the north side and in the shade, a difference due 

 probably to the fact that the thermometer was placed in a situation 

 sheltered from the sun, while the tree was protected from the north winds 

 by buildings adjacent, and was at the same time exposed to the solar 

 rays. Some experiments made-upon other trees established well the prin- 

 ciple of the changing equilibrium of temperature, after more or less 

 time, between the air and the trees, and the more rapidly as the varia- 

 tions of the air are less frequent. The difference in autumn and in 

 winter is at its minimum, and in spring and in summer at its maximum. 



The maximum temperature of the atmosphere takes place, according 

 to season, between two and three o'clock in the afternoon, while in the 

 tree it does not become manifest until after sundown. If we notice the 

 effect of seasons, we find that it is especially in summer that the maxi- 

 mum is more marked, and then it is reached about nine o'clock in the 

 evening. 



The heat disengaged within the organs and tissues of vegetables does 

 not affect, or but very slighly, the temperatures in which they are placed, 

 an d we must seek the principal cause of these changes in solar radia- 

 tion and the temperature of the air. The diurnal variations of temper- 

 ature in the air are easily determined, since this is the difference be- 

 tween the maximum and minimum of the day. But it is very difficult 

 to find this variation in a tree, and we may arrive at this in a manner 

 more or less approximately. 



Observations upon temperature were made at Geneva from 1796 to 

 1800, at sunrise and sunset, and at two o'clock in the afternoon, in the 

 air open to the north, and in a horse-chestnut of 0.6 meter (23.02 inches) 

 in diameter. The maxima and the minima may be had by combining 

 the temperatures at two o'clock and those of sunrise and sunset, the 

 maximum taking place, as we have already shown, near or a little after 

 sunset, and the minimum at about sunrise. The difference will give es- 

 sentially the variations within the tree. By discussing the variations 

 thus obtained in the air and in the tree, it is seen that in the years 1796, 

 1797, and 1798, the variations were, on the average, five times greater 

 in the air than in the tree. 



From observations made at the Jardin des Plantes from December, 

 1858, to July, 1859, it was found that the mean variations of tempera- 



