72 FOREST INFLUENCES. 



Undei- the crowns the temperature is lower in the forest, but above 

 the crowns it is usually higher. The wind reduces the dilterence under 

 and in the crowns, but makes little difference above them. 



Putting- the preceding- results together, it appears that the forests 

 reverse the vertical gradient, and that the gradient grows larger as the 

 surface of the forest is approached. Toward the ground the change is 

 slow; upward it is very rapid, so rapid that the special cooling effect 

 of the forest must disappear at no great height. The surface of the 

 surface of the forest is, meteorologically, much like tlie surface of the 

 meadow or cornfield. The isothermal surface above it in sunshine is a 

 surface of maximum temperature, as is the surftice of the meadow or 

 cornfield. From this surface the temperature decreases in both direc- 

 tions. 



TREE TEMPERATURES. 



The disturbances of temperature under trees, so far discussed, would 

 be the same in kind if the surface covering called a forest were inor- 

 ganic. They are due to the screen formed by the forest against the 

 sun's rays and against the radiation from the soil to the sky. The 

 kind of effect would be the same if the forest were dead, but the quantity 

 might be quite different. The shade under a dead tree is much less 

 than under a live one. This is due to a considerable extent to the fact 

 that its leaves are uo longer spread across the path of the sun's rays, 

 and the heat of the latter is not so much absorbed by the evaporation 

 of water. These, again, may be purely mechanical effects, and it is still 

 left uncertain whether the living tree affects the temperature through 

 its vital action ; whether in the processes of life the tree does not abstract 

 enough heat from the air, or add enough heat to the air, to be meteor- 

 ologically sensible. There is abundant reason to think that it may exert 

 such action, because all vital processes result in the absorption or radia- 

 tion of heat— and so do purely inorganic chemical processes. The tree, 

 as an organic being, is a source as well as an absorbent of heat; the 

 only question is whether the heat it absorbs or emits is enough in 

 quantity to make a sensible difference in the temperature of the air. 

 The analogy of the hot-blooded animal leads one to look first to the 

 tree itself for a solution of this question. Even the cold-blooded ani- 

 mal, when of a large size, has its own temperature slightly higher than 

 the mean temperature of the medium in which it lives. The tree is a 

 living body. Is the same true of it"? 



The most elaborate series of observations of tree temperatures are 

 those taken at Geneva for the five years, 1796 to 1800. The thermom- 

 eter was introduced into the trunk ot a horse-chestnut tree to the depth 

 of inches, or about half of the way to the heart. The temperatures 

 were read three times daily (sunrise, 2 p. m., and sunset), and observa- 

 tions were also taken at the same hours in the air and at several depths 



