Specific Heat

Trees as Organisms.

Trees, considered as organisms, produce in themselves, or in the air, a certain amount of heat, by absorbing and condensing atmospheric vapor, and they exert an opposite influence by absorbing water and exhaling it in the form of vapor; but there is still another mode by which their living processes may warm the air around them, independently of the thermometric effects of condensation and evaporation. The vital heat of a dozen persons raises the temperature of a room. If trees possess a specific temperature of their own, an organic power of generating heat, like that with which the warm-blooded animals are gifted, though by a different process, a certain amount of weight is to be ascribed to this element, in estimating the action of the forest upon atmospheric temperature.

"Observation shows," says Meguscher, "that the wood of a living tree maintains a temperature of +12° or 13° Cent. [= 54°, 56° Fahr.] when the temperature of the air stands at 3°, 7°, and 8° [=37°, 46°, 47° F.] above zero, and that the internal warmth of the tree does not rise and fall in proportion to that of the atmosphere. So long as the latter is below 18° [= 67° Fahr.], that of the tree is always the highest; but if the temperature of the air rises to 18°, that of the vegetable growth is the lowest. Since, then, trees maintain at all seasons a constant mean temperature of 12° [= 54° Fahr.], it is easy to see why the air in contact with the forest must be warmer in winter, cooler in summer, than in situations where it is deprived of that influence."

Boussingault remarks: "In many flowers there has been observed a very considerable evolution of heat, at the approach of fecundation. In certain arums the temperature rises to 40° or 50° Cent. [= 104° or 122° Fahr.]. It is very probable that this phenomenon is general, and varies only in the intensity with which it is manifested."

If we suppose the fecundation of the flowers of forest trees to be attended with a tenth only of this calorific power, they could not fail to exert an important influence on the warmth of the atmospheric strata in contact with them.

In a paper on Meteorology by Professor Henry, published in the United States Patent Office Report for 1857, p. 504, that distinguished physicist observes: "As a general deduction from chemical and mechanical principles, we think no change of temperature is ever produced where the actions belonging to one or both of these principles are not present. Hence, in midwinter, when all vegetable functions are dormant, we do not believe that any heat is developed by a tree, or that its interior differs in temperature from its exterior further than it is protected from the external air. The experiments which have been made on this point, we think, have been directed by a false analogy. During the active circulation of the sap and the production of new tissue, variations of temperature belonging exclusively to the plant may be observed; but it is inconsistent with general principles that heat should be generated where no change is taking place."

There can be no doubt that moisture is given out by trees and evaporated in extremely cold winter-weather, and unless new fluid were supplied from the roots, the tree would be exhausted of its juices before winter was over. But this is not observed to be the fact, and, though the point is disputed, respectable authorities declare that "wood felled in the depth of winter is the heaviest and fullest of sap." Warm weather in winter, of too short continuance to affect the temperature of the ground sensibly, stimulates a free flow of sap in the maple. Thus, in the last week of December, 1862, and the first week of January, 1863, sugar was made from that tree, in various parts of New England. "A single branch of a tree, admitted into a warm room in winter through an aperture in a window, opened its buds and developed its leaves while the rest of the tree in the external air remained in its winter sleep." The roots of forest trees in temperate climates, remain, for the most part, in a moist soil, of a temperature not much below the annual mean, through the whole winter; and we cannot account for the uninterrupted moisture of the tree, unless we suppose that the roots furnish a constant supply of water.

Atkinson describes a ravine in a valley in Siberia, which was filled with ice to the depth of twenty-five feet. Poplars were growing in this ice, which was thawed to the distance of some inches from the stem. But the surface of the soil beneath it must have remained still frozen, for the holes around the trees were full of water resulting from its melting, and this would have escaped below if the ground had been thawed. In this case, although the roots had not thawed the thick covering of earth above them, the trunks must have melted the ice in contact with them. The trees, when observed by Atkinson, were in full leaf, but it does not appear at what period the ice around their stems had melted.

From these facts, and others of the like sort, it would seem that "all vegetable functions are" not absolutely "dormant" in winter, and, therefore, that trees may give out some heat at that season. But, however this may be, the "circulation of the sap" commences at a very early period in the spring, and the temperature of the air in contact with trees may then be sufficiently affected by heat evolved in the vital processes of vegetation, to raise the thermometric mean of wooded countries for that season, and, of course, for the year.