Complete Works of Robert Louis Stevenson (Illustrated) (877 page)

We come next, however, to a second point of difference. 227 In the case of the meadow, the chilled air continues to lie upon the surface, the grass, as Humboldt says, remaining all night submerged in the stratum of lowest temperature; while in the case of trees, the coldest air is continually passing down to the space underneath the boughs, or what we may perhaps term the crypt of the forest. Here it is that the consideration of any piece of woodland conceived as a solid comes naturally in; for this solid contains a portion of the atmosphere, partially cut off from the rest, more or less excluded from the influence of wind, and lying upon a soil that is screened all day from isolation by the impending mass of foliage. In this way (and chiefly, I think, from the exclusion of winds), we have underneath the radiating leaf-surface a stratum of comparatively stagnant air, protected from many sudden variations of temperature, and tending only slowly to bring itself into equilibrium with the more general changes that take place in the free atmosphere.

Over and above what has been mentioned, thermal effects have been attributed to the vital activity of the leaves in the transudation of water, and even to the respiration and circulation of living wood. The whole actual amount of thermal influence, however, is so small that I may rest satisfied with mere mention. If these actions have any effect at all, it must be practically insensible; and the others that I have already stated are not only sufficient validly to account for all the observed differences, but would lead naturally to the expectation of differences very much larger and better marked. To these observations I proceed at once. Experience has been acquired upon the following three points: — 1, The relation between the temperature of the trunk of a tree and the temperature of the surrounding atmosphere; 2, The relation between the temperature of the air under a wood and the temperature of the air outside; and, 3, The relation between the temperature of the air above a wood and the temperature of the air above cleared land.

As to the first question, there are several independent series of observations; and I may remark in passing, what applies to all, that allowance must be made throughout for some factor of specific heat. The results were as follows: — The seasonal and monthly means in the tree and in the air were not sensibly different. The variations in the tree, in M. Becquerel’s own observations, appear as considerably less than a fourth of those in the atmosphere, and he has calculated, from observations made at Geneva between 1796 and 1798, that the variations in the tree were less than a fifth of those in the air; but the tree in this case, besides being of a different species, was seven or eight inches thicker than the one experimented on by himself. The variations in the tree, therefore, are always less than those in the air, the ratio between the two depending apparently on the thickness of the tree in question and the rapidity with which the variations followed upon one another. The times of the maxima, moreover, were widely different: in the air, the maximum occurs at 2 P.M. in winter, and at 3 P.M. in summer; in the tree, it occurs in winter at 6 P.M., and in summer between 10 and 11 P.M. At nine in the morning in the month of June, the temperatures of the tree and of the air had come to an equilibrium. A similar difference of progression is visible in the means, which differ most in spring and autumn, and tend to equalise themselves in winter and in summer. But it appears most strikingly in the case of variations somewhat longer in period than the daily ranges. The following temperatures occurred during M. Becquerel’s observations in the Jardin des Plantes: —

A moment’s comparison of the two columns will make the principle apparent. The temperature of the air falls nearly fifteen degrees in five days; the temperature of the tree, sluggishly following, falls in the same time less than four degrees. Between the 19th and the 20th the temperature of the air has changed its direction of motion, and risen nearly a degree; but the temperature of the tree persists in its former course, and continues to fall nearly three degrees farther. On the 21st there comes a sudden increase of heat, a sudden thaw; the temperature of the air rises twenty-five and a half degrees; the change at last reaches the tree, but only raises its temperature by less than three degrees; and even two days afterwards, when the air is already twelve degrees above freezing point, the tree is still half a degree below it. Take, again, the following case: —

The same order reappears. From the 13th to the 19th the temperature of the air steadily falls, while the temperature of the tree continues apparently to follow the course of previous variations, and does not really begin to fall, is not really affected by the ebb of heat, until the 17th, three days at least after it had been operating in the air. Hence we may conclude that all variations of 230 the temperature of the air, whatever be their period, from twenty-four hours up to twelve months, are followed in the same manner by variations in the temperature of the tree; and that those in the tree are always less in amount and considerably slower of occurrence than those in the air. This
thermal sluggishness
, so to speak, seems capable of explaining all the phenomena of the case without any hypothetical vital power of resisting temperatures below the freezing point, such as is hinted at even by Becquerel.

Réaumur, indeed, is said to have observed temperatures in slender trees nearly thirty degrees higher than the temperature of the air in the sun; but we are not informed as to the conditions under which this observation was made, and it is therefore impossible to assign to it its proper value. The sap of the ice-plant is said to be materially colder than the surrounding atmosphere; and there are several other somewhat incongruous facts, which tend, at first sight, to favour the view of some inherent power of resistance in some plants to high temperatures, and in others to low temperatures. But such a supposition seems in the meantime to be gratuitous. Keeping in view the thermal redispositions, which must be greatly favoured by the ascent of the sap, and the difference between the condition as to temperature of such parts as the root, the heart of the trunk, and the extreme foliage, and never forgetting the unknown factor of specific heat, we may still regard it as possible to account for all anomalies without the aid of any such hypothesis. We may, therefore, I think, disregard small exceptions, and state the result as follows: —

If, after every rise or fall, the temperature of the air remained stationary for a length of time proportional to the amount of the change, it seems probable — setting aside all question of vital heat — that the temperature of the tree would always finally equalise itself with the new temperature 231 of the air, and that the range in tree and atmosphere would thus become the same. This pause, however, does not occur: the variations follow each other without interval; and the slow-conducting wood is never allowed enough time to overtake the rapid changes of the more sensitive air. Hence, so far as we can see at present, trees appear to be simply bad conductors, and to have no more influence upon the temperature of their surroundings than is fully accounted for by the consequent tardiness of their thermal variations.

Observations bearing on the second of the three points have been made by Becquerel in France, by La Cour in Jutland and Iceland, and by Rivoli at Posen. The results are perfectly congruous. Becquerel’s observations were made under wood, and about a hundred yards outside in open ground, at three stations in the district of Montargis, Loiret. There was a difference of more than one degree Fahrenheit between the mean annual temperatures in favour of the open ground. The mean summer temperature in the wood was from two to three degrees lower than the mean summer temperature outside. The mean maxima in the wood were also lower than those without by a little more than two degrees. Herr La Cour found the daily range consistently smaller inside the wood than outside. As far as regards the mean winter temperatures, there is an excess in favour of the forest, but so trifling in amount as to be unworthy of much consideration. Libri found that the minimum winter temperatures were not sensibly lower at Florence, after the Apennines had been denuded of forest, than they had been before. The disheartening contradictoriness of his observations on this subject led Herr Rivoli to the following ingenious and satisfactory comparison. Arranging his results according to the wind 232 that blew on the day of observation, he set against each other the variation of the temperature under wood from that without, and the variation of the temperature of the wind from the local mean for the month: —

From this curious comparison, it becomes apparent that the variations of the difference in question depend upon the amount of variations of temperature which take place in the free air, and on the slowness with which such changes are communicated to the stagnant atmosphere of woods; in other words, as Herr Rivoli boldly formulates it, a forest is simply a bad conductor. But this is precisely the same conclusion as we have already arrived at with regard to individual trees; and in Herr Rivoli’s table, what we see is just another case of what we saw in M. Becquerel’s — the different progression of temperatures. It must be obvious, however, that the thermal condition of a single tree must be different in many ways from that of a combination of trees and more or less stagnant air, such as we call a forest. And accordingly we find, in the case of the latter, the following new feature: The mean yearly temperature of woods is lower than the mean yearly temperature of free air, while they are decidedly colder in summer, and very little, if at all, warmer in winter. Hence, on the whole, forests are colder than cleared lands. But this is just what might have been expected from the amount of evaporation, the continued descent of cold air, and its stagnation in the close and sunless crypt of a forest; and one can only wonder here, as elsewhere, that the resultant difference is so insignificant and doubtful.

We come now to the third point in question, the thermal influence of woods upon the air above them. It will be remembered that we have seen reason to believe their effect to be similar to that of certain other surfaces, except in so far as it may be altered, in the case of the forest, by the greater extent of effective radiating area, and by the possibility of generating a descending cold current as well as an ascending hot one. M. Becquerel is (so far as I can learn) the only observer who has taken up the elucidation of this subject. He placed his thermometers at three points: A and B were both about seventy feet above the surface of the ground; but A was at the summit of a chestnut tree, while B was in the free air, fifty feet away from the other. C was four or five feet above the ground, with a northern exposure; there was also a fourth station to the south, at the same level as this last, but its readings are very seldom referred to. After several years of observation, the mean temperature at A was found to be between one and two degrees higher than that at B. The order of progression of differences is as instructive here as in the two former investigations. The maximum difference in favour of station A occurred between three and five in the afternoon, later or sooner according as there had been more or less sunshine, and ranged sometimes as high as seven degrees. After this the difference kept declining until sunrise, when there was often a difference of a degree, or a degree and a half, upon the other side. On cloudy days the difference tended to a minimum. During a rainy month of April, for example, the difference in favour of station A was less than half a degree; the first fifteen days of May following, however, were sunny, and the difference rose to more than a degree and a half. It will be observed that I have omitted up to the present point all mention of station C. I do so because M. Becquerel’s language leaves it doubtful whether the observations made at this station are logically comparable with 234 those made at the other two. If the end in view were to compare the progression of temperatures above the earth, above a tree, and in free air, removed from all such radiative and absorptive influences, it is plain that all three should have been equally exposed to the sun or kept equally in shadow. As the observations were made, they give us no notion of the relative action of earth-surface and forest-surface upon the temperature of the contiguous atmosphere; and this, as it seems to me, was just the
crux
of the problem. So far, however, as they go, they seem to justify the view that all these actions are the same in kind, however they may differ in degree. We find the forest heating the air during the day, and heating it more or less according as there has been more or less sunshine for it to absorb, and we find it also chilling it during the night; both of which are actions common to any radiating surface, and would be produced, if with differences of amount and time, by any other such surface raised to the mean level of the exposed foliage.