The unseen climate engine: Transpiration and the physics of urban tree cooling

“ It is well understood that the temperature in a forest, a grove, or even a clump of trees, is cooler in summer and warmer in winter than the surrounding country. Man and animals alike seek the shade of groves and trees during the heat of the day, and are greatly refreshed and revived by the cool atmosphere. The difference between the temperature of the air under and among the branches of a single tree, densely leaved, and the surrounding air, on a hot day, is instantly realized by the laborer or traveler who seeks the shade. ”

“ It requires but little knowledge of the physical forces which modify the climate of large areas of the earth's surface to recognize the fact that vegetation plays a most important part. And of the different forms of vegetation, trees, as compared with shrubs, plants, vines, and grasses, are undoubtedly the most efficient. This is due to the vast area of surface which their leaves present to the air on a very limited ground space. The sanitary value of trees has hitherto been practically unrecognized by man. ”

“ Trees not only are objects of beauty—the architecture of Nature—but they promote both health and comfort. Our ardent summers, from June to October, make open, unshaded streets, almost impassable, and reflect heat upon our dwellings from the side-walks and beaten road. ”

“ The conclusion from the foregoing facts is inevitable that one of the great and pressing sanitary wants of New York city is an ample supply of trees. It is, in effect, destitute of trees; for the unsightly shrubs which are planted by citizens are, in no proper sense, adequate to the purpose which we contemplate. Its long avenues, running north and south, without a shade tree, and exposed to the full effect of the sun, are all but impassable at noonday in the summer months. ”

“ Animals and man alike suffer from exposure in the glowing heat. Nothing mitigates its intensity but the winds or an occasional rainstorm. And when evening comes on, the cooling of the atmosphere produced by vegetation does not occur, and unless partially relieved by favoring winds or a shower the heat continues, but little abated, and the atmosphere remains charged with noxious and irrespirable gases. It is evident that shade trees, of proper kinds, and suitably arranged, supply the conditions necessary to counteract the evils of excessive heat. ”

“ For shades that are made by the highest trees, are undoubtedly the coolest, and freshest, by reason it keeps the heat farthest off. Besides this, there are many advantages to be made, in the contrivance of the house; for I see the Planters there, never consider which way they build their houses, so they get them up; which is the cause that many of them, are so insufferably hot, as neither themselves, nor any other, can remaine in them without sweltring. ”

“ Again, the shade of trees protects the earth from the direct rays of the sun, and prevents solar irradiation from the earth. This effect is of immense importance in cities where the paved streets become excessively heated, and radiation creates one of the most dangerous sources of heat. Whoever has walked in the streets of New York, on a hot summer's day, protected from the direct rays of a midday sun by his umbrella, has found the reflected heat of the pavement intolerable. ”

“ The air from outside is drawn into the wood, is cooled by it, and cools us again. And it is not only the air that cools us, but the trees themselves. Observation has shown that the trunks of trees in a wood breast-high, even at the hottest time of day, are 5° Centigrade cooler than the air. We therefore lose considerable heat by radiation to these cooler objects, and can cool ourselves more easily at a temperature of 25° Centigrade in a wood than at a much lower temperature in an open space. When the objects around us are as warm as ourselves we lose nothing by radiation ”

“ By felling the trees which cover the tops and the sides of mountains, men in every climate prepare at once two calamities for future generations; want of fuel and scarcity of water. Trees, by the nature of their perspiration, and the radiation from their leaves in a sky without clouds, surround themselves with an atmosphere constantly cold and misty. ”

“ And if any one doubts the power of trees cultivated in the streets to change the temperature of a city let him calculate the amount of foliage which the seventy-eight thousand trees, when full-grown, will furnish the city of Washington, taking as his basis the fact that a single tree, the Washington elm, at Cambridge, Massachusetts, when in full leafage, equals five acres of foliage, and that one acre of grass emits into the atmosphere 6.400 quarts of water in twenty-four hours, a powerfully cooling process. ”

“ The roots of large trees penetrate beneath the superficial strata, and reach earth of a nearly constant temperature, corresponding to the mean for the entire year. As conductors, they convey the heat of the atmosphere to the earth when the earth is colder than the air, and transmit it in the contrary direction when the temperature of the earth is higher than that of the atmosphere. ”

“ The leaves, flowers and fruits, augmented by certain plants which grow in the shade of the trees, form a layer of mould, or humus, which protects the soil against rapid changes of temperature, and greatly influences the movement of water in it. ”

“ I have said that when marking shade the planters must bear in mind the relation of the earth to the sun during the hottest months, and this caution is very necessary, because if he should happen to be marking trees in January for removal after the crop season is over, and does not remember that the earth is daily shifting its position, he will find that he will have made many mistakes as to the trees which should be preserved, and that a tree that is very well placed for blocking out the hot afternoon sun in January, may be of very little use in March and April. ”

“ It is important to remember that, in many parts of an estate, as the shade trees become lofty the sun will come in, just as it would on a man's head if he carried his umbrella erect, and at the end of a long pole, and I have seen coffee trees so much exposed to the sun as to require fresh shade to be planted near them, not withstanding that some of the coffee trees in question were almost touching the stem of a very tall shade tree. When the planter observes that the sun is thus likely to come in from the shooting up of the shade trees, he should plant fresh shade. ”

“ The streets do not run so straight as those of Philadelphia, and have sometimes considerable bendings: however they are very spacious and well built, and most of them are paved, except in high places, where it has been found useless. In the chief streets there are trees planted, which in summer give them a fine appearance, and during the excessive heat at that time, afford a cooling shade: I found it extremely pleasant to walk in the town, for it seemed quite like a garden: the trees which are planted for this purpose are chiefly of two kinds. ”

“ What a sight would be a road on which one could ride for a mile through an avenue of scarlet oaks, and then for a mile through stately avenue of tulip-trees, and then through lines of scarlet maples, pepperidge-trees, cypress, or long rows of gentlemanly walnut-trees! The time will come when, on the great roads, one may travel a whole day in the shade of stately trees.
It is not enough to plant your own grounds. Every village should line its streets with shade trees. It is not enough to plant shade trees in the streets. ”

“ I think it an excellent measure, and I am sure that during the torrid season the more tree shade you have the fewer will be your cases of heat exhaustion. It is idle to say, as is often said in this country, that trees can not be made to grow in our cities. Under existing conditions the wonder is, not that trees look unhealthy in most cities, but that any of them manage to live at all. It is perfectly well known that the city of Paris has thousands of trees growing vigorously under such surroundings as the American gardener would think impossible. ”

“ Forests and Climate.—Considering the Relation of Forests to Climate and Health, Cleveland Abbe finds that while the forest does not cause increase of rainfall, its tendency is to conserve it. The forest shields the moisture from evaporation and uses less of it for its own growth than would be used for the growth of grasses or herbs, and it also conserves what is left in the soil so as to diminish, or at least regulate, the drainage into the river basins, thereby reducing the danger of destructive floods. ”

“ A dense growth of underbrush, herbaceous plants, and mosses under the larger trees, which will retain the fallen leaves in place, fill the surface-soil with rootlets, checking the flow of water and facilitating its entrance into the earth, is an essential part of a true forest. ”

“ Forests and Climate.—Dr. J. M. Anders, in the "American Naturalist," has carefully examined the influence of forests upon climate and rain-fall. The principal influence exerted by woods upon climate is as windbreaks, in which capacity the service they render is familiar enough. The experiment has been tried extensively in France of planting trees in belts one hundred metres apart, with marked benefit to the climate. Forests may slightly promote the condensation of moisture by inducing an upward movement of the air, as mountains are known to do on an extensive scale ”

“ For, in the case of our shade plantation, if the coffee is short and thickly planted, so as to closely cover the ground, there is necessarily a great want of ventilation, and, when this is the case, rot must, from the great dampness of the ground, have a tendency to increase in the monsoon, while from there being no room for the passage of air underneath the trees, the spores of the leaf disease will be preserved from being dried up and killed during the season of strong and parching winds. ”

“ Much may be done in the first case by judiciously opening channels for air through the shade trees so as to admit a free circulation of air into hollows, and much in the latter by freely handling out the centres of the trees which, in the monsoon, and especially in hollows, are apt to grow a superabundance of young wood, which chokes up the centre of the tree and thus hinders the free circulation of air. ”

“ And some planters, I observe, are by no means satisfied with lopping the lower boughs, but trim off branches fifteen feet from the ground. Under such a system the number of shade trees required is enormous, and the evils arising from the number of boles with their vast mass of large roots will only be the more severely apparent as time advances. By one shade planter in Coorg I have been told that coffee there has already been suffering much from the quantity of boles and tree roots in the land, in consequence of the trimming up system and the quantity of trees required in consequence. ”