A year among the trees

by Wilson Flagg

RELATIONS OF TREES TO TEMPERATURE.

Not long since, in one of my rambles in Essex County, Massachusetts, which is one of the most open and cultivated sections of the country, I entered a little valley near the sea, comprising about fifty acres of well-cultivated land, surrounded by a sort of amphitheatre of hills, which were covered with a dense forest of pines and firs. It was occupied by an intelligent farmer, whose careful observation had taken note of many things which are overlooked by the generality of his class. He remarked that his seed-time and harvest were several days earlier than on the farms in the open country, and that he had crocuses and tulips in his garden, on the south side of the surrounding wood, so early as to astonish his neighbors in the outer world. In regard to the relative temperature of the woods and of the open plain in summer, he remarked that it varied according to the time of day or night. The woods were cooler than the open country, in clear, calm weather, from about nine o’clock in the morning until near noonday; after this time the heat in them increased more rapidly than in the open country, and at the time of dew-fall it was greater in the woods, and continued so during the early part of the night. If the sky were cloudy, not much difference could be perceived at any hour in the temperature of the two situations. In cold and windy weather the woods afforded a comfortable shelter, and this shelter made them apparently warmer, even when the thermometer would indicate no difference.

The theory of my rustic friend contains the general results of all that science has yet discovered in relation to the temperature of woods. But the effects of clearing the forest are so different in different situations as to have given origin to a multitude of theories. This diversity of opinion, however, comes from a partial observation of facts, without their qualifying circumstances. On a hot summer’s day we sprinkle our floors with water, for the purpose of cooling the air of the room. But how can it produce this effect, when by evaporation it carries heat from the floor into the very air that is cooled by it? The fact is easily explained. The greater coolness felt when the air of the room is saturated with the moisture evaporated from the sprinkled floor might not be exactly indicated by the thermometer. The sensation of coolness is caused by the increased power of the air to conduct the heat rapidly from our persons,—the effect of its greater humidity. By the same law we may explain why, after a few clear cold days in the winter, if a south-wind arises, we feel as if the cold were greater, because this wind, while it raises the temperature, charges the air with invisible moisture.

The coldness of the atmosphere over grassy meadows when the sky is clear, after the decline of the sun in summer, is a matter of common observation. As this phenomenon is most evident on the clearest nights, it has given rise to the notion that the moon cools the night air. In our rambles after sunset, we have all felt these constant changes of temperature, which are remarkable when walking over an uneven road, the degree of heat corresponding nearly with our altitude. When we occupy high ground, the air is warm and dry; as soon as we descend into a valley, we feel a sudden chill. These differences are not observed on a cloudy night, or when a clear brisk wind is blowing. But in a calm state of the atmosphere, as the lowest stratum of air contains the greatest amount of moisture, its capacity for retaining heat is proportionally diminished. Consequently the heat from the ground is radiated with great rapidity through this damp stratum of air, while the higher strata remain unchanged in their temperature. Indeed, it has been found by experiment that while the greatest heat at noonday in calm summer weather is very near the surface of the ground, yet after dew-fall the highest temperature is several feet above this surface, increasing in altitude for some hours after sunset.

The action of a wood checks this radiation in the early part of the night. Like clouds in the evening, the trees form a canopy of foliage over the ground, and thereby retain the heat many hours after it has escaped by radiation in the open plain. According to these laws of the radiation of heat, a longer time would be required to cool a tract of forest than an equal area of open space, down to a given point. But, on the other hand, a proportionally longer time is required to raise the temperature in the woods to a given point. Hence it is still a question among meteorologists whether the mean annual temperature of a large tract of country is higher or lower when covered with forest than when generally open and cleared. The sun acts with greater force upon an open country; but the radiation of heat is greater in the same ratio during the sun’s absence.

In considering the effects of clearing, travellers have often overlooked the important advantages of protection afforded by woods to agricultural crops. Even if the mean annual temperature of a country be the same after it is cleared as when it was covered, it may at the same time be too cold for certain plants which were formerly its common productions, because there are no woods to protect them from the winds by day or from the cold caused by excessive radiation at night. Palestine, two thousand years ago, was a well-wooded country, and all the fruits of the sub-tropical climates were raised there to perfection by its ancient inhabitants. The date-palm, the fig-tree, and the olive grew there and bore fruit abundantly. Palestine is now a treeless country, and the same fruits are incapable of enduring its climate; yet recent observations have demonstrated that its climate is not colder than it was in the days of the kings of Israel. But as the country has been despoiled of its forests, these sub-tropical fruits are deprived of their natural conservatories, and cannot be raised without great labor and expense in preparing artificial protection for them. Let the forests be restored to the hills and mountains of Palestine, and, though the temperature of its summers were not increased, the fields would be protected by these forests from the winds, and the tender fruits, thriving under their protection, would again become abundant.

The principles involved in these and similar facts form a distinct branch of meteorological science, and would require a volume for their illustration. I have only hinted at some of the general conclusions. It is evident, indeed, that the same objects that serve to protect us from cold may in an equal degree protect us from heat. The woodcutters will continue their labor in a deep forest without discomfort on a winter’s day, when they could not endure the intense cold of the open country. The earliest flowers of spring, however, are found neither in a wood nor in an open meadow, but under the protection of a wood on its southern border, in little openings that are exposed to the beams of the sun.