Popular Science Monthly/Volume 82/April 1913/The Influence of Forests Upon Climate
|THE INFLUENCE OF FORESTS UPON CLIMATE|
Introduction: Popular Belief in Forest Influences, and its Possible Origin
FAR and wide, the world over, we find a popular belief in an influence of forests upon climate, especially upon rainfall. This is not difficult to explain. Take our own experience, for example. On a summer day we leave the hot, sunny road and walk along a narrow forest path. The trees give shade; the glare and heat of the road are replaced by the soft, dark carpet of leaves and moss; the air seems cool and damp. It is all a great relief, and the impression is inevitable that a forest climate is different from that of the open. Again, on a spring day, when the snow has disappeared from the fields, but when a chilly, wintry wind is blowing, we leave the open meadow and cross a patch of woodland. There is snow still lying deep under the trees; there is welcome protection from the biting wind; it seems pleasantly warm. Has not, we naturally say, the forest a climate all of its own? Once more. We observe, the world over, that where there are extended forests there is heavy rainfall, and we see deserts and treeless areas where the rainfall is light. We infer that the forests have something to do with producing the heavier rainfall, and some of us may even go a step farther and think that the great treeless areas were once forested, and that deforestation has made them dry. Or, to give one more case, we may have noticed the increasing tree growth with increasing elevation on our mountains, and may have concluded that the denser forest is the cause of the heavier precipitation which is generally observable as we ascend our mountain slopes.
Thus it may come about, naturally enough, that people believe in forest influences upon climate. Yet, if we ourselves happen to have based our own belief on any such evidence as the foregoing, we ought to remember that our own sensation of heat, or cold, or dampness, by no means necessarily, or even usually, corresponds with the actual meteorological facts. Further, the great rainy and dry belts of the earth's surface are controlled by a world-wide distribution of temperature, pressure and winds, that is, by the general circulation of the atmosphere, and by conditions of the higher strata far and away beyond the reach of any local effects such as those of a forest. Universally, in response to natural controls, a scanty rainfall is hostile to tree-growth, and forests are favored by heavy rainfall, which gives good conditions of soil-moisture and is generally accompanied by higher relative humidity, more cloudiness and less extreme temperatures than prevail over treeless regions. In the case of mountains, again, it should be clearly in our minds that, as a rule, and up to a certain limit, an increase of altitude involves an increase of precipitation, quite apart from the presence or absence of any forest. We must be careful not to put the cart before the horse. The forests, in other words, are the result of the rainfall, and not vice versa.
Importance of the Subject: its Complexity
That this subject has an important relation to our national conservation policy no one will deny. Unfortunately, the discussion of it has become more or less a matter of semi-political controversy. Much has been written without adequate study of the question. Heated arguments, pro and con, have been advanced in debates and in print. Remarkably divergent views have been, and are to-day, held upon the question. It has been claimed that forests have no climatic influences whatever. On the other hand, some have believed that deforestation in North America has affected the climate of Europe. A recent writer maintains that the principal cause of the "intellectual and industrial stagnation" of the Spanish peasants is to be found in the effects of deforestation in making the climate drier, so that the people are "worked to death to support life." The literature is extended and bewildering. It runs back at least five hundred years. A bibliography published in 1872 contains nearly two hundred titles, and began with Fernando Columbus, who attributed the heavy rainfall of Jamaica to its heavy forests, and a (supposed) decrease of rainfall on the Azores and Canaries to deforestation. It has been said that this whole discussion first came up in really acute form at the time of the French Revolution, when private timberlands were largely destroyed.
The subject is thus greatly complicated by the nature of the discussion. It is, furthermore, by its very nature a complicated problem. On the one hand, climate itself is the complex resultant of many different controls. Among these are the latitude; the elevation above sea-level; the varying influences of land and water; the proximity of ocean currents; the prevailing winds and storms. In this list of controls, but at the end, the last and the least important of all, modifying slightly, perhaps, the total effect of all the other controls, comes the surface-covering of the earth. This may be snow, or grass, or sand, or lava. Here belongs the forest, a special kind of surface covering.
On the other hand, the forests. What do we mean by forests? Do we mean the vast, dense tropical forests of the Amazon, or a grove of trees on a New England farm? Have we in mind evergreen or deciduous trees, or both? Are the forest trees tall or scrubby? Is their height uniform or varying? Is there undergrowth or is the forest clean? Are we considering the forested slope of a steep and lofty mountain or the trees in a valley bottom; a tropical or an extra-tropical forest; a region of heavy or one of moderate rainfall; of much or of little cloud? Clearly, a complex problem is here before us. No wonder that so much diversity of opinion exists with regard to it. Few of those who discuss the question are at all aware of its extent or complexity. They see only one or two small aspects of it, and upon a very insufficient, and often inaccurate, knowledge they base broad and misleading generalizations.
In a matter of such general interest it is most important to proceed carefully, and to see clearly just what we do, and what we do not know. That is the purpose of the present paper: to set forth, as the writer sees it, the status of the "forest and climate" discussion in the light of the available facts. It may be added, parenthetically, that it is only comparatively recently that a scientific study of the subject has been possible.
The Historical Method of Treatment: its Unreliable Results
The favorite method of attacking the problem of forest influences has been the historical method. Probably the large majority of those who believe in such influences are affected, consciously or unconsciously, by the use of historical arguments. A certain region, we hear, was once forested. There are now few or no traces. "People" say that the climate there has "changed." Hence, the disappearance of the forests must have produced the change of climate. This is not an unfair illustration of the historical argument. Sometimes, of course, simple hearsay, and general impressions, are replaced by actual records of the change in area covered by trees, and by rainfall observations (extending over a relatively short period), or by rough accounts of the depth of water in rivers and streams. But, at best, this method of treatment is very unreliable. All the elements in the discussion are uncertain: the early forest conditions; the supposed "change" of climate; the accuracy of any available meteorological observations. Granted that a "change" of climate has actually taken place, was the so-called "change" the cause, or the effect, of the change in forest cover? And may not the "change" have been the result of the well-known oscillations of the climatic pendulum, which bring periods of wetter and then of drier years, and which are, therefore, more, or less, favorable to forest growth?
The historical argument may be illustrated by the following:
Clearly, we have nothing beyond the merest hearsay evidence in all this, and absolutely no facts upon which to base a scientific conclusion. Again, in regard to Greece:
Those with even an elementary knowledge of the climatic zones will recall that Greece, like northern California and northern Africa, lies in the subtropical belt, whose dry, or even wholly rainless summers, depend upon the great controls of temperature and pressure and winds and storm-tracks, far and away beyond the reach of any such insignificant local agencies as a few trees.
This is a good example of the weakness of the historical argument, even when apparently based upon actual observations.
We might cite further the rather hackneyed examples from Trinidad, where the cause of a general but rather slight decrease in the mean annual rainfall for ten-year periods between 1862 and 1891 (from between 66.50 and 67 inches at the beginning of the period to slightly over 65 inches at the end) has been "said to be the disappearance of the forests"; from Kimberley, where the cutting down of trees to supply timbers for the mines is supposed to have had "most injurious effects on the climate," increasing the number of dust-storms, among other effects; from Ismailia, where tree-growth since the opening of the Suez Canal is said to have brought an increased rainfall; from the Peninsula of Sinai, from Syria and from Algeria, in all of which deforestation is said to have changed luxuriant and fertile districts into deserts. One other example, quoted by a recent writer, may, perhaps, be referred to:
There is no need to multiply these examples. They show, clearly enough, why the historical method is unsafe, and why it has given but meager results.
An Essential Consideration: Why Should Forests Influence Climate?
It is a curious fact that so few of those who are firmly convinced that climate is affected by forests, ever seem to ask themselves: "Why should forests influence climate?" We seem to accept it as a fact without asking ourselves why it should be so. If we stop a moment to consider the reasons which come to mind, we shall probably sooner or later enumerate them about as follows:
(a) Because forests must retard and obstruct air movement, favoring calms, and causing the air to ascend slightly over the trees. Both of these effects may be favorable, in a small way, to rainfall. The barrier effect, by reducing the velocity of high winds, ought to moderate the extremes of winter cold.
(b) By means of their shade, trees ought to check the warming of the ground, and of the air, especially in summer.
(c) Because of the retention of moisture in the forest litter, and of the decreased evaporation which may be expected to result from the lessened air movement under the trees, it seems not unreasonab]e to expect that forest air will be somewhat damper than that outside. This, under proper conditions, may also favor rainfall.
(d) The diffusion of the water vapor transpired by and evaporated from the leaves may perhaps increase the opportunity for rainfall.
(e) We may expect the tree cover to diminish nocturnal radiation from the ground underneath, and thus to maintain a slightly higher temperature within the forest than outside of it at night.
(f) Also, there may be some effect from the increased radiating surface due to the presence of the leaves or needles. This must be chiefly effective at night.
(g) The heating of the leaves must be less than that of bare ground, because of the evaporation of much water from the leaves, and because of the slow heating of the water in the leaves. To a certain slight extent, then, a forest cover ought to behave as does a water surface; it ought to warm a little less rapidly and therefore it ought to cool less rapidly.
(h) The process of growth of the trees, and the chemical changes which are going on during their life, must require an expenditure of energy whose effect might possibly be observable in a difference of temperature between the forest and the open. The rise and return of the sap may also be expected to be accompanied by certain slight temperature effects resulting from the transfer of root temperatures upwards and of crown temperatures downwards.
In these, and perhaps in other ways, we may seek for the causes of forest influences upon climate. But, whatever may be the theoretical reasons for believing in such influences, we are here concerned only with the facts as they are at present known. One further word of caution is necessary. It is one thing for a forest to have a climate of its own within its own limits, under or above the trees. It is quite another thing for a forest to affect the climate of the surrounding country, or of distant regions. The latter effect is naturally the one in which the real interest centers. But it is also the one which is by far the most difficult to study. It is clear that nothing more than reasonably local modifications of climate ought to be expected. The special climate of the forest itself—so far as it may appear to have one—can only affect the surroundings by modifying the air currents which pass through or over it, by producing an ascending movement of the forest air to take part in the prevailing wind movement, or by causing, as may happen under especially favorable conditions, local air currents of its own. Most, if not all, of the above-mentioned theoretical effects of forests upon climate have been overestimated.
Forests as Wind-breaks
The most obvious effect of forests is that of the barrier, or windbreak. First, there is far less wind movement within the forest than there is outside. Second, friction on the tree-tops reduces the velocity of the wind blowing over the forest. Third, to leeward of the forest there is a belt of relative cairn which is roughly ten to fifteen times as wide as the forest is high, as has been determined by measurements in Iowa and in the Rhone Valley. More recently, in Roumania, Murat has shown that within 165 feet to leeward the decrease in velocity may be from four to eight miles an hour, and that the effect of the forest in decreasing velocity extends as far as 1,500 feet to leeward. Some years ago, comparative observations in the harbor, city and suburbs of New York and Boston showed a remarkable reduction in wind velocities with increasing distance inland, the velocities in the city being a little over three fifths, and those in the suburbs about one third, of those in the harbor.
Clearly, then, wind-breaks such as those which have been recommended for, and are found in, much of our western treeless area furnish considerable protection, over a narrow strip to leeward of the trees, against the sweep of strong hot or cold winds. Such a reduction in wind-velocity may have beneficial effects in reducing somewhat the extremes of heat or cold, and in diminishing evaporation from soil and from plants, and perhaps also in checking the blowing away of the soil. On the other hand, frost is more likely to occur where there is less air movement. Deforestation, on a large scale, especially on extended level areas, will therefore favor a freer sweep of the wind, which may be hostile to the growth of crops. Over any extended treeless area, exposed to high winds and with a severe climate, the best protection will be found in the planting of narrow belts of trees, alternating with agricultural strips. It should be noted, however, that this very wind-break, by decreasing wind velocity, keeps the air of the forest interior from affecting the atmospheric conditions round about. In other words, the forest diminishes its own climatic influence.
Influence of Forests upon Temperature
There is comparatively little popular interest in any possible influence of forests upon temperature, attention being almost altogether focused on the rainfall factor. Upon their soil temperatures, forests have a slight cooling effect (up to about 5°) attributable to the shade and to the greater moisture of the forest floor; the extremes are retarded and reduced; frost penetrates less deeply. Between evergreen and deciduous forests there is this difference, that in the former sunshine has freer access to the ground, and warms and dries it better than in the latter. In general, a forest climate bears a faint resemblance to a marine climate in having a slightly smaller range of temperature than the open, the extremes being most moderated in summer. In central Europe the mean annual minima are about 2° higher in the forest, and the mean annual maxima are about 4° lower. Individual summer maxima may be 6° to 8° lower in the forest, and individual winter minima 3° higher (Prussia). Conditions in the United States are probably not very different, although our greater extremes of heat and cold here would perhaps lead us to expect a slightly greater forest effect in moderating these extremes. The sum-total effect is, therefore, a slightly cooling one, chiefly because the forest is a little cooler than the open in summer, and about the same, or very slightly warmer, in winter. But these temperature differences in the average of the year are very small, and even in individual cases are certainly usually inappreciable without the use of thermometers. The considerable difference in our feelings of heat and cold ("sensible temperature") within and outside of a forest is probably chiefly due to the combination of the other factors, such as wind movement, moisture, exposure to sunshine, etc. Indeed, a good many of the reported differences between field and forest are probably too large, owing to unfavorable exposure of the thermometers. It is, however, significant that the presence of relatively cool air over forests has been indicated by the fact that balloons, in passing over forested sections, often have a distinct tendency to descend. This cooling effect above the forest is pretty clearly of more importance than any temperature effect within the forest, but we have as yet very little reliable information on this phase of our problem.
It is to be expected that equatorial forests should have more marked effects in lowering the temperature than temperate forests. The high maxima reached over the deserts of the lower latitudes, largely as a result of the excessive heating of the sandy surface, do not occur where the dense equatorial forests shade the ground; increase the radiating. surface by means of their leaves; supply much water vapor through transpiration and evaporation, and possibly also, by favoring fog and cloud formation, cut off sunshine. Woeikof has done good service in calling attention to this important function of tropical forests. We must not, however, suppose that scattering forest patches in our temperate latitudes can have any notable effects upon temperature. As Supan has well stated the case, in speaking of the very "moderate" effect of forests on temperature:
Influence of Forests upon Humidity and Evaporation
Within European forests the relative humidity exceeds that over the neighboring glades or fields by a few per cent. (2-10 per cent.). This is ancondition, and no doubt in part due to the slightly lower average temperature in the forest. The local formation of dew might be favored on this account. It appears, further, that evergreen forests have more influence in increasing relative humidity than do deciduous forests. Evaporation from free water surfaces within forests is a little less than one half of that in the open, a fact which is to be explained chiefly by the decreased air movement, and, to a much less extent, by the slightly lower temperature and the slightly higher relative humidity. In addition to the action of forests in decreasing evaporation, there is the positive effect of supplying moisture to the air through the process of transpiration. The amount of moisture thus given off from the leaves of the forest has been estimated to vary from three times that from a horizontal water surface of the same extent to less than half that from the water. Evaporation is, of course, much the most active under sunshine. In Central Europe the annual amount of transpiration in forests consisting of well-grown beeches and oaks has been estimated to be about one quarter of the total precipitation.
It is apparent that, as rain-bearing winds progress inland from the ocean, their tendency to continue rainy will be favored if they pass over extended forest areas instead of over bare soil, or even over grass or crop-covered surfaces. It is also a well-known fact that a certain portion of the rainfall of continental interiors is supplied from secondary sources not the ocean, such as lakes, rivers, swamps, and to some slight extent even from the forests themselves. But the forests must of course have received the water before they can give it up; they can not supply it by and through themselves. There seems to be no really very good reason for thinking that the rainfall conditions of the interior of North America would be very much changed if all the forests bordering on the coasts were replaced by crops or by grass. It is foolish for us to think that the forests are more important than the ocean in supplying water vapor for rainfall. Without the rainfall supplied by the vapor evaporated from the oceans the existing forests would never have grown at all. The amounts of moisture concerned in the great rain-producing processes of the atmosphere are so large that the local supply from forests can not conceivably play any considerable part. A recent German writer has stated his opinion that
Influence of Forests upon Rainfall: Why do We Expect It?
Thus we come to the phase of the discussion which is of much the greatest popular interest. Do forests increase rainfall? Does deforestation result in a decrease of rainfall? It is almost inevitable that the majority of persons should approach these questions with a fairly strong prejudice on the affirmative side. There is the general and universal impression in favor of such an influence, already referred to in the opening paragraph of this paper. In addition, the theoretical considerations above enumerated turn our thoughts in the same direction. By way of a review, then, let us ask, What are our reasons, at this stage of our discussion, for thinking that forests may influence rainfall? First, the barrier and frictional effect, which, by forcing horizontal air currents to rise, should tend to favor condensation, as cloud, and perhaps also as rainfall. The slackening of the air movement above an extended forest ought to increase the thickness of the stratum of moving air, thus giving it a slight, and local, ascending component. This same slackening effect should produce a tendency to light winds and calms, which are often favorable to showers and local thunderstorms, especially if the air is damp. Second, the damper and slightly cooler air in and over a forest may, at least to a slight extent, affect the passing air currents, especially if these are warm and dry, perhaps increasing the tendency to form local fogs, dew, or even light rain over and to leeward of the forest, provided the existing conditions are already favorable. It has even been held by some that when the process of condensation has been started, it may continue automatically, the liberation of latent heat tending to produce convectional currents.
This perhaps fairly expresses the general view of the average person at this point. However, having seen that the influence of forests upon temperature and upon humidity is so slight, even among the trees, it is unreasonable to expect that the influence upon rainfall over the forest, and especially away from the forest, will be considerable. In the great ascending, damp air masses of a general storm; in the flow of the winds across a mountain barrier; in the active convectional overturning of a summer thunderstorm—what really significant effect can the slightly damper and slightly cooler air of the forest play in the process of producing or determining the amount of the rainfall? We say, "the air over the forest is damper; therefore there will be more rainfall," quite forgetting that the damper air is useless as a source of precipitation unless it is cooled to the dewpoint. Furthermore, this moisture is constantly being carried away by the winds, and distributed through a great mass of air, thereby giving up more and more of whatever rainproducing effectiveness it may have had.
Forests and Rainfall: the Observations and the Difficulties
Whatever may be our personal prejudices, and whatever may be the theoretical considerations in favor of an influence of forests upon rainfall, what we really want is the facts, so far as they are at present available. Obviously, in a scientific study of this problem, the historical method of treatment, previously referred to; all theoretical considerations, and all prejudices, must give way before the results obtained by means of actual observations, made under approved conditions, with accurate instruments. There has been great difficulty in securing absolutely trustworthy observations. Many of the older records are clearly unreliable because of the improper exposure of the rain-gauges, the differences in the elevation or exposure of the instruments being enough to account for all the observed differences in their catch. Some excellent series of observations have, however, been carried on during the past twenty-five years or more in several European countries, by the agricultural and the forest experiment stations. A system of parallel or radial stations has been extensively used, these being located within forests and in the surrounding open country. Simultaneous observations extending over as many years as possible are compared, the greatest care being taken to have the best exposures, and to allow for the effect of the wind on the catch in the gauges.
The proper exposure of rain-gauges is one of the most perplexing problems in observational meteorology. Rainfall has long been known to be very "patchy," that is, there are considerable differences within very short distances. Thus it happens that gauges which are near together and under similar conditions of exposure often record quite different amounts of rainfall or of snowfall. Further, the catch of a gauge is markedly influenced by the exposure. In the open field, for example, where there is a free sweep of the winds across the top of the gauge, more rain-drops and especially more snowflakes, are carried over the gauge than in a more protected location, where the drops and flakes can fall more nearly vertically. Thus, a gauge in a forest clearing where the wind velocity is somewhat reduced by the trees, ought to record more precipitation than one in the open country, although the actual fall might be identical in the two cases. A difference of a few feet in the elevation of a gauge will also often result in a catch varying considerably in two neighboring gauges. Furthermore, forests affect wind directions, and this also may influence the catch in the gauges. An element of great uncertainty is thus inherent in all the earlier results obtained by observation, and indeed to some extent in the later ones also, but it should be distinctly emphasized that every effort is now made to "correct" the results for just such errors. In the majority of places where parallel stations exist, the gauges in the forest have actually shown an excess over those in the surrounding open country. Whether this is a real excess of rainfall, or only a difference in the catch, is the disputed point.
The Lintzel Case
There are four cases which have been frequently cited as showing an influence of forests upon rainfall. There is the famous Lintzel case, first cited by Müttrich. At Lintzel, on the Luneburg Heath, in Germany, the rain-gauges used to show a rainfall smaller than the average at a number of the neighboring stations. In 1877 a considerable planting of young trees was undertaken around Lintzel, until several thousand acres were covered. As time went on, the rainfall at the Lintzel station (in an open field surrounded by the forest) showed an increase as compared with that of the surrounding stations. There are, however, reasons against accepting these apparently conclusive results at their face value. The probability of error, the chance of discovering which is greatly diminished by the "smoothing" of the generalized results; the failure to make allowance for the protective effect of the increasing tree-growth; a recent change in the location of the rain-gauge; the shortness of the record, and the general variability and uncertainty of rainfall as a whole, are all considerations which, on the best of authority, may be urged on the other side.
The Nancy Case
Then there is the Nancy case, from France. This is a case of four stations (in two pairs), two in the forest and two in the open, within a small area, the altitudes and the general condition of one pair being, as one writer has said, "as comparable as stations can be made." These Nancy results showed, for a period of about twenty-five years, and for the best pair of stations, somewhat more rainfall (about one half inch to one inch in the yearly average) in the forest. In the case of the other pair the excess was much greater. This series of comparative observations was unfortunately discontinued a few years since, and although the available data have been widely used, they are, in the opinion of the leading official meteorologist of France, as expressed in private correspondence with the present writer, inadequate to serve as the basis of a serious study.
The Indian Case
The two cases just cited are in the temperate zone. The other two cases are found within the tropics. There is, first, the case of a district in the central provinces of India, where forest protection and reforestation began in 1875, and where the rainfall, as compared with the rainfall of all India, showed an increase of about 12 per cent, in a comparatively few years. This, again, seemed an unanswerable argument in favor of a forest influence upon rainfall. But the complication due to periodic oscillations of climate, various uncertainties and the possibilities of error in the observations, together with the difficulty of "correcting" the catch, acknowledged by the Indian authorities themselves, have led to a feeling that we ought at least to suspend judgment in this case. Nevertheless, because the effect of wind upon the rainfall catch is less in the tropics than in our own latitudes, and therefore the error arising from the increasing protection afforded by the growing forest is greatly lessened, von Hann (1908), the acknowledged authority in climatological matters, is ready to accept the general result of these Indian observations as evidence in favor of an influence of forests in increasing the amount of precipitation at least in the tropics. Dr. G. T. Walker, however, the present director of the Meteorological Service of India, in a recent study of supposed changes of climate in India (1910), does not find evidence of an effect of forests in increasing rainfall.
The Java Case
Finally, we may cite the Java case, which is without question the most striking of all. This case was studied and first discussed a good many years ago by Professor Alexander Woeikof, of St. Petersburg. The facts as given by him are these: There are extensive dense forests in the south of Java, while the north coast has been largely deforested. A station, Tjilatjap, on the south coast, distant from the mountains, has a mean annual rainfall almost twice as large as that of three stations (Batavia, Tegal, Samarang) on the north coast. The difference is, in round numbers, about 150 inches against 75 inches. The north side is the windward side for the northwest monsoon, and during the rainy season (December to March) should have more rain than the south, or lee, side. Yet the fact is that there is about the same rainfall on both coasts at that time. During the southeast monsoon the south (windward) side has a much heavier rainfall than the north (leeward), which is normal. On Celebes, where, according to Woeikof, no such deforestation has taken place, the windward and leeward sides have their normal values of rainfall, the former having a notably larger amount. The case is obviously a very striking one. In reply to a letter from the writer, asking whether newer data from Java tended to strengthen or weaken his previous opinion regarding this case, Dr. Woeikof said:
The Java case remains, then, on the authority of one of the best-known meteorologists, a striking example of forest influence on rainfall. So striking, indeed, is it that one is tempted to ask what other possible controlling factors are here active in producing this surprising result.
Recent European Studies
The careful observations which have lately been made in Europe by several investigators (Schubert, Hamberg, Schreiber and others) in western Prussia, Posen, Sweden, Saxony, France and elsewhere, have clearly shown that rain-gauges at forest stations, and above the forest crowns, do generally catch somewhat more rainfall than do the gauges at the parallel stations in open country at the same elevations. The excess varies roughly, we may say, between 1 per cent, and at the most 10 per cent, of the annual mean. But leading European authorities are pretty well agreed that when definite allowance is made for the effects resulting from differences of exposure, due to the better protection of the forest gauges, the apparent excess within the forest is reduced, by the probability of error, to a very narrow margin indeed. In some cases the margin disappears entirely. Schubert, for example, found a summer excess in forested areas of about 6 per cent. Of these 6 per cent., 3 per cent, he believes to be attributable to the better protection of the forest gauge, leaving 3 per cent. And 2 per cent, of these remaining 3 per cent, he thinks still liable to an error. This leaves but 1 per cent.
Conclusion Regarding Rainfall
It appears, therefore, that we have as yet no satisfactory or conclusive evidence that forests, at least in our own latitudes, have a significant effect upon the amount of rain fall, as distinguished from the amount of the rain catch in the gauge. Nor is there direct and unassailable evidence that our forests increase the frequency of precipitation, although some excellent authorities incline to the view that they do. No one can fairly be called unreasonable if he believes that, after making all proper corrections, there remains no appreciable difference in rainfall inside and outside of our temperate zone forests. Perhaps even the slight remaining differences ought themselves to be "corrected" away. On the other hand, no one can be called unduly optimistic who, knowing the many uncertainties involved in any critical study of rainfall records, gives the forest "the benefit of the doubt" and holds that it really does rain a little more over forests than in the open. But the "little" is, at best, very little, as the latest European observations have shown. We can not, if we will, make it an excess of more than a few hundredths of the total annual rainfall. The margin of difference between the two points of view is thus seen to be very slight indeed. One thing is clear. Granting that all of the observed differences between the catch within forests and outside of forests is due to an actual difference in rain fall, and not largely to the difference in exposure, the excess over the forest still remains but a small proportion of the annual rainfall. In other words, even the uncorrected observations give a maximum value for forest effects which is itself relatively slight. If, at best, forests can only produce such slight differences over and among the trees themselves, we can not suppose that they will have enough effect upon passing air currents to influence the climate of more distant regions. Hellmann has shown that an increase in the rainfall over a forest, resulting from the slackening of the lower air currents and a readier descent of the raindrops, is accompanied by a lessened fall to leeward. Thus there is equalization; simply a slight difference in distribution.
It is not altogether surprising that one writer has expressed the opinion that "no definite and unassailable result can ever be obtained" by means of such forest meteorological observations as those now made in Europe, and that "there would be little to be gained by a further study of the question." Yet this attitude will hardly commend itself to those who are anxious to have the present uncertainty cleared up, so far as possible. In view of what has already been said, it hardly needs to be stated that, in spite of the deforestation, by lumbering and fire, of large sections in the eastern United States, there is no reliable evidence of any decrease in rainfall, nor of any other change of climate. (It is, however, only fair to say that a good deal of this denuded area has been covered by second-growth timber.) Nor, in spite of the prevailing popular impression to the contrary, is there any reliable evidence whatever that cultivation and tree-planting over extended areas of the west and southwest have resulted in any increase in the amount of precipitation. There is, of course, a better conservation of moisture for plant use. We are surely within the bounds of reason when we say that there is no hope that we can increase our rainfall really appreciably or effectively by any amount of tree-planting. A whole ocean of water can not give rainfall if the general pressures and temperatures and winds are hostile to precipitation.
As was pointed out at the beginning of this paper, forests are of many different kinds. We can not, therefore, reasonably expect all forests to have the same effects. There may be a difference between tropical and temperate forests, as has already been suggested in the case of Java rainfall, for tropical weather types and rainfall conditions are different from our own, just as tropical forests are different from our own. Tropical rainfalls, as over the great forested Amazon valley, are largely thunderstorm rains, and as forests tend to check air movement, and calms are favorable conditions for convectional overturning, it appears as if tropical forests might be expected to influence rainfall more than our own. Furthermore, from the hot and damp tropical forest, and from the leaves of the closely-crowded tropical trees, there must come a large amount of moisture which will increase the vapor content of the ascending air and tend to increase condensation and rainfall. Thus Woeikof, whose emphasis on the case of Java has been referred to, believes that in low latitudes the vast tropical forests do increase the amount of rainfall. Von Hann, the leading authority on climate, holds that we may conclude "with considerable certainty that, at least in the tropics, the forest may increase the amount of rainfall." Hettner, also, in his work in the tropical Cordillera, came to the conclusion that the forests in the Cordillera of Bogotá favor the growth of clouds and the production of rain. While this is an interesting phase of our discussion, we have as yet no thorough study of tropical conditions by means of the parallel station method. There is also another point. In low latitudes, where the dense tropical forests are found, the rainfall is already so heavy that it is of little or no significance whether there is a good deal more, or a good deal less. In exactly those regions, therefore, where, if anywhere, forests may have a really appreciable influence on rainfall, little or no economic importance attaches to the question. Woeikof believes that rain often begins earlier over tropical forests, and in Mauritius, Walter has called attention to the fact that the number of rainy days seems to be greater over forested areas.
It need hardly be pointed out that, if rain is already falling, the opportunity for it to reach the earth's surface must be better if it falls through the somewhat cooler and damper air over a forest or a grass-covered surface than through a hotter and drier stratum of air over a desert. In the latter case the loss by evaporation may be so great that the drops do not reach the surface at all. Obviously, the contrasts between these two conditions are greatest in the case of the tropical forests and tropical deserts. It must, however, be observed that this effect is one of the conservation of rain already produced without the action of the forest, not a case of an increase of rainfall directly due to the forest.
Another effect of conservation may sometimes he seen when, after a rain, the low clouds ("fog") continue to hang over a forest, and may give another light shower there while no more rain falls over the fields. In this case, the drops left hanging on the leaves evaporate; the air over the forest may become very damp; a slight cooling will suffice to produce a second falling of the same water which fell previously. This is clearly not a case of an increase of rainfall. It is pretty safe to say that it would rain somewhat oftener, and a little more heavily, over tropical deserts if the surface were covered with vegetation instead of being sandy and therefore heated to a high degree, although the cause of the rain is far beyond the action of desert or forest. But tropical deserts are sandy deserts because the general condition of the atmospheric circulation makes them so, not because they have been deforested.
Influence of Forests in Collecting Moisture from Clouds and Fogs
There is one effect of trees, often observable during dense fogs, which results in the collection and precipitation of water drops which would otherwise not fall to the surface. This is a mechanical collection by trees, or it may be by telegraph and telephone wires, or by the rigging on board ship, of the fog or cloud particles carried against the object in question by the moving air. When the amount of water thus collected is sufficient, drops fall from the collector as a gentle shower. Thus there is an actual increase in the amount of precipitation, although no increase in the amount of condensation. Many years ago, Sir John Herschel, during his residence at the Cape of Good Hope, called attention to the fact that, when low clouds were closely overhead, a shower of rain might be experienced under the trees on the side of Table Mountain, whereas no rain fell outside. The explanation which he gave was inaccurate, but the fact was important. Recently, Marloth has shown that the collection of water droplets from the clouds on Table Mountain is an important factor in supplying moisture for the swamps and springs. A rain-gauge with a bunch of grass fastened on wires around its rim, so that the collected water drops would run into the gauge, gave from ten to thirty-five times as much "rainfall" as an ordinary gauge. Further, the number of horse-power furnished by a stream coming down the mountain decreased more than one half after a fire had burned off the vegetation on the top of the mountain. Abbe has called attention to the "steady dripping of trees enveloped in cloud-fog" on the windward side of Green Mountain, on the Island of Ascension. This mountain owes its name to the fact that it is always green with verdure. From its summit comes the principal water supply for the garrison, this water being contributed partly by "slight showers" and partly by the "steady dripping" just referred to. "Every exposed object," says Professor Abbe, "contributes its drip." Another case, described by William L. Hall, in the Hawaiian Islands, is that of the collection of the drip from the trees in a region of heavy fog (? cloud) in troughs for the use of cattle. In this locality deforestation would, it is stated, "reduce the productiveness of the plantations, if not ruin them entirely."
The present writer has several times, during fogs, noted the dripping of water from the wires above the sidewalks in his own city. The sidewalks being dry at the time, the drops from each wire made a wet line on the pavements. Again, when steaming through the thick fogs on the Grand Banks of Newfoundland, many of us have seen real, though gentle, showers of rain falling from the wet rigging on to the dry decks.
In winter, when the moisture freezes on the trees, the branches and twigs may become heavily covered with "frost." Fischbach has noted the fact that in winters of deficient snowfall in the Black Forest, he has several times observed that the frost shaken off of the trees by the wind has made possible the use of sledges for transporting wood. In one single European case, reported by Wilhelm, the amount of "rainfall" resulting from the occurrence of such a frost deposit on trees was not far below.05 inch.
In this mechanical collection of water particles by a forest, we seem to have a really effective means of increasing the total fall of rain. It is easy to see that if such favorable conditions are often repeated, and where the trees are tall and have many branches, the surface of the ground beneath the forest may easily receive a not inconsiderable supply of moisture. Such action on the part of forests is further aided by the fact that fogs often seem to last longer among trees. Nevertheless, we should remember (1) that the conditions favorable to this particular forest influence are found only locally, especially on forested mountain slopes and tops; (2) that the increase in the fall of rain is limited to the area covered by the forest itself, and is, therefore, not upon soil used for agriculture; and (3) that in the European observations, above referred to, this particular action of forests was at work, as well as all other forest influences, yet the results were, as has been seen, uncertain.
Influence of Forests upon Hail and Other Storms
There has been a widespread impression in parts of Europe that hailstorms avoid forests, and that forests serve to break up and to weaken other storms. The evidence on the question of hailstorms is conflicting, but we may say that the popular impression can be explained on the ground that hail naturally does more damage to tender crops than to forest trees, and that the damage in the former case would attract, in the latter would largely escape notice. Further, as regards storms and high winds in general, forests do, as has been seen, tend to check wind velocity, and thus to reduce the local violence of a gale. On the other hand, however, recent investigations in Germany have shown that in thunderstorms the obliquely descending component of the wind can be but slightly, if at all, affected by forests, whose trees are easily uprooted by these winds.
The Hygienic Influence of Forests
There are several ways in which forests have a hygienic significance, and the location of many of our well-known health resorts in or near extended forest areas is, therefore, well planned and logical. The reduced wind movement; the protection against the severest extremes of summer heat and of winter cold; the marked decrease of dust and of other atmospheric impurities; the grateful shade and lack of glare on sunny days; the relatively small number of microorganisms—all these are helpful, not only to those who are ill or convalescent, but to persons in good health; all these are arguments in favor of wooded parks in and in close proximity to our cities. In addition, but of non-climatic importance, there are the scenic attractions of the forests; the relief from the noise and the bustle of the city; the fragrance of the air among' the evergreen trees, and the frequent intermingling of river and lake and mountain, all of which features contribute to the popularity of forest sanitaria and pleasure resorts. So far as the composition of forest air is concerned, there is no further notable difference between it and the air outside. We can not, therefore, look for any marked curative effects on that account. The much-discussed beneficial effects of the ozone in the forest air seem to lack the support of observation.
The Influence of Forests upon Water-supply, Erosion and Floods
The preceding discussion has dealt with the influence of forests upon climate. Therefore no mention has been made of their relation to the conservation of the water-supply, to erosion and to floods, all of which are non-climatic, or at any rate only indirectly climatic effects. There is still a great deal to be learned about the use of forests in connection with water-supplies; their effects in holding back rainfall and in storing the winter snow; their relation to floods, and ground-water, and springs and erosion. The "last word" in this discussion is to be found in the "Final Report of the National Highways Commission" (Sen. Doc. No. 469, 62d Cong., 2 sess., 1912). From this report we take the following statements, which are of peculiar interest, because they represent the conclusions and recommendations which have been reached after a thorough study of the different phases of this many-sided problem. It is easy to see why different observers, under different conditions, have reached such divergent results.
But these problems do not directly concern the climatologist. He is satisfied if he can make clear, as he sees it, the influence of the forest as a control of climate. If his statements are often disappointingly broad and generalized, it is because he has not the needed scientific basis for making them otherwise.
- In 1882-86 Lintzel had about 90 per cent.; in 1887-91 it had about 102 per cent.; in 1892-96, about 118 per cent.