|Economics of forestry; a reference book for students of political economy and professional and lay students of forestry, by Bernhard E. Fernow|
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Economics of forestry; a reference book for students of political economy and professional and lay students of forestry, by Bernhard E. Fernow
CHAPTER III. THE FOREST AS A CONDITION.
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The earth may be said to be a potential forest. A cover of tree growth more or less dense is or has been the natural condition at least of the larger portion of the habitable earth; and of the entire land surface not less than 60 per cent may be classed as actual or potential woodland.
In the struggle for existence and for occupancy of the soil between the different forms of vegetation, tree growth has an advantage in its perennial nature and in its elevation above its competitors for light, the most essential element of life for most plants. These characteristics, together with its remarkable recuperative powers, assure to the arborescent flora final victory over its competitors, except where climatic and soil conditions are not adapted to it.
The entire absence of tree growth from some localities, such as the northern tundras and the high peaks above timber line, is due both to temperature and soil conditions. Here the two characteristics of perennial life and persistent height growth, become unfavorable, since extreme winter
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temperatures above the snow cover, droughty winter storms, and frosts every month in the year can be endured only by those plants which have a rapid cycle of development, or are sheltered near the ground by the snow cover; the wet soil on the tundras, frozen for most portions of the year, or the thin soil on the Alpine peaks, adds to the difficulties for deep-rooting species in their contest with the lower vegetation. Again, in the interior of continents and other localities unfavorably situated with reference to the great sources of moisture and moisture-bearing currents, deficiency of water, namely scant rainfall or low relative humidity, or both, and excess of evaporation, are inimical to tree growth. Occasionally soil conditions, especially with reference to drainage, and climatic conditions combined, may be more favorable to the graminaceous vegetation, at least for a time, giving rise to pampas, prairies, and savannas; or else the unfavorable conditions combine to such a degree as to give rise to deserts.
In addition, there are other inimical agencies in the animal world, which prevent the progress of forest growth and tend to preserve the prairie: locusts, rodents, ruminants, buffalo, antelope, horses, etc., impede the growth and spread of trees; and especially where compact soil and deficient moisture conditions are leagued with these animals, the change from prairie to forest is prevented, at least for a time.
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Woodlands are the most unfavorable form of vegetation for the life of ruminants, and therefore for the support of the largest number of men. For food production, for agricultural pursuits, man must subdue and remove the tree growth. Hence forest devastation, forest destruction, is the beginning of civilization, its necessary prerequisite.
But while the removal and repression of the wood, as an impediment to culture and food production, is a necessary step toward a higher civilization, the fact that at the same time it furnishes material equally indispensable in building up a civilization requires consideration also, and the necessity for its preservation in part, its continuance in possession of some portions of the soil, is indicated.
Happily, the very soils and situations which are not fit for agriculture are still capable of supporting tree growth; and although the best timber, no doubt, may be grown on land most favorable to agricultural crops, the poorer soils and mountain slopes unfit for plough land will still yield wood crops of useful description.
In reducing, therefore, the woodland condition to one adapted to the highest civilization, the relegation of the different soils and sites to the different uses to which they are best adapted, as fields, pastures, or forest, is a problem of true national economy.
Besides the consideration of a proper proportion of woodlands to furnish the needful supply of wood
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material,--supply forests,--there are other considerations which enter into this problem of the economic use of the soil and of distributing the various conditions of its occupancy. These are based upon knowledge of what we may call forest influences: the influence which the existence of a forest cover as a surface condition of the soil exerts upon soil conditions, temperature conditions, and water conditions, and by virtue of which we may characterize them as protective forests. While the most economic use of the soil for material production necessitates relegation of forests to the poorer soils, protective considerations necessitate its relegation to certain localities.
While our modern philosophy of nature readily perceives that all things are interdependent, and hence no change can take place in one condition without corresponding changes in other conditions, even the oldest civilized men intuitively recognized or at least suspected and appreciated the fact that the forest cover had some influence upon its surroundings, upon climate, health, and water conditions of a country, as is evidenced by many sayings of Mosaic, Roman, and Greek writers, by which farsighted priests prevented their destruction. The consecration of groves to religious use and various mythological conceptions connected with them, point in this direction.
Thus Homer calls the mountain woodlands the habitations of the gods (tεµε?? a&thgr;a?at??), in which
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the mortals never fell the trees, but where they fall from age when their time has come. His tree and woodland nymphs, originating in springs, seem to suggest the suspected relation of forests and springs. The legend of Erichthonios most beautifully hints at the dependence of agriculture and forest cover: when, by the felling of a holy oak, he has offended the dryads, Ceres, the patroness of agriculture, is asked to send one of their number to the mountains of the Camasus to fetch Famine, who takes hold of Erichthonios and kills him.
These relations, thus darkly hinted at in earliest times, became more clearly recognized by philosophical writers. While Aristotle, in his "National Economy," points out that an assured supply of accessible wood material is one of the necessary conditions of existence for a city, Plato, in his "Civitas," writes of the "sickening of the country" in consequence of deforestation. The Roman "Twelve Table Laws," the organic law of the republic, recognizes the necessity of forest protection, and Cicero, in his second Philippica, designates as enemies to the public interest those engaged in forest devastation. Laws prohibiting forest destruction in the mountain forests of the Apennines were generally enforced in the early middle ages; as, for instance, in Florence, where deforestation within one mile of the summit of the Apennines was forbidden, and it was only about the first part of the eighteenth century that these wise provisions
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which had preserved the cover of the higher mountain ranges were abolished and the present sad condition of things was inaugurated in Italy.
Mesopotamia, once praised as the paradise of fertility, where, according to Herodotus, the culture of the grape could not succeed on account of its moisture, has become a sand waste, in which the Euphrates, once an ample source of water supply, is drowned. Most of the springs and brooks of Palestine, and with them the fertility still celebrated in the early middle ages, have gone. Greece shows the progress of a similar decadence; Sicily, once the never-failing granary of the Roman Empire, once well wooded, now entirely deforested, suffers from repeated failures of crops. The so-called fumari, deep gullies in gravel, filled with washed débris, encroach after every rain upon the fertile fields, emptying them of water in a few hours.
The first definite expression of such relations of forest cover to climate appears in a biography of Admiral Almirante, written before 1540, by the Spaniard, Fernando Colon, in the following words:--
"The Admiral ascribed the many invigorating, cooling rains, to which he was exposed while sailing along the coast of Jamaica, to the extent and density of the woods which covered the slopes of the mountains, and adds that formerly Madeira, the Canaries, and Azores enjoyed the same abundance of water, but that since the woods which
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had shaded the ground have been decimated, the rains have become less frequent." Similar language is laid into the mouth of Christopher Columbus in the "Historia de S.D. Fernando Columbo," 1571, which is supposed, however, to be a spurious work.
But it was not until the beginning of the eighteenth century that both in France and Germany voices became loud regarding the evil effects of forest devastation, and then, too, the growing deficiency of material supplies formed a still more prominent argument for action. Thus, in France, where--in spite of Sully's celebrated epigrammatic warning, "La France périra faute des bois," and Colbert's forest ordinance of 1669--only indifferent attention to a conservative forest policy was paid, the members of the académie royale, Buffon (1739), and later the Marquis de Mirabeau (1750), exerted themselves to bring about a better conception of the value of forests.
Buffon expressed himself, as a result of extended observations, that "the longer a country is inhabited, the poorer it becomes in forest growth and water." But the most forcible demonstration of this relation between woods and waters was had as a consequence of the extensive forest devastation which took place during the years of the French Revolution, when an unrestricted people in their greed denuded large tracts of mountain woodlands in the southern mountain districts of
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that country. So soon did the evil effects become apparent, that even in 1792 the governor of the Department of Basses-Alpes reported: "The clearings progress rapidly; from Dique to Entrevaut the mountain slopes have been denuded of the finest forest growth; the smallest brooks have grown into torrents, and several communities have lost by floods their harvests, herds, and houses."
In 1803 the agricultural society of Marseilles complains as follows: "The winters have become severer, the summers drier and hotter, the beneficial rains of spring and autumn fail; the Méjeanne river, flowing east and west, tears away its banks with the smallest thunder-storm, and inundates the richest meadows; but nine months of the year its bed is dry, since the springs have given out; irregular destructive thunder-showers are of yearly occurrence, and rain is deficient at all seasons."
Yet, in spite of these early warnings, which were supported by theoretical discussions of such sound reasoners as Boussingault, Becquerel, and others, action to stem the destruction and to recuperate the lost ground was obtained only within the last forty years, after at least 1,000,000 acres of mountain forest had been denuded, and all aftergrowth had been destroyed by fire and excessive grazing, in consequence of which the mountain streams, turned into torrents, had laid waste about 8,000,000 acres of tillable land, and the population of eighteen departments had been impoverished or driven
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out. Now, although with the expenditure of more than $40,000,000 only a small part has been recuperated, the efficiency of a forest growth in holding the soils of the slopes and retarding the run-off water seems experimentally demonstrated beyond peradventure.
In Germany the greatest exponent of natural philosophy, Alex. von Humboldt, from observations in many parts of the globe, came to the conclusion that forest conditions and climatic conditions are intimately related. Among the causes which tend to lower the mean annual temperature, he cited in his "Cosmos," "extensive woods, which hinder the insolation of the soil by the vital activity of their foliage, producing intense evaporation owing to the extension of these organs, and increasing the surface that is cooled by radiation, and acting consequently in a threefold manner, by shade, evaporation, and radiation;" and in another place he gives expression to his conviction of the relation of forest cover and water conditions in the often-cited words, "How foolish does man appear to me in destroying the mountain forests, for thereby he deprives himself of wood and water at the same time."
In the beginning of this century, when the tendency of dismembering and selling the forest property accumulated by the state governments began to spread, in part as a consequence of Adam Smith's doctrine, those opposed to such a
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policy, especially in Germany, made vigorous propaganda for the theory of the protective value of forest cover, and, as is natural for propagandists, made many sweeping and extravagant claims, and an extensive literature, characterized by vigorous declamation of unsubstantiated facts, and by absence of exact data, was the result.
The condition of Palestine and other Eastern Countries, of Greece, Sicily, and Spain, once fertile, now more or less desolate, was cited, and morals were drawn from these experiences; discrimination as to historic evidences of cause and effect was mostly wanting, so that this historic method of discussing the problem has been largely discredited.
Systematic attempts to establish by experiments and exact methods the truth in the matter, at least as far as climatic influence is concerned, were made only within the last thirty-five or forty years. In France, Becquerel began in 1858 a series of observations on temperatures within and without a forest cover; in 1866, the forestry school at Nancy was engaged in determining moisture conditions at stations in the forest, and later in the open; and several other investigators, both in France and Germany, carried on such observations about the same time. In 1868, the Bavarian government instituted an exhaustive series of observations under Dr. Ebermayer, to determine the climatic conditions within a forest area. Switzerland followed
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with three pairs of parallel stations, and in 1875 Prussia established an investigation, which still continues, with seventeen stations, observations being taken at each on instruments set up within the forest and another set in a neighboring field. In 1884, Austria instituted as series of radial stations at which not only the difference of meteorological data within and without a forest, but the influence of the forest on its surroundings, were to be measured directly.
Although, by these many and long continued observations, some valuable facts have been established, and our ideas as to the elements which enter into the problem have been cleared up, the real object of inquiry, namely, whether and how far forests exercise an influence upon climate, cannot be said to have progressed far to a solution, and it is questionable whether the present methods will ever lead to a solution.
The reasons for this failure are at least three-fold. Both instruments and methods of meteorological inquiry are as yet unsatisfactory. When, for instance, rain gauges will, according to their construction, the manner of their position, and the character of the wind and rain, during the same storm, register amounts varying from 7 to 40 per cent, we are without any means of applying a constant factor of correction, and it would appear that no reliance can be placed on such measurements for the purpose of determining the difference of rainfall
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within and without the forest. The difficulty of finding stations within and without the forest which differ in no other respects than the forest cover, excluding all topographic and other influences upon meteorological phenomena, is well-nigh insurmountable.
Finally, whatever we may be able to do in ascertaining the single meteorological data that give us an insight into the differences regarding these single elements under varying conditions, the difference in their combined effect, which we know as climate, still requires the application of a philosophical mind to the interpretation of the data. Hence we find that not only are the collected data often discordant, but the same data have been used by students of the question both to assert and to deny proof of the existence of forest influences. In other words, the problem is too complicated for our present means and methods to be settled by the mathematical method.
We are, therefore, for the present, thrown back upon the method of general observations in the field and the application of reasoning from well-known physical laws, for this is one of those problems which withdraw themselves from exact mathematical treatment now, and we must rely upon empiricism until we have further advanced in developing the means and methods of meteorological inquiry.
The immaterial influence of the forest is claimed to extend in at least four or five more or less separate,
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yet, nevertheless, more or less closely related, directions, namely:--
This last influence is one which we cannot measure or even argue with any determinable force, but which we ourselves may feel more or less strongly, according to the degree to which our emotions in general are susceptible. In either of the other directions in which an influence of forest cover is asserted, the mechanical obstruction which it represents is the principal effective element; the physiological functions of the living plant playing, to be sure, a part, but of much less importance, probably, than has been often supposed.
It requires no instrument to find out that the effective temperature is higher when the sun has full sway upon our skulls than if we interpose the shade of a densely foliaged tree to obstruct the sun's rays; on the other hand, the cooling breeze, which may pass over the open field, is also obstructed by the forest growth, and its absence may make the air temperature appear higher, even in spite of the shade. Again, it stands to reason
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that a dense old growth, such as one may find here and there on the Pacific coast, with trees towering 250 to 300 feet above ground and so close together that no ray of light reaches the soil, must have a different effect from the low and scanty growth of cedar and piñon which we find on the slopes of the Rocky Mountains and elsewhere, or the young coppice growth of New England, interposing but little shade. Whether the forest lies to the leeward or in the direction of the prevailing wind, whether it be coniferous and evergreen through the year, or only summer-green, will also have to be considered in estimating its protective value.
While the single tree undoubtedly acts in the same manner as a collection of trees, its influence cannot reach very far beyond its surroundings, nor can it be very appreciable. It is also quite evident that neither a few scattered trees and bushes, nor a belt of trees, like a wind-break, nor a small clump of trees in a large open field, nor even an extensive orchard, can act singly as practically appreciable climatic factors, although all these aggregations of trees must have their influence upon their surroundings.
It is the effectiveness with which sun and wind are excluded from the soil, and thereby air temperatures and air humidity are modified, that determines also the degree and distance beyond the limits of the cause to which the modification is felt.
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In other words, while the quality of the influence remains the same, its degree, and especially its effective and appreciable value, must vary as much as there are varying local conditions possible. The size and character of the forest, its density, height, situation, and composition, are of more importance in determining its influence than is usually realized by those who discuss the question.
Another matter which it is also necessary to accentuate, because it is usually overlooked, is that the influence, if any, can only be of local character, it must therefore be discussed only with reference to given local conditions. It cannot be put in comparison with that of the large oceans, the great air currents, the extensive mountain ranges, which determine the general or cosmic climate. The forest can modify only locally the effects of this general climate, in about the same manner as we modify it by building houses around us and heating them, whereby we change the temperature and moisture conditions at least in our habitation; or by building cities, which we know differ, as far as our feeling is concerned, from the climate of the adjoining country.
It may also be proper here to state that, in view of the fact that whatever influence exists, it is dependent on local conditions, the attempt to fix a certain general percentage of forest cover as necessary for a country is childish, and also that there are conditions where the existence of forest
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growth is at least practically prevented by climatic conditions,--although the limits are by no means known,--and hence no expectation can be had of utilizing this influence in these conditions.
Again, since undoubtedly the forest influence on surroundings, as far as climatic factors are concerned, can extend only to a limited distance, the most effective result must be secured by alternations of forest cover and open land, hence the distribution of these two conditions is of as much importance as the relative size of the parcels.
Without going into the detail of the difference of meteorological conditions that may exist in the forest and the adjoining open country, it may be briefly stated that the tendency of a forest cover is to reduce extremes of high and low temperature in about the same manner as does a sheet of water, and this effect is most noticeable in the hot months. But whether and how far this temperature difference is felt outside is not as yet determined. Nor do we know much regarding the important influence on the moisture conditions of the air and on the rainfall. The tendency of a forest growth would be, on account of its cooling effect, to keep the air within and to some extent above it nearer saturation, and as a consequence it might occur that moisture-bearing currents passing over would precipitate their moisture more readily above or near the forest growth. Whether they do is still doubtful, and indeed, to make an appreciable difference
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in the amount of rainfall, it would appear that the forest area must be of considerable extent.
Although some writers have, from existing measurements, argued an influence on precipitation, others have denied it. As stated above, we hold that no reliable rainfall measurements are, as yet, obtainable, and we must leave the question open.
The more readily conceivable effect of a forest growth on moisture conditions of the air is that which it has in common, probably in increased degree, with the so-called wind-break. By breaking the velocity of dry winds and possibly enriching them somewhat with moisture, the rate of evaporation over a neighboring field is considerably reduced, so that, in regions where winds are common, the protection shows itself in increased crops on protected fields.
The same protection against cold winds may make life more bearable, and enable the growing of crops which could otherwise not succeed. Thus it is believed that during the abnormal frosts which a few years ago killed most of the orange groves in Florida, many which had good forest shelter survived. It is also reported that in France the cultivation of the olive has become impossible in the more northern departments, owing to deforestation. On the other hand, it may happen that the opening toward warmer southern winds may modify a severer climate favorably. This consideration again points to the entirely local
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character of forest influences, which may change their value.
As far, then, as forest influence on climate is concerned, we must admit that no satisfactory conclusions have been reached, excepting as to the favorable wind-break effect. That wholesale forest destruction and removal must change the climatic conditions of the denuded area seems an entirely reasonable assumption.
The climatic influence of the forest upon its neighborhood would finally consist in the communication of its own climatic characteristics; i.e. shorter range of thermometrical extremes and more even humidity, in general modifying extremes of winter and summer.
The influence on waterflow, although much fewer attempts at exact determination have been made, seems much more generally admitted. Here, too, extravagant claims have been made as to the efficacy of forest cover, while other factors which influence waterflow have been often given less consideration than they deserve. Thus the topography and the geologic structure exert necessarily a potent influence, which a forest cover may either not be sufficient or else is not needed to modify.
The philosophy of the influence on waterflow rests mainly upon the recognition that the rain and snow waters penetrate more readily a forest-covered soil than one that is bared of this protective cover. The action here is of a threefold nature:
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first, the mechanical obstruction which the foliage offers reduces the amount of the water which reaches the soil and lengthens the time during which it can do so; the foliage, together with the loose litter of the forest floor, also reduces the compacting effect of the raindrops and the drying effect of sun and wind, and keeps the soil granular, so that the water can easily percolate; then the mechanical obstruction which the litter, underbrush, and trunks, and possibly here and there moss, offer to the rapid surface drainage of waters, lengthens the time during which this percolation may take place; and thirdly, the network of deeply penetrating roots, live and decayed, offers additional channels for a change of surface drainage into subdrainage. In addition, it is claimed that, owing to the influence on temperature and moisture conditions of the air, together with reduced evaporation, more water becomes available to the soil, and certainly the fact that the water, by ready percolation, is withdrawn from the dissipative effects of sun and wind must tend in this direction.
The sponge theory so often proclaimed by lay writers is rather a misconception of physical laws and of the behavior of a sponge, although a moss-cover--which is by no means the usual cover of a forest soil--may be of great value in preventing rapid surface drainage. This is attested by Robert Gerwig, the builder of the St. Gotthard railway:--
"One German square mile of moss-cover," he
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says, "can retain 1,000,000 to 1,500,000 cubic meters of water (1 English square mile will hold 14,000,000 to 20,000,000 cubic feet). It will, in many cases, depend on a difference of 20 to 30 cubic meters (700 to 1000 cubic feet) per second of waterflow from the surface of a square mile, whether a flood will be dangerous or not. The bare slope would give up these 20 to 30 cubic meters per second, and deliver the 1,000,000 to 1,500,000 cubic meters in 15 hours. If it is remembered that damaging flood-waters are of short duration, it becomes evident how even moderate assumptions regarding the amount of water retained in the moss-cover (or in the forest litter and soil of a forest) produce favorable results."
It stands to reason that in this direction the condition of the forest cover must have much to do with the degree of its effectiveness, and that in this connection the condition of the forest floor is of more moment than that of the leaf canopy. Hence we may find that while the tree growth may be left intact, yet, if the loose litter and under-brush has been burned off and the soil been compacted by the tramping of sheep and cattle, the effectiveness in regulating waterflow is much impaired.
It is also apparent that with heavy rainfalls and on steep declivities on compact and sparsely fissured limestone rock, even the best-kept forest growth may not be capable of retarding the surface
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drainage long enough to prevent a resultant flood in the river.
Particular interest in this connection attaches to the influence of forest cover on the melting of snow masses, which gives rise to spring floods. In the dense forest, the snow is usually less deep, a part being intercepted by the crowns of trees and evaporated, and lies more uniformly, owing to the absence of drifting winds. It is a well-noted experience that it will lie in the shade of the woods from one to two weeks longer, i.e. melt so much more slowly. These elements of distribution in space and time must have an influence upon the rapidity of surface flow, and if the soil is not frozen, time is given for percolation and gradual removal.
Here, again, weather conditions may be unfavorable, the soil remaining frozen and the melting proceeding rapidly, when the forest effect may be lost. Nevertheless, while the forest effect may become powerless in exceptional cases and under special conditions, the tendency of changing surface drainage into subterranean drainage must be beneficial in the majority of cases. It may also happen that the soil conditions, by their loose structure, as in cinder cones, lava, or loose sand hills, are such as to permit percolation readily, when the office of the forest cover can be dispensed with.
The value of the change of surface drainage into subterraneous drainage becomes apparent in
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the more even riverflow. While the waters that run off over the surface collect rapidly and are carried away in floods, giving rise to high water stages, the percolated water finds its way into the river slowly by underground channels, feeding, on its way, springs and brooks, or is collected as ground water by seepage at lower levels.
This distribution of the water, which lengthens the time during which the atmospheric precipitation can be usefully employed, and which, under circumstances, may lengthen the supply for years, the water reaching the river years after it fell on the mountain top, renders the riverflow independent of wet and dry seasons, and equalizes its flow,--a condition of most importance for all industries dependent on water-power, navigation, irrigation, etc.
This forest effect on the run-off of terrestrial waters is naturally greatest and most important in mountainous regions, where the water has the tendency to collect quickly and to be carried off rapidly, but it also exists in the level plain, where it has the tendency to elevate the general ground-water level and thereby make a reserve available during times of drouth.
In close connection with these effects of forest cover upon the flow of water stands its influence on the stability of the soil. The tendency of the rain waters falling on hills and mountains is to carry in their descent to the valley loose particles of soil
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with them, and as the little rivulets run together and acquire force, gravel, stones, and even large rocks and boulders are broken loose and moved to lower levels by the torrent. This action, known as erosion, takes place everywhere more or less rapidly, according to the presence or absence and character of the soil cover, and no better and more efficient protection against it is to be found than a dense forest cover.
A grass cover may also protect the soil underneath against the erosive action of the waters, whenever the declivity is not too steep, but since the rains do not penetrate through the dense greensward of the mountain meadows, and hence are carried off superficially, they acquire a momentum which finally leads to the same gullying and erosive action which a naked soil experiences.
The forest alone is capable of obstructing the mechanical effect of the rainfall upon the soil, and retarding the rapid surface drainage which becomes the carrier of the débris. Here, again, the condition of the forest floor, rather than the tree growth, is the effective element.
If it is considered that, in the United States, the amount of erosion at present may be estimated at 200 square miles per year, rendering thereby large areas of fertile soil unfertile and at least temporarily useless for human occupancy, the economic importance of a conservative policy for the mountain forests may be readily apparent.
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The experiences of France in this particular are incontrovertible arguments, and furnish, in later years, experimental evidence of the effectiveness of a forest cover in arresting the progress of erosion. France, too, furnishes perhaps the most striking and most extensive example of how the loose, shifting sands, the dunes and sand hills in the plain, may be changed by a forest cover from a useless, nay dangerous, condition into one of profitable occupation.
Regarding the sanitary influence of forests, there have also been many claims made which cannot be substantiated. The original principal claim was that the physiological action of the foliage, in absorbing carbonic acid from the air and exhaling oxygen, made forest air healthier, but it has been calculated that the amount of oxygen so exhaled is insignificant in proportion to the needs of human respiration, and is probably offset by the increase of carbonic acid resulting from the decomposition of organic matter in the forest.
Then it was claimed that by the transpiration of the foliage wet ground may be drained, and thus made healthier, and in this connection the Eucalyptus plantations at the monastery of Tre Fontane in the Campagna Romana are frequently cited as having removed the malarial conditions of that region. As a matter of fact, the fevers still occur, even under the Eucalyptus plantation, although more rarely. This comparative improvement seems
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attributable mainly to the rebuilding of the old Roman drainage canals, which had been allowed to collapse, and the malaria-breeding mosquitoes have been reduced thereby. In any case, where drainage is to be secured, artificial canalization could probably be made more effective than forest planting. Nevertheless, a sanitary influence exists, as every one can experience, but it is mainly of a negative character: absence of smoke, dust, obnoxious gases, and of strong winds which characterize the air of cities, and which to some extent (at least dust and winds) occur in the open, renders a forest region more healthful.
Furthermore, it has been found that forest air is more free from pathogenic microbes. Especially those bacilli which develop in the soil, like the cholera, typhus, and yellow fever bacilli, find in the forest soil less favorable conditions for development, and, owing to the absence of strong winds, are less apt to be carried into the air, where they would be breathed by man. In fact, in the dense forest, where the variation of soil moisture is small and decomposing humus keeps the soil acid, no pathogenic microbes have as yet been found. Here, too, to be sure, the degree of effectiveness must depend on the condition of the forest and especially of the forest floor.
It is also not impossible that the opening of large swampy forest districts may improve health conditions by changing moisture conditions; this
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especially with regard to malarial diseases. These are not produced by bacilli, but by parasitic protozoa (Plasmodium malariæ), which seem to thrive in the swamp conditions. As long as the water covers the soil, there is no danger, but as soon as the water recedes, the plasmodia develop, and with the assistance of mosquitoes or by other means are communicated to man.
A further indirect sanitary influence must not be overlooked in our modern economy of city life. The recuperation of bodily energy and of spirit which an occasional sojourn in the cool, bracing, and inspiriting forest air brings to the weary dweller in the city must not be underestimated as an element in the general health conditions of a people. In addition, the question of a good water supply is being recognized as more and more dependent upon the condition of the sources of supply.
Knowing that a large number of diseases are bred in soils, it becomes essential that the drinking water carry as little soil particles as possible, and although, by artificial means of filtration and sedimentation, the river water may be freed of sand and bacilli, we would have more assurance of freedom from disease, if the water came from a well-forested region, where, as we have seen, no pathogenic bacteria are produced, and if the washing of the soil into the river on the way to the reservoirs were prevented by proper attention to preventing the erosion along its banks.
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Summarizing the present knowledge of forest influences and viewing it from the standpoint of the practical economist, it will appear that there is sufficient evidence of the value of properly located forest areas, as affecting at least water and soil conditions in a marked degree, and in a minor degree health and climatic conditions, to make the subject of forest conservancy one of great importance. Especially is this the case with the forest cover on mountain sides and in the hill country, where the destructive tendencies of the water are apt to gather force, if not modified by the obstruction of the forest floor.
It is always to be kept in mind that not the extent, so much as the location and condition of the forest cover is of greatest importance, and that the effect can be determined only with reference to local conditions in every particular case.
The protection of the soil cover at the head waters streams thus becomes a concern of state activity, and the establishment of forest belts in drouth-ridden countries, or the fixation of sand dunes and drifting sands, becomes a public work of internal improvement.
In the Appendix will be found further details regarding the measured forest influences, in the form of a resumé, taken from Bulletin VII, Forestry Division, U. S. Dept. of Agriculture, entitled "Forest Influences," 1893, in which this question is exhaustively discussed.