Popular Science Monthly/Volume 19/October 1881/Forest-Culture in Alpine Ravines
|FOREST-CULTURE IN ALPINE RAVINES.|
WHATEVER differences of opinion may exist respecting the meteorological influence of forests, it is generally agreed that in mountainous countries they play an important part in regulating water-courses and in preserving the soil on the slopes of the hills. This function, which has been observed for a long time, was presented in a clear light by M. Surrell, engineer of bridges and highways, whose fine work on the torrents of the high Alps ("Torrens des Hautes-Alps"), published in 1841, has been the point of departure for all studies and all legal projects respecting rewooding. While the author had in view only the restoration of the French Alps, his conclusions are applicable, although in different degrees, to all mountainous countries; but the phenomena which he considers are manifested with the most intensity in the Alps, and the renewal of the woods there is imposed as a real measure of public policy.
In respect to vegetation, nature has divided the Alpestrian mountains into three zones: creating on the summits, around the rocks and glaciers, pasture-lands on the middle slopes, forests; in the lower valleys, lands suited to occupation by agriculture and by villages. Unfortunately, this natural division has been too often disturbed; the inhabitants, leaving the valleys, have established themselves in the higher regions, have cut down the forests around their houses, and devoted to cultivation lands which, disintegrated by the plow, are incessantly cut up into ravines by every rain; or the zone of pasture-lands has encroached on that of the forests, and has been increased by the daily devastations of the shepherds. Extending its borders every year lower down the mountain, it has finished by taking possession of the slopes and despoiling them of their wood. Gradually the grass itself, no longer protected by the cover of large trees and continually fed upon by hungry flocks, has disappeared, leaving behind it only the denuded flank of the mountain, an easy prey of which the torrents have not been slow to take possession.
The mountain-torrent is not an ordinary brook, but is a stream with characters of its own and peculiar ways. Originating in a narrow basin, the bed of which is very steep, it is subject to sudden variations; often wholly dry, it becomes a flood after a storm, and overcomes all the obstacles that oppose its course. There are clear torrents and muddy torrents. The former, which are the torrents of eruptive regions, carry but little matter with them, and are characterized by sudden freshets, which are due to the fact that the waters running over impermeable rocks, are precipitated immediately into the ravines and collect in considerable masses. The torrents of the second class have formed themselves beds in the loose soil, are continually washing away the bases of their banks, provoking slides, carrying with them solid matters derived from the degradation of the hills, and discharging them in the lower valleys and covering the fields with a thick mud. The bed of the torrent is washed out more and more, and the banks increase at the same time; new ravines are formed, and branches of them, thus eating away the spur of the mountain, which is gradually destroyed, or which, undermined at the base, occasionally slides bodily into the valley, which it closes up.
Attention has long been given to devising means to limit the ravages of these torrents, which ruin the land, threaten estates, destroy roads, and sometimes even compromise the existence of villages. Walls have been built along the banks to protect them, or across the streams to allay the force of the waters. The most efficacious means, however, as yet discovered, has been to maintain the woods on the slopes of the mountain. The effect of cutting away the trees in promoting the formation of torrents has not been doubted by the inhabitants of mountainous regions, and is clearly set forth by M. Surrell, who says: "When we examine the tracts in the midst of which torrents of recent origin have been formed, we perceive that they have in all cases been despoiled of their trees and bushes. If, on the other hand, we examine hills whose sides have been recently stripped of wood, we observe that they are cut up by numerous torrents, which have evidently been formed very lately. Here is a remarkable double fact: wherever there are recent torrents there are no longer forests, and wherever the ground is cleared these torrents are formed; and the same eyes that see the woods fall on the declivity of a mountain, may see appear there immediately a multitude of torrents."
The disastrous consequences of removing the woods from the Alps began to attract attention in the last century, and have since been discussed in many publications and official reports. In 1853 the prefect of the department of the Lower Alps said in a report to the Minister: "If prompt and energetic measures are not taken, it will be almost possible to designate the precise moment when the French Alps will become a desert. The period from 1851 to 1853 will produce a new diminution in the number of the population. In 1862 the Minister will remark a continuous and progressive reduction in the number of hectares devoted to agriculture; each year will aggravate the evil, and in a half-century France will count more ruins and one department less." The departments of the Upper and Lower Alps actually lost thirty thousand inhabitants, or one ninth of their population, between 1851 and 1876. A law for recovering the mountains with wood, which had been prepared by M. Forçade de Rouguet, director-general of the administration of the forests, was adopted by the legislative bodies in 1860, and was put in operation shortly afterward.
In some cases, the work of planting woods is left optional, but is encouraged by the offer of rewards to the communes or individuals who undertake it. In other cases, where the public interest is at stake, the state determines the area of land to be planted, and allows the proprietors to perform the work if they will; if they refuse to do so, it attends to the matter itself, taking care that its proceedings shall be as inoffensive to the people as they can be made, and be at the same time effective.
The first question that is presented in dealing with a hill that has been cut up with ravines is to determine the perimeter of the lands to be restored. The area should not be limited to the banks of the torrent and its branches, for these banks, being continually undermined and always changing, would continue, by giving way, to enlarge the basin if they were not themselves fixed by vegetation. The justice of the rules which M. Surrell laid down on this point in 1841 has been confirmed by experience. We should begin, he said, to trace along either bank of the torrent a continuous line which shall pursue all the inflections of its course, from its remote origin till it issues from the gorge upon the lower valley. The tract included between each of these lines and the crest of the banks would form what I would call a zone of defense. The zones of the two banks would meet at the upper end, following the contour of the basin, and would thus surround the torrent, like a belt, in its whole extent. Their width, which should vary with the degree of the slope and the consistency of the soil, may be as little as fifty yards at the lower end, but should increase rapidly as the zone rises up the mountain, and should end by including spaces of five or six hundred yards. This delineation is applicable not only to the principal branch of the torrent, but also to the secondary torrents that empty into it, and to the smaller ravines departing from each of these torrents, and thus, pursuing one branch after another, should not stop till it reaches the source of the most remote rill. As these zones of defense go on enlarging up the mountain, they will join, and even merge into each other toward the summit, so as to form a continuous band around the upper part, leaving no void in it.
The perimeter of the land to be rewooded having been determined, the next step is to prohibit pasturage, in order to prevent further disintegration of the soil by the feet of the sheep, and allow the grass to recover. The ultimate result is assisted by cutting the bushes down to the stump, and planting willow-cuttings in horizontal rows about two yards apart, to hold the earth on the almost vertical slopes, and then sowing the seeds of grasses in the intervals. Concurrently with these preliminary operations, the only object of which is to prepare the soil for the reception of the forest-growth to be planted later, the torrent itself is attacked with works intended to impede its course, hold back the drift-matter, and prevent further undermining of the banks. For this purpose wattles and bars are inserted along the stream and its smallest ramifications, beginning generally at the upper parts, where the water, not having acquired its full force, can be more easily stopped, and the suspended matter may be more easily retained. Green branches of willow and hazel are woven around stakes in the ravine, take root in the soil, and become a living obstacle which perpetuates itself. If the wattles are close enough together, they will transform the ravine into a kind of staircase, by the agency of which the violence of the water is allayed at each step, its force is lessened so that it does not wear upon the soil, and it is made to run almost clear.
More energetic measures are required lower down, where the torrent exercises a more destructive action. Here dams of masonry are inserted in the banks, provided with an arched channel in the lower part to permit the outflow of water at moderate stages of the stream; they serve to hold back the stones that are worked out from the mountain, to promote the growth of alluvions, to break the fall of the torrent and diminish its violence by enlarging its bed. Some of these dams are real works of constructive art, and have cost as much as eight or ten thousand dollars.
The real replanting of the woods is done after the ground has become settled and the torrent has been subdued. Nurseries of young trees suitable for the purpose are previously established near the locality of the works, which are drawn upon as the plants are needed. The species vary according to the nature of the situation and the soil. Generally pines and firs of different kinds are best adapted to the higher situations, deciduous trees to the lower ones. Use has also been made of several species of shrubs and bushes, which with their branching roots are wonderfully fitted to fix the earth, and by reason of their rapid growth quickly furnish a shade to the bare surface. The planting is begun at the top of the elevation and is conducted downward, in such a manner as to leave no places vacant. The young trees, protected against the sun by the grasses which were previously sown and by the willow-cuttings which have already taken root, soon begin to grow with vigor. An effort was made, in accordance with the law of 1864, to substitute regrassing for replanting with wood in the interior of the perimeters; but it did not answer the purpose of consolidating the soil, and was abandoned. Sometimes the communes have shown themselves hostile to the execution of these works on account of the interdiction of pasturage. Such was the case in the communes of Orres and Saint-Sauveur, whose inhabitants drove away the workmen in 1864. The work was resumed three years afterward, after it had been completed in an adjoining commune, and the people were conciliated by being employed in it. The upper valley of the torrent of Vachères, which was formerly considered one of the largest and most violent torrents of the Alps, once all cut up into ravines, is now covered with vegetation, and the torrent itself is described as extinct; once the terror of the country, it has been changed, at an expense of twenty-four thousand dollars, into an inoffensive river.
Like results have been obtained wherever works of the kind have been executed. M. Gentil, an engineer, reports of one district: "The aspect of the mountain has been changed all at once. The soil has acquired such stability that the violent storms of 1868, which caused great disasters in the high Alps, fell harmlessly on the regenerated perimeters. The mountain has at the same time become productive; where a few sheep could hardly live by destroying everything, may now be seen abundance of grass fit to mow. This method of improvement is remarkable for giving the people what they need most, and for giving it to them with only a brief delay. The pastoral inhabitants find food for their flocks in the grasses and hay of the planted areas and in the foliage of the trees; and the acacias that have been caused to grow there will soon furnish them the wood they will need in their vineyards. The torrential character of the stream has disappeared; the water is less turbid, even in time of rain, and is better adapted to purposes of irrigation. It is no longer loaded with solid matter when it reaches the lower valleys, and naturally keeps its bed. . . . Diversions from the regular course are less liable to occur and less dangerous, and the people on its banks can protect themselves with slight expense." M. Gentil relates several examples of torrents formerly very dangerous which have been fully and permanently subdued by means of such works as have been here described, giving protection to highways that were often interrupted before, and security to valleys that were often in danger, and adds: "Immense benefits have accrued to the lands situated in the lower valleys near the discharging basins of the streams. Not only are the inhabitants delivered from the expense of keeping up costly and precarious dikes; their property, also, being no longer in danger of being suddenly buried under a flood of gravel, has acquired a fixed value. They are able to till their land hopefully, and with the assurance that they will enjoy the crop. This certainty is a blessing of enormous value. The proprietor, able to rely upon the future, will no longer think of leaving the country."
So far as the work of restoration has been executed, its success—in respect to the processes employed and the results obtained—has been complete. The chief question concerning it which remains is as to the extent to which it shall be systematically carried out.—Revue des Deux Mondes.
- [Some instructive reports have recently been published in France concerning the progress that has been made in rewooding the slopes of the Alps, which, having been stripped of trees, are most exposed to the ravages of torrents. While the Alps present features which have no counterpart in the settled regions in the United States, the problems which had to be solved in reclothing the lower parts of the mountains with wood and staying the processes of devastation involve principles which may be applied as well in our own hilly and mountainous districts.—Editor.]