THE PARTNERSHIP OF WOOD AND WATER.

Forest and Bird, Issue 27, 1 July 1932, Page 2

THE PARTNERSHIP OF WOOD AND WATER.

(By Dr. Hans Burger, Swiss Forest Institute, contributed to American Forests.)

FOREWORD BY HON. SECRETARY.

The following article contributed by Dr. Hans Burger, Swiss Forest Institute, to “American Forests,” is particularly applicable in that New Zealand is a hilly and mountainous country similar to Switzerland. No part of this southern land is far removed from the sea, which runs to great depths in close proximity to our coasts, thus the all-too-meagre top soil, when washed off owing to forest depletion and over-grazing, is finally deposited under the ocean and docs not form additional coastal land to any extent.

Historians and geographers teach us that the ancient cultivated countries on the Mediterranean were far more fruitful when their hills and mountains were still covered with vigorous forests. The quickly increasing population of those days, however, mercilessly destroyed the forest in order to increase the' area of the cultivated land and to obtain wood for houses and ships in the easiest possible manner. Spain, Portugal, Italy, Greece, Asia Minor and parts of Northern Africa, which in earlier times seem to have been paradises of fruitfulness, are now partially laid waste. Canaan, the land that flowed with milk and honey, is to-day an arid and desert district. Of the cedar forests of Lebanon, with whose wood Solomon built the temple, there are now only sparse remnants. The forests in all these countries have, to a large extent, disappeared. Mountains and hills are covered chiefly with poor pasture land, whose hardened surface does not allow the precious rain to penetrate into the soil. In the rainy seasons mountain brooks and rivers become swollen torrents, carve out deep river beds, cause landslides, harmful floods and cataracts of stones and boulders. Little water enters the soil, almost all flows away on the surface and soon after the rainfall the river beds have turned to arid deserts and the few springs are sealed up. Even the ancient investigators, with real feeling for nature, established through observation that these changes in climate, this desolation of districts and countries which had earlier been paradisically fruitful, these extremes of drought and flood, could only be the result of the reckless destruction of the forests.

In Switzerland, too, floods and landslides have increased greatly in the course of the centuries. Because of the increase in population, the forests in the plains and on the larger slopes of the mountains have given way to cultivated lands. In the mountains the herds of goats, sheep and cattle, always increasing, have hindered natural sowing and regeneration in the forest (particularly in localities where the natural conditions had already been rendered unfavourable through other causes). By examination it was proved that through the influence of men and their cattle the upper forest boundary had, in the course of the centuries, been pushed back in many valleys about 300 to 1,000 feet, and that in other districts mere ruins of the once luxurious forests remained. To the destruction of part of ' our forests is attributed the growing damage done by inundations and landslides. Our Swiss Forest Research Institute has undertaken to examine this question. At the beginning of this century it chose two small valleys in the Emental mountains-—or, as they call them there, two “Graben”—the Rappengraben and the Sperbelgraben. The Rappengraben is only one-third wooded, the Sperbelgraben almost completely covered by forests. For over thirty years rain and snow have been measured here with special instruments; automatic installations have noted continuously the volume of water that drained away, and in specially built basins all .the stones and boulders washed down in each valley were held back and measured. Observations were made as to how the water moved in the earth, and the number of landslides which occurred in each valley. It was established that in great storms the mountain stream of Sperbelgarben never rose so quickly and only reached to one-third or one-half of the high water mark of the Freildandbach in the Rappengraben. Also in thaws in winter, or when the snows melted in the spring, the wooded brook never rose as high as the unwooded brook. The wooded brook also brought down fewer stones than the unwooded brook, and on the wooded slopes of the Sperbelgraben there were no landslides, while in the pastures of the Rappengraben fresh landslides broke away every year and the piles of stones in the valley grew larger. How can one explain the beneficent levelling influence of the forests on the stream flow and on the amount of soil wash and gravel ? Let us imagine a valley completely overlaid with lead, so that nearly all the water of one rainfall runs off at once. None can soak in and only very little can evaporate. The rain water would in a very short time reach the bottom of the valley from the highest watershed. Each fall of rain would quickly fill the stream

bed to high water mark, and very soon after the rain had ceased the stream would have run dry. There would be no springs at all. If the ground in the same valley were still non-porous, but planted with a vegetation of grass, bushes and woods, then at least part of the falling shower would be caught by the plants and would evaporate. The total leaf surface of grasses and herbs of the pasture and of natural meadows that can catch and hold moisture is small; they can therefore not hold so much water on their stems and leaves as bushes, and especially as trees or whole forests. After many examinations it is found that the trees of a forest, according to the species of tree and the density of the forest, will hold back twenty to forty per cent, of the rainfall, so that it never reaches the ground but evaporates and increases the humidity of the air. The covering of plants has still another influence. Stems, trunks and the leaves on the ground form obstacles to the water that is trying to flow away on the surface. Thus the surface flow is hindered. The water from the higher slopes reaches the stream beds later, high water is not so great because the supply of water is spread over a far longer period. If we have to do with a natural valley, we have to take into consideration the fact that the ground is more or less loose and porous and therefore allows part of the rainfall to soak in. The volume of water which is not withheld by the vegetation divides itself into the water that runs away over the surface and the water that soaks into the ground. High water and floods are caused chiefly by the water flowing away on the surface. So the looser and more porous the ground of the valley, the more rainwater will soak into the earth and the danger of floods be diminished. What happens to the water that sinks into the ground? Part of it remains in the surface soil and clings to the capillary pores as though in a sponge. It is therefore called captive water, or capillary water. Another part of the water that sinks into the earth is drawn down by gravitation through larger pores and channels into the lower layers of the ground. This water is called “sinking” water, or gravitation water. The captive or capillary water that remains in the higher layers of the earth partly evaporates directly in dry weather, and is partly used and given off by the plants. This water, therefore, never flows away. It enriches the air by its moisture and helps to make sufficient dew. The “sinking” water in the lower layers of the ground moves only very slowly through the earth. It is in a measure treasured up. It emerges according to the circumstances either at the end of some hours, days or even months into the streams. The gravitation water, therefore, does in the end flow away, but

usually after the surface rainfall of one shower has long since run off. Ihe gravitation water forms springs and ground water, which fill our streams and rivers in the long periods of drought or frost. If the subterranean reservoirs are large, they can take in more water in the rainy seasons, thus lessening the danger- of floods and ensuring larger, more enduring springs to bubble up during the droughts and frosts. When nothing but a thin layer of weather worn earth lies on the hard, impermeable rock, the water reservoir is of course small, and relatively little gravitation water can be held; a great deal of water runs quickly off; high water is soon reached and the few poor springs soon seal up and allow the rivers and streams to dry. If, on the contrary, the underground rock is furrowed and porous, the reservoir space is large and a considerable quantity of the rainfall can be taken up. The fear of flood is lessened and continuous springs feed the rivers and streams in dry or frosty weather. The size of the water reservoirs for '‘sinking” water is therefore not dependent on the vegetation, but is established by the geology of each district. The best reservoir is, however, useless when the channels to it are stopped up. Here lies the chief influence of the forest on the surface flow. When the ground is naturally thickly covered with plants, it has roots cutting through the soil. The roots form by their growth pipes and channels in the earth, so that it becomes looser and more porous. It is easy to see that the thin grass mantle of a pasture, with its small, weak roots which hardly make any impression on the earth, cannot loosen the ground. The small grasses and herbs of the pasture also offer very little protection, and any sign of a natural loosening of the earth is alwavs destroyed by the continual trampling of the grazing cattle, so that the pasture ground always remains hard and impermeable. In the hay meadows conditions are a little better. The plants are larger, send their roots deeper, the hardening of the ground through the trampling of cattle is less likely, and during part of the year the high grasses protect the earth from the direct beating of the rain. In a thickly planted hay field a large part of the shower of rain is caught by the high, dense grass stems, the mamstalks hinder the rapidity of the surface flow of the rainfall, whereby the sinking in of the rest of the water is made easier. Even more favourable is an uncultivated natural meadow. If a district in Switzerland is no longer in any way cultivated, it soon seeds itself with larger herbs and bushes and finally, if left entirely to nature, becomes forest land. The roots of the forest trees penetrate into and loosen the soil, according to circumstances, to a depth of one, two or three metres. The surface of

the earth is prevented from hardening by the covering of fallen leaves and needles, and through the crowns of the trees. The ground of a well-kept forest is loose and porous, and can absorb a great deal of “sinking” water.

Many hundreds of experiments, carried out all over Switzerland, show that pasture lands are almost non-porous. Three to five times better is the permeable earth of the hay field and ten to thirty times better that of good, uncultivated forest floors. If heavy rain falls on a pasture, very little water remains on the vegetation and it can only with difficulty penetrate into the earth and reach the subterranean reservoirs because of the hardness of the ground. Most of the water runs quickly over the surface and causes floods in the streams and rivers. If the same shower falls on well cared for forest land, most of the water can sink into the loose earth, move slowly through the ground and emerge only through springs,, when the stream flow has long since run off. In this way the forests can greatly diminish the danger of floods. If it rains for many days at a time, the storage capacity of the forest soil can on exceptional occasions be exhausted, and therefore as much water flows away from a wooded district as from an unwooded district. This, however, occurs very rarely. In their upper course all rivers have a tendency to carve out deeper river beds. The banks therefore become steeper and steeper, till they eventually begin to slip or to be worn away. The river carries the sediment down with it on its course, depositing it at last in the valley, where the water moves more slowly. It thus raises the river bed to such an extent that the water breaks out of the sides and floods fruitful districts, depositing piles of the sediment. Only by building barriers across the river can one make it impossible for the river to cut a deeper bed. The banks can be held together if they are always completely covered with plants. Every shower of rain sweeps earth off a bare surface, till there is nothing left but the rocky foundation. The rivers always carry a great deal of soil washed from the pasture lands because the earth is partly cut by the trampling of cattle and because most of the rainfall flows away over the hardened ground. Thickly planted grass fields can hinder the fruitful earth from being carried away. Even better are shrubs, trees or vigorous forests, which drain the earth and at the same time bind it with deeply penetrating roots. Slopes planted with woods can withstand far steeper river banks than unwooded slopes. For a successful struggle against mountain torrents and inundations of the plains, a complete collaboration of engineers, agriculturists and foresters is an absolute necessity.

It will be asked why Switzerland, with good forest laws for the protection of her mountain forests, still has devastating floods in the rivers from time to time. The answer is that the percentage of wooded land in the mountains is comparatively small, because a large part of the catchment basins lie above the limit of the growth of forests. The Rhine district in Graubunden is only eighteen per cent, wooded and forty-three per cent, of the area lies above the forest limit. The whole of the Engadin down to the Swiss frontier is only thirteen per cent, wooded and seventy per cent, of the district lies so high that no forests can grow there. The catchment basins of the Reuss up to the Vierwaldstattersee has only nine per cent, forest and sixty-six per cent, of the area lies above the forest limit. The Swiss Rhone valley as far as the Lake of Geneva has sixteen per cent, forest, and fifty-four per cent, of the district is so high that it could never be wooded. In the last fifty years Switzerland has newly afforested 16,956 hectares of land in dangerous catchment areas, which, however, amounts only to about 0.4 per cent, of the land area. It is evident that with 0.4 per cent, of new forest not all floods can be prevented. It has also to be taken into consideration that Switzerland produces only one-quarter to one-third of her food provisions herself, and it is therefore difficult to withdraw large tracts of cultivated land for afforestation. But the Swiss nation has realised the importance of protecting its forests. A law, adopted by popular vote, demands that the wooded areas of Switzerland never be diminished. In the protection forests of the Alps even the private proprietor cannot fell a tree without the permission of the forest superintendent. As a compensation for this reduction in the rights of possession the States subscribes large sums to the building of roads and to afforestation in the protection forest districts. Just because, for economic and climatic reasons, the wooded area of Switzerland cannot be much increased, it is absolutely necessary that we protect and care for our present forests. We must see to it that they regenerate properly and that the youngplants are not destroyed by grazing animals. We must also take care that the forest floor does not become hard and must therefore never allow cattle in the protection forest except in time of real necessity. Only a vigorous and well kept forest, made up of the proper spedes of tree, can lessen the fear of flood and only a well kept forest can ensure keeping the earth in a correct condition to conduct large volumes of quickly sinking water to the underground reservoirs which feed our rivers in time of drought or frost, work the mills, the sawmills, the factories and the electric power plants.