This section is from the book "Economic Tree Planting", by Birdsey Grant Northrop. Also available from Amazon: Economic Tree-planting.
But on the question of the influence of forests on climate and the permanent water supply, there is a growing unanimity among practical foresters and professors in the forest schools of Europe. Their theories and observations plainly show that the wholesale clearing of forests has an injurious effect on both, while the extensive planting of trees on arid regions has ameliorated the climate, prevented mountain torrents,and rendered the water supply more permanent. These investigations show that the general destruction of forests has rendered the climate dryer, more changeable and trying, and that forests on the one hand tend to lower the general temperature of a country and promote the fall of rain at more regular intervals, and on the other hand they ward off sudden meteorological changes which result in heavy falls of rain and disastrous floods.
It is well known that houses too closely surrounded by trees are damp. Beautiful and healthful as shade trees are, they may stand too near the house. Dense evergreens growing so close as to shut out all sunlight, are harmful. It is an old Italian proverb, that " where the sunlight cannot come the doctor must;" and sometimes the wisest direction of the physician to his rheumatic patient is, to cut down the tree which too densely overshadows the house and excludes all sunlight. The wetness of roads completely overshadowed by. trees, shows how forests affect the humidity of the ground they cover. Mr. Marsh says: " One important conclusion at least is certain and undisputed, that within their own limits and near their own borders forests maintain a more uniform degree of humidity in the atmosphere than is observed in cleared grounds." Speaking of the indiscriminate clearing in America, he says : " with the disappearance of the forest, all is changed. At one season, the earth parts with its warmth by radiation to an open sky, and at another receives heat from the unobstructed rays of the sun ; hence the climate becomes excessive, and the soil is alternately parched by the fervor of summer and seared by the rigors of winter."
Wm. Cullen Bryant says: " Our summers are becoming dryer and our streams smaller. Take the Cuyahoga as an illustration. Fifty years ago large barges loaded with goods went up and down that river. Now, in an ordinary stage of the water, a canoe or skiff can hardly pass down the stream. And from the same cause—the destruction of our forests—other streams are drying up in summer." Almost every work on forestry abounds in evidence that extensive forest denudation has everywhere diminished the flow of springs. The case of the famous spring in the Island of Ascension is often cited, which dried up when the adjacent mountain was cleared, but reappeared in a few years after the wood was replanted. Several lakes in Switzerland showed a depression of their level after a general devastation of the forests. Siemoni says : " In a rocky nook in the Tuscan Apennines there flowed a perennial stream from three adjacent springs. On the disappearance of the woods around and above the springs the stream ceased, except in rainy weather, but when a new growth of wood again shaded the soil, the springs began to flow." Marchand says: " The river that from time immemorial furnished ample water-power for the factory at St. Ursanne dwindled so much when the surrounding woods were cut that the factory was at last obliged to stop altogether." President Chadbourne says that Salt Lake contains nearly twice as much water as it did when the Mormons came, and that the water supply is increasing throughout the territory, not by an increase of rain, but cultivation and extensive groves of trees have checked the influence of drying winds and lessened evaporation.*
* Near my residence (Woburn, Massachusetts,) there is a pond upon which mills have been standing since the early settlement of the town. These have been kept in constant operation until within thirty years, when the supply of water began to fail. The pond owes its existence to a stream which has its source in the hills stretching some miles to the south. Within the time mentioned.
I visited the planted forests of the Duke of Athole—whose estates, beginning near Dunkeld in Scotland, extend forty miles by ten—in company with Captain Campbell Walker, now the Conservator of State Forests in New Zealand, who was long employed in the same service in India. He said he had personally observed the drying up of springs and decrease of the average amount of water in some of the mountain forests in India, in which extensive clearing had taken place, and that such clearing had unquestionably lessened the regular supply for springs and permanent flow in the streams and rivers. While I was in England, the terrible famine in India resulting in the starvation of over seven hundred and fifty thousand people—more than the entire population of Connecticut and Rhode Island—was a prominent theme of public thought and talk and sympathy. Captain Walker, Dr. J. C. Brown, and other foresters expressed the view that forest denudation, diminishing the springs and lessening the former sources of artificial irrigation, was the leading cause of this terrible calamity. Under the early rule of the East India Company, there was a wide-spread devastation of the forests, and in later years the construction of extensive railway and telegraph lines have created a new demand for timber. Recently the English Government has adopted energetic measures for re-foresting the mountains, and placed the remaining forests under the supervision of competent foresters.
In a paper read to the Vienna Geographical Society in 1875, Herr Wex, Counsellor of State, and Director of the Government Works for the regulation of the flow of the Danube, affirms that in the last fifty years the decrease in the average level or comparison of the highest and lowest flow of the Elbe and Oder has been seventeen inches, the Rhine twenty-four, Vistula twenty-six, Danube at Orsova, fifty-five. These measurements, embracing the greatest flood heights, the lowest flow, and the medium average flow, show that the floods are unquestionably higher than in former years, and the contrast between the highest and lowest flow is greater, and that these higher floods are no compensation for the diminution of the medium and low flood, and that many manufactories built during the last fifty years have experienced a marked diminution in the water supply of their streams, and steam-engines have been employed to meet the deficiency of water-power, once ample to do the same work.