Professor Humphreys, of the Weather Bu reau, says: "These universal slow climatic changes that for thousands of years have been modifying the glaciers and changing the inland seas might very well have led to extensive for est destruction; but that it itself was the effect and the destruction of the trees the cause seems most unlikely." In the Lop Basin of Chinese Turkestan, Huntington found vast areas covered with the dead skeletons of trees. He says: "It has of ten been asserted that the destruction of forests has been the cause of the diminution in rainfall. In the Lop Basin the opposite appears to be the case; the supply of water has diminished, and therefore the forests have died." The late Prof. Cleveland Abbe says: "In this day and generation, the idea that forests either increase or diminish the quantity of rain that falls from the clouds is not worthy to be entertained by rational, intelligent men.) Ga'es exposed over forests catch more rain than similar gages exposed at the same eleva tion over upwooded adjacent areas. This is due to the fact that the forests restrict the move ment of the wind, and not to a difference in the precipitation.
The covering of tobacco plants with cheese cloth results in establishing a local climate which will continue so long as the cloth remains in position. Extremes of temperature, both heat and cold, are reduced, and the resulting climatic change produces a marked effect upon all vegetation grownunder the artificial con ditions. A vegetable growth may have a heavy frost deposited upon it, while the same character of soil that grew the vegetation im mediately adjacent, at the same level, but covered with two inches of sand may receive no frost. This is due to the difference in specific heat of the two surfaces, the sand gaining a higher temperature during the day with the same insolation than the vegetation or the un sanded surface, and radiating heat at a greater rate during the cold hours of night, thus keep ing at a higher temperature the. air immediately next the ground.
The erection of a tent, of a barn, of a dwelling-house, of a village, or the growth of a great city, respectively, influence the local climate in proportion to the area that is cov ered, modified by the character of the materials used in the constructions, each different form of matter having a different coefficient of ab sorption, reflection and radiation. Likewise the vegetable covering of the earth may have a local appreciable effect. The flooding of an area, the cutting away of forests and the covering with sand may have either minute or considerable effects upon local climates in pro portion to the magnitude of the areas affected; but this does not mean that there is any great difference in the climatic effect between a forest covering and one of bushes, of grass or of growing crops; and it does not signify that there is sufficient change in the thermal con ditions, due to the activities of man, as to make an appreciable difference in the tempera ture at an altitude of 100 or 200 feet, or to effect the general climatic conditions, or to cause storms to be more frequent than formerly, or of greater severity, or to increase the amount of precipitation.
Rainfall records for 100 years in New Eng land and 60 years in the Ohio Valley do not show any appreciable effect on rainfall as the forest areas have been changed to cultivated fields. The precipitation for the last half of each period is practically the same as for the first half, but the rain curve in both cases shows pronounced oscillations in periods that vary from 8 to 11 years.
Effect of Forests on Floods.— Until recent years it has been quite generally believed that the effect of forests on the movements of water was to diminish the frequency and intensity of floods and to maintain higher stream-flow dur ing dry periods. But a careful study by emi nent scientists of the history of floods in the principal rivers of Europe for the period cov ered by the past 200 to 500 years, of the run off of the streams of Wisconsin by Prof. D. W. Mead, of the University of Wisconsin; of the streams in the Yellowstone Park and elsewhere in the Rocky Mountains by Chitten den; of the Missouri, Mississippi and Ohio rivers by army engineers; and of the rivers of the Ohio Valley by the writer; do not confirm this view. Opinons differ. For esters and many geologists take the affir mative side of the question, while meteoro logists and constructinif hydraulic engin eers generally take the view that the broken, permeable soil of seedtime, and the growing crops of summer and fall, each year, and the covering of bushes that quickly shades the ground when trees are cut, may be as potent as the forests themselves in the conservation of moisture and in the restricting of run off.
It is not clear how the planting of trees at the headwaters of streams can be effective in minimizing or controlling floods, as it requires only a superficial survey to show that of the various basins that catch precipitation and shunt it down into the valleys where it may accumulate and cause floods, only a minute area lies at the head-waters of streams. If it be granted that the planting of trees, or their preservation, reduces the destructiveness of floods, which it will not, then the trees must be placed where the flood waters fall in order to be effective, which is on the slopes and al luvial plains some distance from headwaters, and it is not practicable to turn these food producing areas back to the wild forested con dition without doing a direct and positive injury to humanity.