190 REPORT OF THE FORESTRY COMMITTEE 



precipitation results from the fact that the bulk of the forests are in the moun- 

 tains. Altitude, as is well known, has a definite relation to the amount of precipi- 

 tation, and unless this influence is eliminated that of the forest can not be clearly 

 determined. 



Precipitation increases with altitude, but it would be a mistake, however, to 

 accept, as it has been commonly believed to be for many years, that the rainfall 

 chart is practically identical with the contour chart. The influence of the forest 

 upon precipitation increases with the increase in altitude. Therefore, while it is 

 true that mountains affect precipitation, wooded mountains affect it to a still 

 greater degree. This effect is especially marked during the summer months, and 

 the effect is greater over coniferous than over broadleaf forests. 



Practically all observations then tend to show that there is an increase in the 

 total amount of precipitation over wooded areas as compared with that over bar- 

 ren or deforested ones. One reason for this is undoubtedly the tendency of 

 moisture-bearing currents to precipitate their moisture more readily above or near 

 the forests than over bare or cultivated fields at the same elevation, due to the 

 dampening and chilling effect of the forest upon the atmosphere, which induces a 

 greater condensation of the water vapor. 



That the air over forests contains a much larger amount of moisture than that 

 over bare or cultivated fields is today a proven fact, based both on actual conserva- 

 tions in the upper air strata and determinations of the quantities of water evap- 

 orated by the forest. 



That the vertical influence of the forest extends to a height far greater than 

 100 or 200 feet has been proven by observations taken during balloon ascensions. 

 Thus Renard, commander of engineers and subdirector of the Central Military 

 Balloonist Institute of France, states that the effect of the forest upon the tem- 

 perature of the upper strata of air has been repeatedly felt during ascensions at 

 an elevation of nearly 5,000 feet over the forest of Orleans, which has an area of 

 75,000 acres. This influence is scarcely felt over field crops, and it is obvious that 

 the difference can not be due to the greater height of the trees, which at best 

 reach but a little over 100 feet on an average. It can be accounted for only by the 

 greater amount of water given off by the forest and the lower temperature above it. 



The condensation of vapor on the surface of leaves in the form of dew, hoar- 

 frost, etc., in northern latitudes, according to C. E. Ney, is from 0.4 to 0.8 

 inches a year. It is much more in southern latitudes, especially in tropical forests. 

 The condensing capacity of many tropical forests, because of the extreme damp- 

 ness of the air within them, is so great that during every clear and still night drops 

 of dew fall continuously from the leaves as in rain. (This is also the case in the 

 redwood belt on the Pacific Coast.) Thus part of the moisture which is evapora- 

 ted from the leaves during the day is condensed during the night, and the dews in 

 the forest in all latitudes are so heavy that they dampen the soil under the leaves. 



Another reason for greater precipitation over forests may be the mechanical 

 action of the trees themselves. When a cloud in the mountains passes through a 

 forest, the branches and the leaves of the trees retard its movement. It comes, 

 therefore, into a state when it can no longer retain its moisture in suspension, just 



