FIFTH NATIONAL CONSERVATION CONGRESS 185 



intercepted by the forest in the form of mist or drops of dew or crystals of hoar- 

 frost on the branches and foliage of the trees. 



Forests in broad continental valleys enrich with moisture the prevailing air 

 currents that pass over them, and thus enable larger quantities of moisture to 

 penetrate into the interior of the continent. The destruction of such forests, es- 

 pecially if followed by weak, herbaceous vegetation or complete baring of the 

 ground, affects the climate, not necessarily of the locality where the forests are de- 

 stroyed, but of the drier regions into which the air currents flow. 



Forests in the mountains, while they have a marked influence upon local pre- 

 cipitation, their influence upon the humidity of regions lying to the leeward on the 

 whole is not very great. 



FORESTS AND STREAMFLOW 



THE effect of forests upon streamflow in level countries differs from that 

 of forests in hilly or mountainous regions. 

 In a level country where there is no surface runoff, forests, in com- 

 mon with other vegetation, act as drainers of the soil. Hence the importance in 

 draining the marshy lands and improving hygienic conditions, In such countries 

 the effect upon streams is unimportant. 



In the plains and in level country the forest : 



(1) Constitutes an effective means of draining and drying up swampy lands, 

 the breeding places of malaria, and swamp fevers. The reforestation of the 

 Landes, Sologne, the Pontine marshes, and a hundred other examples prove 

 this. 



(3) Draws moisture from a greater depth than does any other plant or- 

 ganism, thus affecting the unutilized water of the lower horizontal strata by 

 bringing it again into the general circulation of water in the atmosphere and 

 making it available for vegetation. 



(3) Lowers to some extent the subterranean water level, but it has no 

 injurious effect upon springs, since these are practically lacking in the level coun- 

 tries with horizontal geological strata, where its lowering influence has been chiefly 

 noted. 



(4) Refreshes the air above it and increases the condensation of moisture 

 carried by the winds, thus increasing the frequency of rains during the vege- 

 tative season. 



In hilly and mountainous country forests are conservers of water for stream- 

 flow. 



In the mountains, the forests break the violence of rain, retard the melting of 

 snow, increase the absorptive capacity of the soil cover, prevent erosion, and 

 check surface runoff in general, thus increasing the underground seepage and 

 so tend to maintain a steady flow of water in streams. 



Forests in hilly and mountainous country, even on the steeper slopes, create 

 conditions with regard to surface runoff such as obtain in a level country. The 

 steeper the slope the less permeable the soil, and the heavier the precipitation the 

 greater is the effect of forests upon streamflow. 



