MECHANICAL EFFECT OF FORESTS. 41 



by the crowns of the trees, of which a considerable part 

 evaporates and is thus lost to the soil. On the other hand, the 

 roots of the trees take considerable quantities of water from the 

 soil ; hence forest soil may be less moist in the root region than 

 soil in the open. At the same time, open land covered with 

 grass and weeds, as it usually is when not cultivated, allows 

 less water to penetrate into the soil than forest soil, while field 

 crops take considerably more water from the soil than forest 

 trees. The conclusion is, therefore, justified, and it is proved 

 by every-day experience, that forests act most beneficially 

 upon the preservation of moisture. In this respect a great 

 dilference is produced by altitude. In lower situations, 

 evaporations from the soil and by the leaves may consume all 

 the water which reaches the ground. With rising altitude, 

 the percentage of the rainfall thus consumed becomes smaller, 

 so that an increasing proportion of the rainfall sinks into the 

 ground and becomes available for the feeding of springs. 

 Hence mountain forests have from time immemorial been 

 looked upon as the preservers of moisture. The presence or 

 absence of springs in a given locality, however, depends 

 chiefly on the geological formation. Sandstone and chalk 

 formations are poor in springs, because the water sinks 

 rapidly to a considerable depth, while other formations are 

 generally rich in them. 



3. Mechanical Effect of Forests. 



The mechanical effect of forests makes itself felt chiefly in 

 regard to the distribution of the rain water, the preservation 

 of the soil on sloping ground, the binding of shifting sand, 

 the prevention of avalanches and the moderation of air 

 currents. 



a. FeeiTuxij of Sprimjs and Rivers. 



Most of the rainwater falling on a bare slope rushes down 

 into the nearest watercourse in a comparatively short time, 

 thus causing a rapid rise in the level of the stream. Only a 



