582 FOREST LITTKK. 



and the rainfall is slight, it may happen that it retains all the 

 water and allows none to reach the soil. This usually happens, 

 however, late in the summer when assimilation is ahout ended 

 for the year. 



The soil-covering finally acts by protecting the soil from the 

 evaporation of the water it contains. The water ascendinj^ 

 by capillarity from the soil finds in the larger interstices of the 

 soil-covering an impediment to the continuance of this action 

 up to the surface ; it therefore collects in the lower strata of the 

 soil-covering and is re-absorbed by the soil as the atmosphevt- 

 cools down in the evening. Ebermayer* has experimentally 

 shown that the evaporation of water from a forest-soil protected 

 by litter is 21 times less than when the litter has been removed. 

 In this respect the difference between dead leaves and moss 

 should be noted. Wallney has shown that a soil-covering 

 composed of beech leaves is the best preservative against 

 evaporation ; it is much more effective than the mossy soil- 

 covering of coniferous forests, which dries up rapidly in 

 summer. 



Whenever the soil is otherwise supplied with sufficient 

 moisture, either owing to its hygroscopic power or a more than 

 usual supply of water — as in sweating sand {Schuitzsand) , when 

 the level of subsoil-water is high ; in narrow valleys, mountain 

 terraces, depressions in plains and plateaux, &c. — a soil-covering 

 of humus and litter is of less importance in this respect. It 

 may even prove injurious in localities, which, without its aid, 

 are too wet. In all other cases its importance is the greater, 

 the less hygroscopic the soil — in sandy and calcareous soils, 

 all shallow soils, loose gravels and masses of boulders — which 

 can only retain water when covered with humus and litter. 

 It is also obviously more important on slopi'S than on level 

 ground. 



(b) Influence on porosity of the soil.— The activity of a st)il 

 also depends on its porosity, which affords interstices in it for the 

 circulation of air and renewed supplies of oxygen. Litter and 

 humus keep the soil loose and prevent its becoming caked b\ 

 the rain. 



* Din I'iivsikalischeii Eiiiwirkuii''eu des Waldes auf Lull uud Bodeii. 



