87 



April and until late in May ; that the moisture falls and is low during 

 July and August, with a mean of 24 per cent, of saturation for these 

 months ; but that in October the soil is again at or near the point of 

 saturation. 



The blanket of humid air which accumulates under a cover of vesfe- 

 tation, retards evaporation and conserves soil moisture. The denser 

 the vegetation the more marked is its influence. The litter — the or- 

 ganic debris in an early stage of decomposition — on the forest floor 

 has the same tendency, and has even a greater water capacity than the 

 soil itself. On the other hand, a forest is a powerful desiccator; as 

 Zon ('13:71) has recently put it: "A soil with a living vegetative 

 cover loses moisture, both through direct evaporation and absorption 

 by its vegetation, much faster than bare, moist soil and still more than 

 a free water surface. The more developed the vegetative cover the 

 faster is the moisture extracted from the soil and given off into the air. 

 The forest in this respect is the greatest desiccator of water in the 

 ground." This drying effect is shown particularly near the surface 

 of the soil, where roots are abundant and where drouth is so marked 

 that it may prevent the growth of young plants here (cf. Zon and 

 Graves, '11 : 17-18). 



Warming ('09:45) says: "It may be noted that, according to 

 Ototozky, the level of ground-water invariably sinks in the vicinity of 

 forest, and always lies higher in an adjoining steppe than in a forest ; 

 forest consumes water." 



McNutt and Fuller ('12) have made a study of the amount of soil 

 moisture at 3 inches (7.5 cm.) and at 10 inches (25 cm.) below the 

 surface in an oak-hickory forest, at Palos Park, Illinois. They found 

 that the percentage of water to the dry weight of the soil at the 3-inch 

 level averaged 18.9 per cent, and at 10 inches was 12.5 per cent, of the 

 dry weight of the soil. The greater moisture near the surface is due 

 to the humus present in this layer. The grazed part of the forest 

 possessed less soil moisture, and shows the conserving effect of vege- 

 tation. (Cf. also Fuller '14.) 



The artificial control of soil moisture is well shown by the effect of 

 windbreaks. Card ('97) studied the moisture content of the soil to 

 leeward of a windbreak and found that in general there is a "de- 

 crease in the per cent, of water as the distance from the windbreak 

 increases." As the physical conditions leeward of windbreaks are 

 similar in many respects to those in forest glades and forest margins, 

 it is very probable that the conditions of soil moisture also will be very 

 similar in these places. 



