■ 86 



less heat than the surface of the fields. The same screen acts, how- 

 ever, in the reverse direction by preventing radiation to the sky, thus 

 retaining more of the heat than do the open fields. The balance of 

 these two processes, it seems from observation, is in favor of the first 



and the average result is a cooling one The dififerences of 



temperature at the depth of 6 inches (152 mm.) are more than half a 

 degree less than at the surface. In this is to be seen the specific effect 

 of the forest litter; it adds a covering to that possessed by the sur- 

 face, so that while the deeper layer is cooled as much by the protec- 

 tion from the sun's rays as is the surface, it is not cooled so much by 

 radiation of heat to the sky. Its temperature is, consequently, rela- 

 tively higher, and approximates somewhat more the field tempera- 

 tures." 



"The forest soil is warmer than that of the open fields in winter, 

 but cooler in the other seasons, and the total cooling is much greater 



than the warming one The forest, therefore, not only cools 



the soil, but also moderates the extremes of temperature" (p. 46). 



The character of the forest, whether evergreen or deciduous, in- 

 fluences the temperature conditions of the soil, as is seen by a com- 

 parison of these conditions in the forest and in the open. The two kinds 

 of forest are much alike in winter; during the spring the soil warms 

 up more rapidly under conifers. Temperature variations are slightly 

 greater under deciduous trees. 



6. Soil Moisture and its Relation to Vegetation 



The moisture in the soil is derived largely from precipitation, but 

 part of it, in some localities, comes directly from the adjacent deeper 

 soils or rocks, and thus only indirectly from precipitation. As Illinois 

 lies at the bottom of a large basin, there must be some subsurface flow 

 from the adjacent higher regions, but to what extent is not known. 

 McGee ('13a: 177) estimates that the general ground-water level — 

 the level at which the soil becomes saturated — has, since settlement, de- 

 clined 10.6 feet in Illinois. This decline is not limited to drained re- 

 gions but is a general condition. In addition to these changes of level 

 there are seasonal fluctuations. Sherff ('13a: 583) observed in Skokie 

 Marsh that the water-table was at or above the surface in May, then 

 declined until early September, and then rose rapidly to the surface by 

 the middle of October. The wet prairie at Charleston has undergone 

 just such changes as these ; the ground-water level has been lowered 

 and there are marked seasonal changes. 



Harvey ('14) has recently shown that the soil of Bryngium-Sil- 

 phium prairie at Chicago contains a large amount of water during 



