DIFFERENCES OF SOIL TEMPERATURE. 43 



Some of tliese will be discussed later; the material on hand does not 

 permit the discussion of the others. They are instructive, however, 

 in showing- how great is the difference in action of diiierent forests. 

 For instance, the effect on the temperature of the surface at INIelkerei 

 (_3".95) is nearly five times as great as at Hadersleben (— 0".S4). This 

 difference of action of the forest is fsir-reaching, for it extends through 

 a layer of 4 feet of badly conducting soil. The ditference at this depth 

 at St. Johann (— 3".03) is about tliree times that at Eberswaldo (— 1".0G) 

 or at Hadersleben (— 1".04), and is five times that in the young forest 

 at Lintzel (— O^oS). The average of the seventeen stations (represent- 

 ing about two hundred years of observation) should give us good and 

 significant results. It shows for the surface— 2".59, for a deptli of G 

 inches (152 mm.)— f.ST, and for a depth of 4 feet (1.22 m.)— 2".02. The 

 influence of the forest on the soil, then, is a cooling one, on tlie aver- 

 age, and for central Europe the cooling amounts to about two and a 

 half degrees for the surface. The cooling is due to several causes : The 

 first is the shade; the foliage, trunks, branches, and twigs cut off much 

 of the sun's heat, absorb and utiliz<' it in vegetative processes, or in 

 evaporation, or reflect it away into si)ace. Thus the surface soil in the 

 forest receives less heat than the surface of the fields. The same screen 

 acts, however, in the reverse direction by preventing radiation to the 

 sky, thus retaining more of the heat than do the open fields. The bal- 

 ance of these two processes, it seems from observation, is in faAT)r of 

 the first and tlie average result is a cooling one. 



But the thatch of living foliage is not the oidy screen possessed by 

 the forest soil. It has in addition the screen of the forest litter, and. 

 this is in a condition to be even more effective than that of living 

 foliage. It lies in contact with the ground, preventing the dissipation 

 of the heat of the soil by moving air, and at the same time lying so 

 loosely as to tV»rm air spaces, which act as insulators in the way of 

 preventing the exchange of heat between the forest soil and air. 

 These cooling influences are enforced by the moist condition of the 

 forest soils. It does not warm so easily as the drier field soil, and more 

 of the heat which reaches it is used in evaporative processes than in 

 that of drier soil. The balance of all these processes is, it appears, in 

 the direction of cooling. And the cooler forest soil will cool, to some 

 degree, the air in contact with it, and this air, flowing off to some 

 other place, will take this cooler temperature with it, so that the cool 

 ness of the forest soil will make itself felt over the vicinity. 



The differences of temperature at the depth of G inches (152 mm.) 

 are more tlian half a degn^e 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 surface, so that while the deeper layer is cooled as 

 much by the protection 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, relatively higher, and it approximates somewhat more 

 the field temperatures. 



