DIFFERENCES OF SOIL TEMPERATURE. 



47 



explanation would sutticr for the slight retardation of the fall of the 

 curve for deciduous trees. There is one feature, howeAcr, that is unex- 

 pected. It is, that the difference between the two sorts of forests is not 

 especially marked in winter. As a matter of fact, in the high latitudes 

 in which the observations are taken, the sun is so low in winter that the 

 bare trunks and branches are fairly efficient screens against his rays. 



The darker color of these bodies also permits them to be more 

 warmed up when the sun does appear. Besides, the persistent cloudi- 

 ness of the winter and the covering of sikow in these latitudes tend to 

 equalize the effects of the soil of these two types of forests at this 

 season. The annual means for the two kinds of forest differ but little. 



Fir;. 4.— Diflference.s of soil temperatiur (wood.s and open fields). Compari.son ofdeciduon.s and ever- 



For the surface they are —I'^Su for deciduous trees, — 2<^.0li for ever- 

 green trees; at 6 inches they are-io.89 and-io.82; at 4 feet— 2o.05 and 

 —20.19. They are represented in the diagram for the 4-feet depth by 

 the two straight lines, to show the slight difference between the two 

 types of forest cover. The difference in amplitudes is somewhat 

 greater. The amplitudes are: 



Surface. Six iuclios. Four foet 



For deciduous trees . 

 For cvfriireeii trees . 



80.50 



6°. 04 

 00.04 



50.12 

 40.37 



