Southeastern Washington and Adjacent Idaho. 



53 



A comparison of a large number of temperature readings at 

 one foot on the two slopes reveals the fact that at this depth the 

 soils on the south slopes are from 3 to 5 F. warmer in early 

 spring than those on the north slopes and that these differences 

 may increase by late summer to 7-io F. In Table VIII the 

 soil temperature from i to 5 feet on the two slopes at the base 

 station is given. 



TABLE VIII 



SOIL TEMPERATURE AT 1-5 FEET ON APRIL 18 AND AUGUST 15, 1914, ON A 

 NORTHEAST AND SOUTHWEST SLOPE, RESPECTIVELY 



The higher soil temperature prevailing on the south slopes 

 has much to do with the earlier seasonal activities of many 

 plants here than of those on north hillsides. 



Summarizing briefly the habitat conditions in the prairie, we 

 find the north and northeast slopes less xerophytic than the south 

 and southwest slopes. This is due in part to actually greater 

 precipitation caused by blowing snow, and in part to soil texture, 

 which is more open, has more humus, and a greater water hold- 

 ing capacity. These factors are reflected in the greater amount 

 of soil water and in lower temperatures. Likewise, these slopes 

 are sheltered from the drying southwest winds, and from the 

 perpendicular rays of the sun. This is reflected in slightly lower 

 air temperature and greater humidity, and especially in the lower 

 evaporating power of the air. 



These differences in habitat are reflected in the vegetational 

 cover in a number of ways. The resumption of growth in late 

 March or early April is often from 10 to 17 days earlier on the 

 warmer exposed slopes. Likewise, the same species flower and 

 dry up correspondingly earlier than on the sheltered slopes. The 

 vegetational cover is less dense on the exposed slopes, often 10 ta 



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