126 THE ST UDY OF PLANT COMMUNITIES * Chapter VI 



the open and ten degrees higher at night. Soil temperatures under 

 forest are lower than in the open during the growing season and 

 usually higher in winter. However, soil temperatures under de- 

 ciduous forest are subject to considerable winter variation. 





FIG. 66. Effect of slope exposure is apparent in the desert, as elsewhere. 

 Although species differences are not great, the south-facing slope at right sup- 

 ports a much sparser, more widely spaced stand of sagebrush than the oppo- 

 site slope. Washoe County, New. -Photo by W. D. Billings. 



Soil temperatures are further modified by dead or living cover 

 on the surface. Any such cover reduces the range of extremes and 

 the speed of variation. This amelioration of temperature may be 

 important in the viability and germination of seeds and the sur- 

 vival of seedlings. Particularly affected are the physical and physi- 

 ological processes involving water, its movement and availability 

 in the soil, and its absorption and transpiration by the plant. Also, 

 when soil is frozen, the runoff from heavy rains is much increased. 

 Studies in Arizona 123 showed daily minimum soil temperatures to 



