122 THE STUDY OF PLANT COMMUNITIES ' Chapter VI 



true mean is more nearly approached by averaging the hourly 

 temperatures for twenty-four-hour periods. 



Annual mean temperatures are almost useless ecologically, for 

 they do not indicate seasonal variation and duration. Temperate 

 desert regions may have amazingly high annual mean temperatures 

 and yet have winter frosts, which constitute an important limiting 

 factor in the survival of certain species there. Subarctic areas may 

 support forest vegetation because of the warm summers, yet mean 

 temperatures may be so far below freezing that they suggest that 

 little if any plant life would survive. It can be seen that mean 

 monthly temperatures are desirable for evaluating ecological con- 

 ditions, and this is equally true for monthly mean maximum and 

 minimum values. Collectively, these indicate the extent of the 

 growing season and the extremes to be expected during that time. 



Temperature Variations.— Since fluctuations of insolation result 

 in fluctuations of temperature, seasonal and daily temperature 

 changes, as with insolation, can be expected to follow a general 

 pattern for any region. The pattern follows that of insolation but 

 with temperature responses lagging behind changes in radiation. 

 A daily maximum of atmospheric temperatures usually comes in 

 midafternoon, and minimum temperatures occur just before sun- 

 rise. Soil temperatures lag even more, for their maxima may not 

 occur until 8:00-11:00 P.M. and minima may not be reached until 

 8:00-10:00 A.M. This is, of course, due to the fact that soil is a 

 poor conductor of heat. For the same reason, the soil surface, if 

 unshaded, produces the highest temperatures for an area and like- 

 wise has the widest range of temperatures. It is the subsoil tem- 

 perature that follows the trend indicated above. With increasing 

 depth, daily fluctuations are reduced until at two or three feet 

 they are not apparent. Seasonal air temperatures also lag as is indi- 

 cated by the usual hot days of July and the cold of January, both 

 extremes coming after the June and December solstice. Soil tem- 

 peratures follow seasonal atmospheric trends with a further lag. 



Since the total insolation decreases with distance from the Equa- 

 tor, temperatures likewise decrease. Temperature zones, therefore, 

 tend to run east and west, and the greater the latitude the lower 

 the temperatures to be expected. 



There are, however, local and generalized exceptions. Large 



