& 



136 HEAT 



have been accumulated by the United States Department of Agri- 

 culture and the United States Weather Bureau. The thermal con- 

 stant for corn computed from the mean daily temperature at the 

 average date of planting to the beginning of harvest is about 1600° 

 to 1800° F., depending on the variety of corn grown, in the principal 

 corn-producing regions. The thermal constant for cotton is about 

 1900° F. and temperature is the principal limiting factor in de- 

 termining the northern limit of successful commercial production of 

 cotton. If a variety of cotton could be developed which would 

 mature with a few hundred day-degrees of temperature less than 

 1900 the area of cotton production in the United States could be very 

 greatly extended. 



85. Fire.— Fire produces very high temperatures for short periods 

 of time in very local areas. Its most common effect is the destruc- 

 tion of vegetation. The greatest losses from fires occur in forest 

 regions where hundreds of acres of valuable timber are destroyed 

 each year (Fig. 61). Forest fires are started by various agencies. 

 Fires are left unextinguished by campers, or smokers throw away 

 burning matches or cigarette stubs, but many fires are also started 

 by electrical storms. 



In addition to the destruction of vegetation there are various other 

 effects that may be produced by fire. The seed-bearing cones of the 

 lodge pole pine {Piiius Murrayana) often remain closed and hang 

 on the tree for years. The heat from a forest fire, however, will fre- 

 quently cause these cones to open up and allow the seeds to fall. This 

 phenomenon is often a deciding factor in determining the type of 

 reproduction on a burned-over forest area. In forests where the 

 humus layer is deep and very dry, fire may destroy the entire layer 

 of organic matter and may thus convert a dry forest into a swamp 

 or shallow lake. Recent studies have indicated that in some grass- 

 lands the vegetative growth of the grasses is stimulated rather than 

 injured by periodic burning but at the same time there are efl^ects 

 upon the soil fertility and upon animal life that are detrimental. 



REFERENCES 



BuRKHOLDER, Walter H. : The Effort of Two Soil Temperatures on the Yield 



and Water Relations of Healthy and Diseased Bean Plants, Ecology, 1, 113- 



123, 1920. 

 Buxton, P. A.: The Temperature of the Surface of Deserts, Jour. Ecology, 



12, 127-134, 1924. 

 Chapman, R. N., et al.: A Comparison of Temperatures in Widely Different 



Environments of the Same Climatic Area, Ecology, 12, 305-322, 1931. 



