684 



JOURNAI, OF FORESTRY 



HEAT 



Graphs of height growth and mean air temperature for different 

 periods are presented in figure 3. It is recognized that a better ex- 

 pression of temperature would be furnished by the product of duration 

 and intensity of efficient temperatures. Since, however, the Hmits of 

 efficient temperature, as far as yellow pine is concerned, have not been 

 established, it is felt that little would be gained by such summations. 

 Moreover, the mean temperatures, which may be safely taken as a rough 



Figure 3. — Seasonal air and soil temperature in relation to height growth of western 

 yellow pine saplings. All temperatures in degrees F. above Z'^. a, Mean air temperature, 

 July-October; b. Mean air temperature, April-June; c. Annual air temperature; d, .Annual 

 height growth; e, Mean soil temperature at 2 feet depth, May-June. 



index, show that air temperature plays a subordinate role in the prob- 

 lem at hand. The only really satisfactory expression of temperature 

 is a measure of the heat absorbed by the plant — data which unfortu- 

 nately are not readily obtainable. 



The July-October temperatures are for the year preceding the growth 

 with which they are correlated. Since height growth is completed by 

 July I, it is evident that the temperature after that date could have no 

 effect upon the current year's height growth. It might, however, affect 

 the height growth of the following spring. The storage of carbo- 

 hydrates during the after-summer and autumn, for use in the period 

 of greatest vegetative activity the following spring, is quite universal 



