10 



BULLETIN 1263, U. S. DEPARTMENT OF AGRICULTURE 



Reactions in these tests were dissimilar to those in the first, because 

 a dry atmosphere introduced the factor of transpiration. In all cases 

 the soil was moist enough so that wilting could not be ascribed to 

 lack of soil moisture. There was no constant relation between air 

 and soil temperatures, the ratio of soil to air temperature increasing 

 with the period of exposure. For the critical air temperatures above 

 approximately 140° F.« the soil temperatures were from 15° to 50° 

 less. In only one instance did the soil temperature exceed 140° ; 

 144.8° was secured with an exposure of 30 minutes, which produced 

 an air temperature of 161° F. Under ordinary field conditions,, 

 where the soil is not commonly so moist as under these experimental 

 conditions, a higher temperature is likely to be recorded by a ther- 

 mometer at the surface of the ground than in the air above the 

 ground; and the seedlings are subject to wilting, not only in the 

 leaves but also in the stem where it comes in contact with the surface 

 of the ground. 



Table 3. — Third test of heat resistance: Exposure of seedlings that survived 

 second test to radiation from electric coil 6-'f days after so icing. 



[Six seedlings of e^ach species used in each pot.] 



Pot No. 



Average. 

 Total.-. 



Grand total. 



Maxi- 

 mum 

 temper- 

 ature 

 in air. 



Maxi- 

 mum 

 soil 

 temper- 

 ature. 



Expos- 

 ure. 



> F. 

 147.2 



150.8 



152.6 



150.8 



150.4 



' F. 

 102.0 



110.0 



107.0 



106.3 



Min. 



100. 3 



Number of seedlings dead, mortally injured, 

 slightly injured, and uninjured, by'species. 1 



Western 

 yellow 

 pine. 



Lodgepok 



pine. 



0-2-3-1 



B(l) 

 1-0-2-3 



B (2) 

 1-1-3-1 



B (3) 

 3-0-2-1 



0-0-0-6 

 0-0-0-5 



B (1) 

 0-1-2-3 



Douglas Engelmann 

 fir. spruce. 



A (6) 

 4-2-0-0 



B(6) 

 3-2-1-0 



B (5) 

 3-2-1-0 



B (6) 

 4-0-0-1 



25-12-21-36 



A (6) 

 1-0-3-2 



2-0-1-3 



B(4) 



2-0-2-2 



1-1-0-4 



1 Capital letters indicate immediate condition of seedlings following exposure; figures in parentheses 

 indicate seedlings so affected, as follows: A = slightly cooked; B = seriously cooked. The four numbers 

 under each species indicate the final condition, in the following order: Dead seedlings; injured seedlings 

 that will die; injured seedlings that will live; uninjured seedlings. 



2 Only five lodgepoles transplanted in this pot. 



3 Of five firs in pot 4, the one escaping injury was very short stemmed. 



Throughout the tests at high temperatures, the fir was most visibly 

 affected. The needles appeared to be both shriveled and blistered, 

 for which superficial effect the word ' ; cooked " has been used. The 

 spruce was affected in a similar manner, but less markedly. The 

 pines apparently took care of themselves by excessive transpiration 

 up to the limit of their ability to supply water to the leaves. In 

 view of the fact that the yellow pine seedlings were considerably 

 taller than the others, and thus more exposed, they resisted well. 

 When the danger point was reached their needle tips shriveled first. 

 Lodgepole may have received sonic protection from the taller seed- 

 lings: bul its general high resistance in all the tests discounts this 

 factor, especially since the spruce seedlings of approximately the 

 same height suffered much more severely. 



