KELATIVE RESISTANCE OF TEEE SEEDLINGS TO EXCESSIVE HEAT 5 



was in the nature of a rapid drying, and that the various degrees 

 of injury were controlled more by soil differences than by any other 

 variable in the habitat or in the species. Trees on sandy soils of 

 relatively small water-holding capacity were most injured, and those 

 on loamy soils of relatively high capacity were best able to with- 

 stand the unusual demand for water at the leaves. Therefore, the 

 conclusion seems justilied that this instance does not give an illustra- 

 tion of heat injury, in the strict sense of the term, and it is difficult 

 to conceive of any temperature recorded in naturally heated moving 

 air being high enough to produce heat injury, at least so long as the 

 foliage retained any moisture whose evaporation might have a 

 cooling influence. 



DESCRIPTION OF EXPERIMENTS. 



PRELIMINARY TESTS. 



The first actual experiments in heat resistance made by the present 

 writers, in 1919, appear to have given what have been described 

 above as "border-line " cases, involving drying injury quite as much 

 as or more than direct heat injury. These tests have been described 

 and reported by Bates (3) and, therefore, need be only briefly 

 mentioned to give the reader a full understanding of the problem. 

 Seedlings of the four Rocky Mountain species which have been 

 mentioned, as well as the eastern white pine, were grown in a 

 number of pans, which after germination had taken place were 

 kept at different moisture contents, varying from 3 to 14 per cent. 

 The pans were heated by sunlight in a greenhouse, where the air 

 temperatures were sometimes raised by artificial heat. Maximum 

 temperatures of about 130° F. were recorded by blackened ther- 

 mometers resting on the surface of the soil. The pans were watered 

 early each day so that, by the time the maximum temperature was 

 attained, they had had considerable opportunity to dry. The most 

 heavily watered pans were never in a dry condition, however, and 

 the moistest soil did not permit any loss that could possibly be 

 ascribed to heat injury. The losses increased quite definitely with 

 decreasing moisture, so that the influence of this factor is evident. 

 Most of the injury took the form of stem wilting, similar to that 

 described by Hartley (-5). Douglas fir and yellow pine showed the 

 lowest percentages of injury, followed by white pine, Engelmann 

 spruce, and lodgepole pine. The severe injury to lodgepole was 

 ascribed in part to its slow germination, so that the seedlings were 

 younger and more frail than those of other species when the first 

 severe exposures were given. It seems quite certain that the poor 

 showing of both the lodgepole and spruce seedlings was due to their 

 frailty, small mass to resist drying, and relatively small root systems 

 to supply moisture. 



PLAN OF TESTS IN 1922. 



The second series of experiments was planned to test the resistance 

 to heat injury exclusively of the various species at the ages of 30, 

 45, 60, and 90 days. It was conducted in the greenhouse and labora- 

 tory at the Fremont Forest Experiment Station. 



