EELATIVE RESISTANCE OF TREE SEEDLINGS TO EXCESSIVE HEAT 13 



difficult to determine with any precision either the effects of different 

 periods of exposure under the same temperature conditions or of 

 different temperatures for similar exposures. 



Regarding the effects of different periods of exposure, as many 

 comparable data as are available have been examined, and it is found 

 that beyond a period of five or six minutes, which was the shortest 

 employed in the radiation tests, there is not so much additional in- 

 jury with added time as might be expected. This confirms the idea, 

 which has been previously emphasized, that in nature it is probably 

 the momentary maximum temperature which determines the degree 

 of injury. It is also in agreement with results obtained in 1919, 

 which showed that seedlings surviving one severe exposure were in no 

 danger from a repetition of the same thing. However, it is entirely 

 conceivable that transpiration which affords protection may tem- 

 porarily attain a rate which can not be maintained indefinitely, and 

 that the protection will therefore break down. 



To arrive at an' approximate measure of the effects of different 

 temperatures, it has been necessary to counteract the large number 

 of variable factors by averaging the results in three temperature 

 groups without regard to the time factor. The temperature groups 

 cover respectively all the tests below 140° F. which caused any injury, 

 those between 140 and 160° F., and those above 160° F. In order to 

 obtain for each test a summation of all the degrees of injury, the 

 classes of injury shown in Tables 2,, 3, and 4 have been rated as 

 follows: One seedling killed, 16| per cent injury; one seedling in- 

 jured, will die, 12J per cent injury; one seedling injured, will live, 

 4J per cent injury. This rating is based on six seedlings in a group, 

 and is increased proportionately for smaller numbers, so that the pos- 

 sible sum for a group is 100 per cent. 



By averaging the temperature and degrees of injury for all of the 

 tests in each temperature group, three ratings have been established 

 for each species. The lower temperature groups show greater aver- 

 age age of seedlings, but also longer exposures; and it is believed 

 that these two factors will be just about compensating, so that the 

 effects of different temperatures are brought out well enough for 

 comparative purposes. 



The curves drawn through the three points for each species are 

 shown in Figure 1. The marked differences between the four species 

 are clearly brought out. While too much significance should not 

 be attached to the shapes of the several curves, it is apparent that 

 Douglas fir is much more sensitive than the other species at com- 

 paratively low temperatures. At about 150° F. the spruce shows a 

 more marked sensitivity. The shapes of their curves suggest that 

 the pines might resist a very high temperature before all individuals 

 would be killed ; but there is every reason to believe that at a slightly 

 higher temperature than any recorded in these tests, first the yellow 

 pine, and soon thereafter the lodgepole, would show complete break- 

 downs. 



CONCLUSIONS. 



In considering the general application of these experiments with 

 possible forms of heat injury to seedlings of the four most important 

 trees of the central Eocky Mountains, the following observations are 

 of value : 



