12 BULLETIN 1263, U. S. DEPARTMENT OF AGRICULTURE 



of temperature are likely to occur, its natural range at high eleva- 

 tions greatly reduces the probability of its having to resist tempera- 

 tures as high as those experienced by yellow pine, and its total heat 

 requirements are apparently less. Its preference for bare mineral 

 soils as a germinating bed is an expression of its need to secure 

 readily available soil moisture. In the light of results given here, 

 a failure of lodgepole seedlings under excessive temperatures must 

 be construed as being due to relatively short root length and corre- 

 sponding inability to secure necessary moisture. 



Douglas-fir seedlings suffered most severely under the high tem- 

 peratures, considerably more so than the spruce seedlings of frailer 

 structure, and far more than the yellow pines. This reaction corre- 

 sponds to some extent with common occurrences noted in the open. 

 Douglas-fir seedlings require, to a decided degree, some form of 

 protection during the first two or three years of their existence, or 

 until safely established. It has been observed where the two species 

 occur together and the soil is reasonably moist that spruce seedlings 

 are more often found without protection than those of fir. 



Very few fir seedlings recovered when visibly injured. It ap- 

 peared on even casual observation that lodgepole and spruce seed- 

 lings recovered from slight injury much more rapidly than the 

 fir. Western yellow pine was intermediate in this respect. TVTiere 

 no visible " cooking " was observed immediately, but later observa- 

 tion showed that the seedling was injured, a portion of the stem 

 or the bases of the needles, rather than the tips, were likely to 

 he affected; and where such injury existed, the seedling usually 

 succumbed. 



INFLUENCE OF AGE OF SEEDLINGS. 



A portion of the data in Table 4 is summarized in the last three 

 lines so that the results of this test may be compared in part with 

 the earlier tests on a basis of similar time and temperatures. Thirty- 

 five and six-tenths per cent of the seedlings were killed or severely 

 injured at 46 days of age, 39.4 per cent at 64 days of age, and 23.3 

 per cent at 92 days of age. The mean temperatures recorded in the 

 subtests, which are thus compared, were 147.9° F. for 9.3 minutes, 

 150.4° F. for 9 minutes, and 149° F. for 13.5 minutes. The higher 

 loss in the second period is fully accounted for by the 2.5° additional 

 temperature; and there may, of course, have been variations in 

 atmospheric conditions or in the arrangement of the apparatus which 

 made the recorded temperature more effective at one time than at 

 another. However, the average result for the third period shows 

 clearly that the seedlings become more resistant with age. This 

 seems to be true of all the species considered, but is possibly true in 

 a slightly greater degree for the spruce and lodgepole seedlings, 

 which at the outset are extremely small and frail. 



TEMPERATURE SCALE FOR EACH SPECIES. 



It is evident that some individual seedlings offer much greater re- 

 sistance to heat injury than do others of the same species, age, and 

 genera] origin. Such variations are to be expected With all organ- 

 isms. Owing to the comparatively small number of seedlings tested 

 for each age, period, and temperature, these variations make it very 



