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1924* 

 UNITED STATES DERi 



RICULTURE 



DEPARTMENT BULLETIN No. 1263 



Washington, D. C. 



September 3, 1924 



RELATIVE RESISTANCE OF TREE SEEDLINGS TO EXCESSIVE HEAT 



By Caelos G. Bates. Silviculturist, and Jacob Roesek. Jr., Forest Examiner, 

 Fremont Forest Experiment Station, Forest Service 



CONTEXTS. 



Page 



The problem 1 



Literature available 2 



Description of experiments 5 



Preliminary tests 5 



Plan of tests in 1922 5 



Results in moist air 30 days 



after sowing 7 



Page 



Description of experiments — Contd. 

 Results in dry air 46, 64, and 



90 to 92 days after sowing 9 



Influence of age of seedlings 12 



Temperature scale for each spe- 

 cies 12 



Conclusions 13 



Literature cited 16 



THE PROBLEM. 



The studies of forest types and of the limiting factors in the dis- 

 tribution of the Rocky Mountain forest trees, which have been car- 

 ried on at the Fremont Forest Experiment Station near Manitou, 

 Colo., and elsewhere in the central Rocky Mountain region since 

 1910, indicate that excessive temperatures due to direct insolation 

 may often be a deciding factor as between one species and another. 



In nature, however, and under the usual experimental conditions, 

 it is almost impossible to attain an intense heat or an injurious tem- 

 perature without rapid and usually thorough drying of the sur- 

 face soil. For this reason it has been very difficult to draw safe con- 

 clusions as to whether seedlings which succumb under direct insola- 

 tion and extreme heating of the surface soil were in fact first in- 

 jured by the temperature or by this dry condition. 



With a moist condition, the first injury under intense heat may 

 be to the leaves. But in direct sunlight the surface of the soil, un- 

 less it is very moist, reaches a higher temperature than the leaves 

 of the seedlings. If. therefore, the high temperature is approached 

 gradually with opportunity for soil drying and heating, the first in- 

 jury may be to the stem of the seedling, which wilts at the ground line 

 and causes the seedling to collapse. This constitutes a " border- 

 line ? ' case between heat injury and moisture shortage. More pro- 

 longed heating and drying will cause a general wilting, plainly due 



91204°— 24 



