16 BULLETIN 1263. U. S. DEPARTMENT OF AGRICULTURE 



peting vegetation and reasonable rainfall. However, spruce seedlings 

 often come in under stands of pine, where pine seedlings will not 

 grow at all, and thus ultimately supplant the pine. 



4. The resistance of all seedlings becomes slightly greater with 

 aging and accompanying hardening of tissues and with increase in 

 the number of needles, some of which no doubt protect others. Older 

 trees are also much less liable to heat injury than young seedlings, 

 because of the stem protection furnished by bark and the greater 

 distance of the foliage from the ground surface. 



5. TYhile in nature high temperatures are seldom entirely dis- 

 sociated from a more or less arid condition, it is evident that ex- 

 treme heat must often kill seedlings which would be able to cope 

 with the existing drought conditions. The scale of heat tolerance 

 for the four species suggested in conclusion 3 (lodgepole pine, yellow 

 pine, spruce, and Douglas fir) is the same as the inverted scale of 

 resistance to transpiration, given on page 1:27 of Bates' (3) discus- 

 sion, as an average for two sets of results obtained under slightly 

 different conditions. The relative heat tolerance of the species is 

 somewhat at variance with their relative heat requirements for 

 optimum growth as this may be judged from the prevailing tem- 

 peratures of their habitats. All of these factors, as well as light 

 requirements, are important for the forester to have in mind. 



The observations presented above bring out the fact that under 

 certain conditions, perhaps seldom occurring in nature, maximum 

 temperatures may be the critical factor preventing natural repro- 

 duction. TThere this is the case, planting may be necessary or the 

 establishment of a given species may not be possible until the heat- 

 ing action of direct sunlight on the site has been somewhat modi tied. 



LITERATURE CITED. 



(1) Bates. C. G. 



Experiments in sandhill planting. Proe. Soc. Am. Foresters. Vol. V. 

 No. 1. 1910. 



(2) 



Transect of a mountain valley. Ecology, Vol. IV, Xo. 1, Jan.. 1923. 



(3) 



Physiological requirements of Rocky Mountain trees. Journ. Asr. Re*.. 

 U. S. D. A.. Vol. XXIV. No. 2, Apr. 14. 1923. 



»3aj 



Forest types in the Central Rocky Mountains as affected by climate and 

 soil. U. S. Dept. Agr. Bui. Xo. 1233. 1924. 



(4) Ceislar. Dr. A. 



Einfluss der Lichtstarke und der Tempera turhohe auf die grosse der 

 CWorophyllassimilation einiger Waldbamne. Note in Forestry Quar- 

 terly. Vol. VI. Xo. 3, Sept., 190S. 



(5) Hartley. CarL 



Stem lesions caused by excessive heat. Journ. Agr. Res.. Vol. XIV, 

 No. 13. Sept. 2, 1918. 



(6) Korstiax. (\ 1\. and X. J. Fetherolf. 



Control of stem girdle of spruce transplants caused by excessive heat. 

 Phytopathology, voL 11, Xo. 12, Dec, 192L 



(7) Mi-nxs, E. X. 



High temperatures and eucalypta Journ. of Forestry, Vol. XIX. No. 1, 

 Jan.. 1921 

 (Si MacDougal, D. T.. and E. R. Working. 



Another high-temperature record for growth and endurance. Science, 

 Vol. LIV. 1921 



(9) Shkeve. E. B. 



A thermo-electrica] method for 'he determination ol leaf temperatures. 

 The Plant World. voL 22, No. 4. Apr.. 1919. 



WAG PaiK PIKG t. FWU 



