RELATIVE RESISTANCE OF TKEE SEEDLINGS TO EXCESSIVE HEAT 3 



the sun and which are quickly and frequently dried out, at least in 

 the superficial layer. The pine, therefore, has the quick, deep-root- 

 ing habit and the large seeds which make such rooting possible even 

 before the cotyledons have unfolded. Pihon pine (P. edulis), which 

 grows in even warmer situations than yellow pine, has this habit 

 developed to a high degree, opening its cotyledons and shedding the 

 seed coat with the greatest deliberation while a strong tap-root is 

 being formed. 



It is, therefore, easily seen that in investigating the present reac- 

 tions of seedlings of the different species, not only the fundamental 

 physiological requirements must be taken into account, but adapta- 

 tions of form and habit which fit the trees for certain combinations 

 of conditions commonly occurring in nature. Western yellow pine 

 and Douglas fir, as shown by the large size of their seeds and the 

 deep rooting of the seedlings, are clearly adapted to sites whose 

 moisture supply changes rapidly, whatever their total requirements 

 for moisture may be. Lodgepole pine and Engelmann spruce seed- 

 lings come from much smaller seed, and are about equally frail, 

 shallow rooting, and unadapted to quick drying of the surface. 



The fact that the few references on heat resistance that have been 

 found are taken from observations in the more arid portions of the 

 United States, seems to indicate the close dependence of excessive 

 temperatures upon a dry condition of the atmosphere, and a con- 

 sequent aridity of the soil surface. The problem, as understood in 

 this discussion, arises from the great intensity and heating power 

 of sunlight, due both to the aridity and high altitude of much of 

 the Kocky Mountain region. In moister regions the problem is no 

 doubt less serious. 



The first specific evidence of direct heat injury to tree seedlings 

 appears to have been noted by Hartley (S) and Bates (1) in 1909, 

 in the conduct of planting experiments in the Nebraska sandhills, 

 where the occurrence in a single day's observation of a soil surface 

 temperature of about 60° C. led Bates to the belief that the losses in 

 plantations on south exposures, where much higher tempera cures 

 doubtless occur at times, might be primarily due to this factor. This 

 appeared especially probable in cases where the small trees had been 

 partly or completely buried by moving sand; but it was also noted at 

 this time that many trees, which had not been covered and did not 

 die, had considerable enlargements of their stems at the ground line. 



Hartley (-5) in 1918 published his observations of heat injury 

 noted at about the same time in Nebraska and elsewhere. He con- 

 cluded that lesions which were common on the stems of very young- 

 seedlings in the nursery were undoubtedly due to excessive heating 

 of the surface soil, while similar lesions on the stems of trees as 

 much as 4 years old were probably due to the same factor. Ex- 

 periments were conducted in the laboratory which showed that stem 

 injury very similar to that noted in nursery beds could be produced 

 by heat applied in several ways, but more readily above than below 

 the surface of the soil. 



Elsewhere it has been frequently noted in the handling of nursery 

 stock that trees growing close together had normal stems, while those 

 spaced widely enough to permit much sunlight to reach the ground 

 showed lesions on the stems. Korstian and Fetherolf (£), for ex- 

 ample, found that by inclining spruce seedlings slightly to the south 



