FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 109 



and coagulation of the protoplasm, or results primarily from an 

 excessive transpiration rate induced by the high temperature. 



In ''Physiological Requirements of Rocky Mountain Trees " {6) 

 the results of a heating experiment are reported, which show that 

 seedlings of Douglas fir and yellow pine, on the one hand, are relatively 

 much more resistant to this form of injury than the seedlings of 

 Engelmann spruce and lodgepole pine. In this test superheating 

 was induced by direct sunlight in a warm greenhouse, and the high 

 temperatures were usually maintained for several hours each day. 

 A temperature of about 130° F, as measured by a blackened ther- 

 mometer laid on the surface of the soil, appeared to represent the 

 critical point. The form of injury was invariably wilting, first 

 shown by the collapse of the stem of the seedling at the ground-line. 

 In view of the fact that the pans containing the wettest soil showed 

 little injury, the conclusion can hardly be avoided that in this test 

 excessive transpiration and to some extent desiccation of the soil 

 surface and of the stems at this level, were the primary causes of 

 injury. Therefore, yellow pine and Douglas fir seedlings withstood 

 injury because they are stouter and more deeply rooted than spruce 

 and lodgepole pine seedlings of the same age. 



Illustrating the complexity of the problem that is encountered 

 when the effects of surface temperatures in the field must be noted, 

 a more recent test may be cited. 10 In this test very high tempera- 

 tures were secured by placing the pots of seedlings in close proximity 

 to an electric heating coil with its radiation of long wave length. 

 With exposures of about 10 minutes, little or no injury was shown 

 until the pots were brought close enough to the coil so that a tem- 

 perature of 140° F. was recorded by a thermometer at the level of 

 the cotyledons. Temperatures as high as 130° F. could be endured 

 for much longer periods without marked injury. It therefore ap- 

 pears that a temperature of 140° F. or slightly higher, is directly 

 injurious in a period too short to permit much transpiration loss. 

 Under these conditions the cotyledons of Douglas fir shriveled 

 markedly, and usually with fatal results to the entire seedling; 

 yellow pine seedlings were next in order, but the injury was usually 

 localized in portions of the leaves, and not so likely to be fatal; 

 spruce seedlings shriveled completely, like those of Douglas fir, but 

 much less frequently; within the range of the tests lodgepole pine 

 seedlings were practically immune to this heat injury. 



With this view of the problem, and a realization of its intricacy 

 and the need for most careful and complete field records, there can 

 be but one object in introducing the available data on the surface 

 temperatures of sites in the Pikes Peak region, namely to show the 

 magnitude of the variations in surface temperatures, as compared 

 with the variations in the temperatures at a depth of 1 foot. After 

 considering these few data, and the facts which have been stated 

 above relative to the resistance of the several species to direct and 

 indirect heat influences, it will be apparent that while high tempera- 

 tures bring out a nice distinction between yellow pine and Douglas 

 fir, the line between fir and spruce is probably drawn on a different 

 basis, most probably on the basis of the moisture relations alone. 



The determination of the exact temperature of the surface soil, 

 during insolation, is next to impossible. It is practically possible 



10 u. S. Dept. 13 ul. I2&; Reiniivo Resistance of Tree Seedlings to Excessive Heat. 



