FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 105 



available, are seen by considering even a few original observations. 

 For example, in a yellow pine site at Fremont, which has a very 

 slight tilt to the south, and averages 1° to 2° cooler than the control 

 station because of shading, while the soil temperature, as recorded 

 at 8 a. m., was continuously below 32° from November 28, 1920, to 

 February 27, 1921, a period of 91 days, there was during this period 

 only one decade in December and one in February in which no sur- 

 face temperatures above 32° were recorded at 8 a. m. Nor does 

 this by any means speak for the highest temperatures of the day; 

 and even the two exceptional decades showed maximum air temper- 

 atures above 40°. 



The conclusion is inevitable, therefore, that the evaporation stress 

 during periods in which soil moisture is completely nonavailable is 

 far less for well-insolated sites than is indicated by Table 30. On 

 the other hand, a continuously frozen soil, not only for the average, 

 but in each individual year, is a practical certainty on every north 

 slope and even in the canyon bottoms, which might be reached by 

 the sun, but where the summer moisture supply is adequate to develop 

 very dense stands. Table 30 may possibly, exaggerate the evapora- 

 tion stress by prolonging the period a little after the time when snow 

 melting is rapid enough to provide the surface roots with water, but 

 this very melting greatly retards the warming of the deeper soil. 

 This difference between insolated sites which generally produce open 

 stands and cooler sites which encourage denser stands must be further 

 accentuated when it is considered that the evaporation measurements 

 have nearly all been taken close to the ground. On a warm site the 

 evaporation is likely to be little, if any, higher at a considerable 

 elevation than near the ground. This is true not only because the 

 open forest permits very good air circulation at all levels, but also 

 because, in the winter period, radiation close to the ground may 

 frequently cause thawing which would not otherwise be possible, and 

 this will be an important element in the total evaporation. On the 

 contrary, it is self-evident that in a close stand, the higher the eleva- 

 tion in the midst of the crowns, the greater will be both the insola- 

 tion and the wind movement. The evaporation at the ground, 

 therefore, is no measure at all of the stresses to which the more exposed 

 crowns are subjected. 



With the quantities shown by Table 30 as a guide, then, but with 

 the factors which influence the relative values taken into considera- 

 tion, the following observations may be safely made: 



(1) The evaporation stresses to which seedlings may be subjected 

 in a dense forest, even in a region of high winter-sunshine percent- 

 ages, are appreciable for the total period of soil freezing, out un- 

 doubtedly become wholly insignificant with a snow blanket. On 

 the other hand, the dense forest, as it commonly is on a slope facing 

 the north, creates for its larger and more exposed trees a winter 

 drought of long duration coupled with considerable evaporation 

 stress. It is believed that the capacity for resisting the consequent 

 drying out is, perhaps, a better measure of drought resistance than 

 any ability which has, in the course of this study, been shown during 

 warmer periods, because no severe summer drought has been encoun- 

 tered. The winter drought, as it affects the older trees in a dense 

 stand, not only is of yearly occurrence, but seems to be almost inde- 

 pendent of precipitation, except as the presence of a general snow 

 blanket may increase the atmospheric humidity. 



