136 BULLETIN 1333, U. S. DEPARTMENT OF AGRICULTURE. 



Broadly speaking, evaporation rates are almost as high at the 

 highest as at the lowest elevations in summer, temperature and wind 

 movement being almost compensating. In the winter, however, 

 evaporation is almost nullified by very low temperatures in the higher 

 forest types (spruce and lodgepole) where the vapor deficit becomes 

 very small, and high wind is, therefore, noneffective. In the middle 

 and lower portions of the Pikes Peak region winter evaporation is 

 an important factor because of the frequent combination of high 

 temperatures with strong winds. In the Wagon Wheel Gap locality 

 the evaporation is relatively high for corresponding sites except in 

 winter when the temneratures are very much lower tnan at Fremont. 



If measured near the ground the evaporation in the various forest 

 types during the growing season is found to be much more closely 

 controlled by local insolation and temperatures than by the moisture 

 or movement of the atmosphere, and in consequence it can be said 

 that the well-insolated sites are much more conducive to transpira- 

 tion losses than are the cooler ones. The soil temperatures in this 

 period give a fair indication of the evaporation stresses, which are 

 particularly important in dissipating the soil moisture. Ecologists 

 nave generally considered that a high evaporation rate during the 

 growing season constituted a barrier to the success on the site of the 

 more moisture-demanding species. This may be true in the extreme 

 case in which the rate of transpiration is likely to put a serious tax 

 upon the ability of the plant to secure moisture. In the present 

 study it has not been found that the moisture reservoir was seriously 

 depleted on any site. It has been seen that the species of greatest 

 moisture demands according to the results of this study succeeds on 

 the site which produces the greatest evaporation stresses currently 

 during the growing season and in the aggregate. The conclusion is 

 inevitable that, so long as the moisture supply is fairly good, high 

 evaporation can not be considered deleterious in the usual sense and 

 that it can not be a selective factor as between species. Rather it is 

 merely evidence of heat conditions which encourage the species that 

 is most likely to transpire freely, and which are injurious only to the 

 species that for physiological reasons does not transpire so readily. 



On the other hand, during the winter the moisture supply may not 

 be available; it is obviously less available at high elevations than at 

 low because of the much lower air temperatures; it is less available 

 on northerly than on southerly exposures because of the poorer inso- 

 lation of the former; it is less available where the ground is bare than 

 where it is covered by a snow blanket laid on early enough to prevent 

 freezing of the soil. In view of the last-named fact the Pikes Peak 

 region presents a unique situation, a situation in which the very 

 existence of establishea stands of evergreen trees is threatened almost 

 yearly by drought of the soil and air during the period of vegetative 

 dormancy. 



WINTER SOIL TEMPERATURES. 



It has been stated that soil temperatures during the growing season 

 supplement or may be substituted for air temperatures as measures 

 of the growing conditions for seedlings. Perhaps the temperature 

 of the soil near its surface is the best measure of the heat, and espe- 

 cially of the maximum temperature, to which the seedling is subjected 

 at a critical age, and no doubt this measurement should receive more 

 consideration than it has in the present study. 



