138 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE. 



These computations show a maximum evaporation of about 100 

 units in the thrifty yellow pine forest, during the mean period of 

 frozen soil. Yellow pine barely exists, in mixture with limber pine 

 and Douglas fir, where the evaporation may be 350 units. Nearly 

 300 units are possible in an opening of the north-slope Douglas fir 

 forest, but within the forest the total stresses are less than one-third 

 as great. The same type at YVagon Wheel Gap shows the possibility 

 of about 300 units, but only half as much on a well-insolated south 

 slope. The spruce sites show from 60 units (low canyon type) to 

 400 units at timberline. The greatest loss noted occurs on a high, 

 windswept ridge, where limber pine is now the only claimant of the 

 ground. The lodgepole pine type in southern Wyoming, although 

 soil temperatures are slightly below freezing for five months, has 

 evaporation stresses barely exceeding those of the yellow pine type. 

 Possibly the rapidly increasing exposure to wind at high elevations 

 and the increasing length of the period of soil freezing would set a 

 limit to the upward extension of spruce, though the limit of ability to 

 resist the mechanical effect of wind and snow may be reached first. 

 Regarding drought in alpine situations and the degree of xerophytism 

 which it develops, the following citation from Cowles (11^ is of 

 interest: 



The distribution of alpine plants, however, is apparently due in large degree 

 to edaphic conditions. The timberline in general may probably be referred to 

 atmospheric conditions, but the marked gaps and oscillations which usually 

 occur are due in a large measure to soil relations. While xerophytes increase 

 in the alpine parts of mountains, it is to be observed that edaphic as well as 

 climatic factors become more xerophytic upwards. While changes occur as one 

 traces one type of edaphic formation upwards, these changes are far less marked 

 than are those observed in passing from one edaphic formation to another. 



Although soil freezing does not appear to be a generally critical 

 factor in relegating the species of the central Rocky Mountains to 

 their respective sites, it would seem to have a marked influence on the 

 distribution of yellow pine in view of the sensitiveness of the species 

 to winter drying. ''Winter killing'' rarely does more than defoliate 

 the established tree, and a few trees of this species have become 

 established where the exposure must be very severe. However, the 

 heavy loss of seedlings of yellow pine during the winter, in both 

 plantations and natural stands, suggests that on sites which do not 

 hold a snow blanket the fatal effects are most likely to be felt by very 

 young trees, both because there is less opportunity for storage of 

 moisture in the tree itself, and on account of the nearness of the foliage 

 to a reflecting surface which would greatly augment transpiration. 



PRECIPITATION AND SOIL MOISTURE. 



The annual precipitation at the control station is about 22 inches, 

 of which nearly one-half occurs during the growing season. In 

 the Pikes Peak region this annual amount decreased to 14 inches 

 at the base of the mountains, and increases to nearly 2h inches in the 

 ce zone. There is, then, an increase of about 2.4 inches per 

 1.000 feet increase in elevation. In the Rio Grande region the pre- 

 cipitation seems to be somewhat Less at a middle elevation, but is 

 fully as much at high elevations, hi southern Wyoming the lodge- 

 pole pine type, slightly higher than tin* control station, has only 15.6 

 annually, and only one-fourth of this amount occurs during the grow- 

 ing season. 



