FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 137 



Soil freezing. — The fact about soil temperatures which stands out 

 preeminently is that the soil which has a low mean temperature or a 

 low summer maximum is likely to be frozen for a much longer period 

 than the soil which, either because of its low elevation or on account 

 of direct insolation, attains higher mean and maximum temperatures. 

 The period and extent of soil freezing may, however, be considerably 

 modified by a blanket of snow. Evervone knows that an early fall 

 of snow may completely protect the soil from freezing, though Warm- 

 ing, (22) considering only one phase of the matter, says: ''Only where 

 there is a covering of snow is the temperature of this (the soil's) 

 surface, also its daily mean temperature, lower than that of the air." 

 Evidence might be introduced to show that under a deep snow cover- 

 ing the soil temperature may remain stationary for weeks at, say 

 32°, while the air temperatures go to 0° or lower. Bouyoucos (7) 

 has convincing data on this subject. 



The data available show that the mean temperatures of soils at a 

 depth of 1 foot in pure yellow pine types are likely to be below 32° 

 for two or three decades; 12 that the continuance of this temperature 

 for 1 1 decades does not prove fatal to pine but does tend toward its 

 replacement; that in Douglas fir sites it varies from 7 to 17 decades; 

 and that a period of 15 to 19 decades may be somewhat characteristic 

 of the higher spruce types, where the mean soil temperature for the 

 year is 32° or less. Thus there are marked variations for any one 

 species, but it should be borne in mind that within certain limits the 

 conditions which produce soil freezing are likely to be concomitant 

 with conditions which reduce transpiration. 



The gradation from spruce to pine sites in the severity of soil 

 freezing is really more marked than the average soil temperatures 

 indicate. The liability to freezing which will permit injury of the 

 aerial portions of the tree is better measured by the products of 

 periods and greatest depressions, which, as roughly computed, vary 

 from 3,600 at timber line to 30 in the yellow pine type. Where the 

 mean temperature for any decade is never very much below freezing 

 the continuity of the freeze is likely to be broken on any warm day 

 and the possibilities of such relief decrease just about as the amount 

 of depression below 32° increases. At the highest stations there can 

 be found no source of relief until almost the close of the freezing 

 period, when snow water may penetrate the surface soil in advance 

 of thawing at 1 foot. 



Evaporation during soil freezing. — In a measure, the longer period 

 of soil freezing at higher elevations and on poorly insolated slopes 

 should be compensated by the decreased opportunities for water 

 loss by evaporation. It is evident that the important thing is not 

 the period of soil freezing, nor the temperature reached, but the fate 

 of the water stored in the stem and foliage of the tree, which must be 

 held during this period. 



The best computations which it has been possible to make indicate 

 that the higher forest types are subject to the greatest total loss by 

 evaporation, notwithstanding the much lower daily or monthly 

 losses. A large part of this total loss is likely to come near the enci 

 of the period of soil freezing, when air temperatures rise much more 

 rapidly than soil temperatures. 



U It is assumed that at a recorded temperature of 32° F. soils will never be frozen because of the 

 depression of the freezing point by solutes and surface tensions. 



