102 BULLETIN 12.33, U. S. DEPARTMENT OF AGRICULTURE. 



upward extension of the forest and in permitting the initiation of 

 single specimens only in sheltered nooks, where there is no room for 

 a self-protecting group or stand and where there is indeed insufficient 

 protection for the individual when it reaches above the level of rocks 

 or of the usual snow blanket. 



A mean annual air and soil temperature of 32° may be approxi- 

 mately limiting to growth on fully exposed sites, although there is 

 little question that some excellent spruce forests exist and produce 

 very satisfactory wood increments where this temperature is closely 

 approached. Low temperatures may directly preclude normal devel- 

 opment of stands during the growing season, and they may react, 

 through the long period of soil freezing, to create a very great winter 

 exposure; but it becomes evident that it is the winter exposure which 

 directly creates a a timberline." 



(5) The single lod^epole pine station shows mean soil temperatures 

 similar to those of the spruce type, but its winter temperatures are 

 more nearly those of the western yellow pine type. 



(6) To summarize: Soil temperatures are more responsive to the 

 local effects of insolation than air temperatures are, and even the soil 

 temperatures at a depth of 1 foot are probably more representative 

 of those conditions at the surface of the ground that affect germina- 

 tion and the young seedling than are air temperatures as they are 

 usually and conveniently measured. Consequently, soil temperatures 

 bring out more closely than air temperatures the growing-season con- 

 trasts between north and south slopes, between forested and open 

 sites, and between the heat of bare or rocky soils and older or better- 

 protected soils. Every soil-temperature difference in a single locality 

 such as the Pikes Peak region is accompanied by a change in the 

 character of the forest reproduction, although in some cases it is 

 doubtful if the soil temperature is the controlling condition. Yellow 

 pine, limber pine, and bristlecone pine all appear to enjoy warm soils 

 and to reproduce on sites where very high temperatures are to be 

 expected, at least for short periods. Douglas fir, on the other hand, 

 reproduces well only with sufficient shade to greatly modify these 

 high temperatures. This is true in the Pikes Peak region even on 

 northerly slopes where the sun's rays strike obliquely. The distinc- 

 tion between Douglas fir and Englemann spruce does not seem to be 

 so much a matter of temperatures which might injure young seedlings, 

 as of soil or light conditions which in the dense forest gradually starve 

 the fir. This distinction may be more logically discussed after con- 

 sidering the soil moisture data. 



Evaporation during ike period of soil freezing. — In accordance with 

 what has been shown in Table 29 as to the variation in the extent of 

 soil freezing in the different types, and with the suggestion that this 

 period should be measured, not in days, but in terms of the induce- 

 ment to evaporation, an effort is made to show, in Table 30, the total 

 probable evaporation Cor each such period. 



