FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 101 



4 feet, is very discouraging to any kind of reproduction, but permits some bristle- 

 cone pine as well as fir. The entrance of bristlecone rather than yellow pine 

 under these conditions may, perhaps, be taken as evidence of dissimilar soil 

 preferences of the two pines. 



(e) Additional density of cover on a north slope (F-9) reduces the extremes 

 of soil temperature 1° or 2° without appreciably affecting the period of soil 

 freezing. This change of temperature extremes, which would be much more 

 apparent if absolute maxima and minima nearer the surface could be considered, 

 is conducive to better germination of both spruce and fir, but results in an 

 undergrowth controlled by spruce. 



(/) A site not unlike the last in its air-temperature conditions (W-Al) may 

 show much lower soil temperatures at all seasons (4° to 6°) without appreciably 

 changing the character of the reproduction. The two sites are most similar 

 during the growing season, both in absolute soil temperature and in having soil 

 temperatures much (8° to 10°) below the air temperatures. They vary widely 

 in the severity of their soil freezing; but, when atmospheric conditions in this 

 period are considered, they are found to be not so unlike. (See Table 30.) 



(4) The soil temperatures of the four spruce sites here considered 

 cover a wide range, the mean annual temperatures for the 4-foot 

 depth varying from about 38° on the two sites at 9,000 feet elevation 

 to 33° at or near timberline. The timberline station at Fremont and 

 the north-slope station at nearly the same elevation at Wagon Wheel 

 Gap show essentially the same mean temperatures; but temperatures 

 for the timberline station go to much greater extremes, owing to the 

 lack of forest cover at the observation point, and the lack of a snow 

 blanket in winter. 



When the canyon spruce sites (F-3 and F-5) are compared with 

 the north-slope fir site of the same locality in which spruce repro- 

 duction is encouraged (F-9), it is found that the former have equal 

 or higher soil temperatures during the growing season and warmer 

 and less deeply frozen soils during the winter. Of all sites, these 

 canyon bottoms would be the last on which fir would be found. The 

 spruce may also control the lower and, perhaps, the coldest portion 

 of the fir slope. It is then evidently not the soil temperature 

 conditions which exclude fir and permit excellent reproduction and 

 development of the spruce. The character of the soil at the foot of 

 the slope and in the canyon bottom is suggestive, as something in 

 common for these two sites, and after so3-moisture conditions are 

 considered, it must be admitted that soil quality and moisture and 

 not soil temperature seem to be the controlling factors in this 

 situation. 



The timberline station on Pikes Peak (F-16), where the soil receives 

 full insolation, attains to summer soil temperatures ahnost equal to 

 those at the lower (canyon) limit of spruce. In view of this fact, as 

 well as the great intensity of sunlight at the high elevation, it is 

 evident that the heat conditions for a Reasonable growth each season 

 are fully satisfied. By comparison with a similar elevation at Wagon 

 Wheel Gap, however, the effect of the canopy in modifying tempera- 

 ture extremes is plainly seen and it becomes evident that near 

 timberline the forest may have a tendency to choke itself by creating 

 soil conditions not conducive to vigorous reproduction. In other 

 words, at a certain point the temperature will not permit a perma- 

 nently dense stand. The result is that in thinner stands each indi- 

 vidual tree during the long winter is subjected to an exposure which 

 can not be tolerated beyond a certain point. The temperature con- 

 ditions at different seasons are thus all interrelated in limiting the 



