FOREST TYPES IX CENTRAL ROCKY MOUNTAINS. 131 



At the other extreme stand the canyon and lower-slope spruce 

 sites, whose soil is mainly transported. This soil is of finer quality 

 than that of the slopes, is rich in humus, which may blow into the low 

 places as well as wash in, and consequently is of the greatest water- 

 holding capacity. This great capacity and the usually high leyel of 

 the moisture content encourage dense stands. Only spruce can 

 grow in the densest stands; other species are excluded not by lack of 

 moisture, or by too much moisture, but by lack of insolation. It 

 seems possible that these yery sites may, after unusually prolonged 

 drought, proye to be the driest. The difficulty which is often expe- 

 rienced in starting plantations in gulches which encourage heayy vege- 

 tation is testimony to the extreme desiccation of such soils when rain 

 is withheld. In 1909, the writer definitely found the bottom situa- 

 tions to be the driest of all the sites in late summer in the Nebraska 

 sand hills. 



The Douglas fir sites on north slopes are almost as moist as the 

 lower spruce sites, and probably do not dry out to so great a degree. 

 The surface conditions, especially, remain fairly favorable because 

 of the lack of drying insolation. On the other hand, the evidence of 

 the south-slope fir site at Wagon Wheel Gap, and the occurrence of fir 

 seedlings almost on the roots of yellow pine on south slopes, shows 

 convincingly that the need of this species is not for great moisture, 

 but rather for moderate temperature conditions. 



RECAPITULATION. 



GENERAL CHARACTER OF THE CLIMATE. 



The mountain climatic conditions, which have been portrayed 

 mainly through the relatively long-term records at the Fremont 

 Forest Experiment Station, at an elevation of 8.836 feet on the eastern 

 slope of Pikes Peak, may be characterized as follows, in terms which 

 apply generally to the forest types (fig. 10). 



(1) The summer or growing-season conditions are not unfavorable 

 to the growth of vegetation that has very moderate heat requirements. 

 The mean growing-season temperature is about 55°, with the period 

 from June 10 to September 10 free from freezing temperatures. 

 Almost 50 per cent of the total precipitation occurs during the grow- 

 ing season (June 1 to September 10, as here considered) following 

 a brief drought in May or early June; hence the moisture of the 

 deeper soil stands throughout the season at a favorable point, rarely 

 if ever approaching the nonavailable content. This period is never 

 characterized by high winds, and great atmospheric dryness occurs 

 only when the drought period extends well into June. 



(2) With the cessation of daily showers, which rapidly decrease 

 in number and volume after August 15, a period of clear, dry weather 

 occurs which rapidly depletes the moisture of the surface soil and 

 which may, by the end ol October, cause the death of a large propor- 

 tion of the new seedlings of the season, most of which will not have 

 germinated before July, and will therefore be very immature and 

 poorly rooted. In tnis wholesale elimination, which doubtless 

 makes some distinction between species, the early frosts may have 

 some influence, but do not appear to be so important as drought. 



