FOREST TYPES IX CENTRAL ROCKY MOUNTAINS. 81 



dryness plays such an important part, the contrast between a shaded 

 north-slope site and an open south exposure is far less marked than 

 at Fremont, except for the coldest part of the year. 



(3) The high spruce burn at Wagon Wheel Gap (W-D) shows, 

 for the growing season, the highest ratio to the control station of all 

 the stations studied. During the winter months its ratios are rela- 

 tively much higher than those for the timberline station on Pikes 

 Peak, due no doubt to the fact that the latter receives negligible 

 insolation. It has already been noted that Station W-D shows at 

 all times somewhat greater wind movement than the control station. 

 The importance of this factor, combined with full insolation and act- 

 ing in the presence of low air temperatures, is plainly shown in the 

 not insignificant evaporation of the winter period. 



The very low evaporation rates in the two local spruce stands 

 (F-3 and F-5) reflect the influence both of heavy shade and of the 

 close growth of the stand which reduces the air movement almost to 

 nothing. Here again, however, the potent effect of atmospheric 

 humidity may be observed; for, although the insolation at either 

 station is probably not more than 10 per cent of that received at the 

 control station and the wind velocity is reduced in almost as great 

 proportion, the evaporation is about three-tenths as great as at the 

 control station. It should be noted that at Station F-3, as at the 

 north-slope fir stations, the evaporimeter is sometimes completely 

 covered by snow. This does not occur at Station F-5. 



(4) The single lodgepole pine station (F-ll) is marked by moderate 

 evaporation at all periods. The winter rate is not excessively low, 

 as the temperatures might suggest, because the wind is at this time 

 a very strong factor. 



(5) The local limber pine type (F-6) shows clearly the effects of 

 winter winds which strike the slope veiy squarely, the winter evap- 

 oration rates being among the highest notwithstanding the lack of 

 sunlight on this northwest slope. The higher limber pine site (F-13), 

 which has been depicted as presenting nearly timberline conditions, 

 is, so far as the available record can be depended upon, even more 

 severe in winter than is the actual timberline site, the former being 

 considerably warmer and better insolated. 



(6) If the three major types are compared, for which the data are 

 adequate, it is seen that, with anything like normal cover, there is a 

 decided tendency for the evaporation rate to grade down from pine 

 to Douglas fir and from the latter to spruce. This decrease accom- 

 panies the decreasing temperatures which are characteristic of the 

 types. It is undoubtedly augmented by the reduction of insolation 

 due to increasingly dense stands in the higher zones. Although free 

 air movement evidently increases generally with increase in elevation, 

 the greater density of the stands tends to nullify any effect from this 

 source. 



It may, then, safely be said that the typical conditions for the 

 reproduction of the species are evidently less severe, as regards the 

 tendency toward evaporation, in the spruce than in the fir or pine 

 types. This statement is not in the least weakened by the fact that 

 in very open situations there is practically as much evaporation in 

 one zone as in another, as is evidenced by comparing Station M-l 

 with the control station, or Station W-B2 with Station W-D. It 



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