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



Douglas fir or even to yellow pine. The Black Hills in general, being 

 " hilly" rather than mountainous, encourage yellow pine on all sites; 

 but in the more mountainous portion, and where the erosion of sedi- 

 mentary formations has resulted in long and steep slopes, the Black 

 Hills spruce is found, as might be expected from this description. 



In order to complete the picture of a valley cross-section, it is only 

 necessary to add that, except in narrow canyons whose wails shade a 

 great area, the creek bottom facing the south is likely to be of a very 

 different character. That area which is not shaded when the sun is 

 highest will immediately take on the character of the south slope, as 

 described in paragraph 1, with this important exception, that the 

 soil deposition, seepage, and run-off which this area may receive, may 

 encourage a denser stand than characterizes the upper slope, and in 

 this denser stand it is not uncommon to find stragglers of the more 

 tolerant Douglas fir, and even of spruce. 



(5) The east slope at middle elevations. — As has been clearly shown, 

 the east slope in trie Pikes Peak region receives a large proportion of 

 the direct insolation, for the simple reason that the mornings in 

 summer are generally cloudless. This means that surface tempera- 

 tures may be attained almost, if not quite, as high as on south 

 exposures, it being understood that the maximum effects of insola- 

 tion are approached very quickly. This direct insolation is, however, 

 of less duration than on south slopes, and its drying powers are pro- 

 portionately less. The evaporation may be rated between 0.70 and 

 0.80. The result is of course a better conservation of moisture. 

 This is without doubt augmented by a fairly good soil, built up by 

 the wind deposits of material from west exposures. 



The existence of direct insolation, creating excessive tempera- 

 tures in the morning if only for a short time, means that the pioneer 

 trees must be largely yellow pines. The conservation of moisture 

 however, permits heavier stands than can be supported on south 

 slopes. The result is that on easterly exposures the succession of 

 pine by Douglas fir is more prompt and more certain than on south 

 exposures. 



Under certain conditions there may even be no pine stage. A good 

 illustration is that of an extensive east slope below a middle elevation, 

 which after being denuded by fire produced directly a heavy stand in 

 which fir is much more prominent than yellow pine. This entire 

 slope, however, is boulder-strewn, and these boulders perform the 

 same function as pine trees; that is, they supply the necessary shaded 

 spots for fir seedlings. The boulders are much to be preferred to 

 pines as nurses, because they use no water. This phenomenon may 

 occasionally be noted even on south exposures. 



(6) The west slope. — The winter winds make this aspect by far the 

 most difficult of any at middle elevations to cover with vegetation of 

 any sort. A west slope does not in the aggregate receive so much 

 insolation as an east slope, yet on exceptionally clear summer days it 

 may easily become very warm, and is with certainty fitted only for 

 the limber pine and yellow pine. In summer the evaporation rate is 

 moderate, and the moisture conservation would be good were there 

 any soil to retain it. In winter the evaporation rate is nearly as great 

 as on a south exposure. Probably a more severe strain is presented 

 here because the soil is thawed much less frequently. Along with 

 the desiccating effects of the west wind, which is a factor only in 



