FOREST TYPES IX CENTRAL ROCKY MOUNTAINS. 147 



winter, must of course be mentioned its ability to remove the soil- 

 building materials. 



The physiological studies have indicated that limber pine is as low 

 as yellow pine in photosynthetic capacity, and perhaps lower; yet a 

 more important factor in its distribution is the probable fact that it 

 never exposes itself to rapid water losses through the stomata. Lack- 

 ing the vital qualities which make our forest trees what they are, this 

 weed is apparently satisfied to " close up shop" whenever the condi- 

 tions are in the least unfavorable, and thereafter it is practically 

 immune to desiccation by wind. It is for this reason that limber pine 

 becomes established on west slopes where yellow pine can not. 

 Although it is a valuable pioneer, it is intrinsically so worthless that 

 its tenure might well be temporary. Unfortunately, a cover of limber 

 pine on a westerly slope is usually scant, and neither the wind 

 exposure nor the insolation is very much modified thereby. So far 

 as the writer's observation goes, only spruce may be expected to 

 supplant the limber pine, and this probably so slowly that centuries 

 may be consumed in the succession. 



(7) Lower elevations. — The general relationships among the six 

 sites described above, must of course prevail at any elevation. The 

 fact of the matter is, however, that even when, topographically 

 speaking, these sites are duplicated near the foot of the mountains, 

 the conditions are found to be very different. At the foot of the 

 mountains there is less precipitation; higher temperatures prevail the 

 year round, tending further to dissipate this precipitation; the atmos- 

 phere is relatively drier at the eastern foot; and perhaps most impor- 

 tant, the higher air temperatures further augment insolation in pro- 

 ducing unbearable conditions at the surface of the ground. To a 

 certain extent these higher temperatures must be counterbalanced 

 by weakened light intensity. 



The effect of less precipitation and its more rapid dissipation must 

 of course be to make impossible on any site such dense stands as 

 prevail at higher elevations. When to the greater openness of stands 

 is added the greater heating opportunity per unit of area, it is readily 

 seen why there should be, first, the entire elimination of the heat- 

 sensitive Englemann spruce, with Douglas fir occupying the most 

 sheltered sites, then the confinement of the forest to northerly 

 aspects, and finally its disappearance where the mountains merge into 

 the plains. 



It is not necessary or relevant to do more than mention blue spruce 

 (Picea parryana), white fir {Abies concolor), or pinon pine (Pinus 

 edulis) y which are evidently more heat-tolerant than their mountain 

 counterparts and succeed in slightly extending the group ranges. 

 These trees have little place in the economy of the forest and have 

 been given no careful study. 



There is not much question in the writer's mind that soil quality is 

 an important factor in the sudden termination of the forest at low 

 elevations. Not only in the Pikes Peak region but generally through- 

 out the Rockies, the mountains proper are composed of igneous rocks 

 and the foothills of sedimentaries. The former produce open, more 

 or less gravelly, well-drained soils, the drainage being further insured 

 by the strong relief. The sedimentaries produce fine, compact soils; 

 and as the foothills are almost invariably devoid of springs and per- 

 manent streams it is evident that their soils are rarely subjected to 



