FOREST TYPES IX CENTRAL ROCKY MOUNTAINS. 



99 



F-4 the soil normally is frozen at the 4-foot depth for 5 or 6 de- 

 cades. The Nebraska sandhill soil has not, so far, closely approached 

 freezing at the 4-foot depth. 



One of the reasons, therefore, for the crowding out of yellow pine 

 by Douglas fir and for the producing of a " succession" in the truest 

 sense, may be the shade of the mother canopy, which induces a pro- 

 longed winter drought for the small trees. In~the one situation where 

 this is fairly represented (F-4) the condition of the parent trees, as 

 well as the composition of the reproduction seems to show the sever- 

 ity of the strain so produced. However, it should be observed that 

 a heavy canopy at the same time so reduces the exposure of each 

 tree or seedling that the immediate and radical effects of winter 

 drying, commonly described as " winter-killing," are not observed 

 in the fairly dense forest. 



(2) It may be well to examine next the limber pine sites, repre- 

 sented by F-6 and F-13. Notwithstanding the fact that the second 























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Fig. 8.— Relation of soil and air temperatures with and without forest cover north slopes. 



station represents almost the upper limits of the species, its mean 

 temperatures _ are almost equivalent to those of the lower station, 

 and its growing-season temperatures slightly higher, because of a 

 bare and well-insolated soil. The relatively high values at either 

 station give emphasis to the argument that limber pine seedlings 

 thrive under intense radiation, and that the necessary heat for 

 growth, at elevations where the air temperatures are generally low, 

 is secured on burns and rocky ridges through purely local absorption 

 of heat by the ground. It is probable that the maxima attained in 

 the soil and immediately above it will fall little short of those for 

 yellow-pine sites. Also the winter soil temperatures of these sites 

 are considerably higher than the air temperatures, especially at the 

 higher elevation. In mean soil temperatures the sites correspond 

 closely to the quasi-vellow-pine sites, but in neither of the limber- 

 pine sites is freezing to a depth of 4 feet to be expected. This eon- 

 elusion for Station F-13 is reached after a consideration of the 

 relative winter temperatures at 1 and 2 feet. 



