FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 107 



Fremont, is not without its significance. (Compare W-Al with 

 F-9, W-A2 with F-2.) It is due both to a drier atmosphere and to 

 greater prolongation of the soil freezing as a result of lower tempera- 

 tures at Wagon Wheel Gap. Although the forest conditions at the 

 north-slope stations are not dissimilar in the two localities, it is 

 believed to be true, generally speaking, that both spruce and fir are 

 pushed to somewhat lower levels in the Wagon Wheel Gap locality. 

 On the south exposures, there is at Fremont a yellow pine forest 

 with conditions slightly encouraging Douglas fir, and at Wagon 

 Wheel Gap, a Douglas fir forest with considerable bristlecone pine 

 of all ages. It is believed that the generally lower temperatures at 

 Wagon Wheel Gap would not alone explain the complete absence of 

 3^ellow pine, but that the greater possibility of destructive winter 

 evaporation does explain it in part. 



(4) Examination of the data for yellow pine sites, in the light of 

 what has been shown as to probabilities of relief through frequent 

 thawing, indicates that this species thrives best where the danger of 

 winter drying is least. There is a slight suggestion that the lower limit, 

 as approached at Station M-l, may be reached through an increase in 

 this risk; but there is insufficient data on the plains conditions to 

 justify this statement. It is known that the level plains freeze for 

 considerable periods, 9 and that during such periods the evaporation 

 may be relatively high; but as to the severity of this combination 

 no estimate can as yet be made. It is evident that the Nebraska 

 sandhills are not extraordinarily severe in this respect, and there is no 

 evidence of winter killing in the established yellow pine plantations 

 there. Stations F-12 and F-4 both appear to represent conditions 

 which, as a result largely of the shade cast by the pine trees themselves, 

 are approaching too great severity for the reproduction of that 

 species. 



The importance of this factor of winter evaporation with reference 

 to yellow pine is clearly shown by the frequency with which pine 

 forests in various parts of the range are injured by winter killing, 

 which is rarely "killing" except to the foilage, but must detract 

 greatly from the vitality of the trees affected and ultimately decrease 

 their chances for predominance. It has been seen currently that the 

 very lowest evaporation stress occurs on an open, smooth, southerly 

 slope (F-2), and under these conditions the stand is almost pure 

 yellow pine. It is significant that on other parts of this slope and on 

 other similar slopes, which are strewn with boulders, a great deal 

 more injury from winter drying has been noted; and that all such 

 boulder-strewn areas develop a large proportion of Douglas fir, which 

 is Jess seriously injured than is the western yellow pine. 



To sum up : Successively lower temperatures in the types from pine 

 to spruce, particularly well measured by the soil temperatures which 

 show the effects of insolation or its lack, lead to more continuous, 

 more severe, and more certain periods of soil freezing each winter. 

 Consequently, other conditions being at all equal, the fully exposed 

 trees growing on the coldest sites or in the coldest types are most 

 liable to a winter drying so extensive as to be injurious to the trees 

 and probably to affect the composition of the forest. Exactly the 

 reverse is likely to be true with respect to conditions affecting young 

 seedlings small enough to be protected by the snow blanket. Al- 



9 At Laramie, Wyo., the soil temperature at a depth of 3 feet is normally below freezing for 3 months, 

 January to March. 



