THE MONTANE ZONE 151 



the nights usually are cool, almost to frost, and the usual daily 

 temperatures of July and August about 75 to 85 degrees. Engel- 

 mann spruce is sometimes planted in Missoula, but does not 

 thrive even under careful summer irrigation, although the Col- 

 orado blue spruce (Ficea Parryana) and the Norway spruce do 

 very. well. Within the limits of its range the abundance and 

 dominance of Engehnann spruce is conditioned upon its seeding 

 habits and tolerance. As a competitor with other species it has 

 the distinct advantage of being strongly shade resistant, and 

 this quality is its sole compensation for its slow growth in height. 

 Field seedlings gain in height less than 4 inches in 4 years under 

 conditions apparently favorable. Other data on growth are to 

 be found in Tables 22 and 23. 



The growth of this species may easily be checked by fires, 

 as the bark is very thin and the trees easily destroyed. Fires 

 are less frequent, however, in this kind of forest than in some 

 others, owing to the moister conditions under which the tree 

 usually grows. The species is subject to few serious parasites 

 among the fungi, though some among the insects operate to re- 

 duce its vitality or shorten its life. 



The most frequent companion of the Engelmann spruce at 

 all elevations, but especially in the higher ones, is the alpine or 

 sub-alpine fir (Abies lasiocarpa) . It has almost as wide a range 

 of vertical distribution as the spruce, but in altitudes above 6,000 

 feet it becomes much more conspicuous and abundant, probably 

 owing to the lessened frequency of other species. 



The salient characteristics of this species are, in the posi- 

 tive direction, a liberal production of seed from early youth, 

 facility of dispersal and free germination. The species is tol- 

 erant and resistant to cold. Negatively its seeds have transient 

 vitality, the tree requires much moisture both in soil and air, its 

 upper temperature limit is low, its early growth is slow and it 

 is not resistant to fires (61). 



The alpine fir is the most typical tree of the high mountains 

 in Montana, and from the highest slopes it descends to 2,000 

 feet in canyons in the northwestern part of the State. The most 

 favorable situations for this species are to be found in the 

 sheltered basins at altitudes from 5,000 to 7,000 feet. It is 

 found usually upon slightly elevated knolls of sloping ground 



