THE MONTANE ZONE 149 



of both the Hudsonian and Canadian zones. It is found plenti- 

 fully both east and west of the divide, forming 11% to 21% 

 in the Belts and Absaroka, 13% in forests of the Kootenai Val- 

 ley and more or less in other parts. At lower elevations the 

 species finds its best conditions; on bottom lands and in cool 

 canyons and frequently reaches 3 or 4 feet in diameter. Pure 

 forests of Engelmann spruce occur and such are found in Gla- 

 cier Park, over limited areas, in the upper Avalanche Basin and 

 a good, uniform stand on the slopes of Brown Pass over the 

 Continental Divide between Waterton and Bowman lakes. 



Certain factors are conspicuous in the nature and habits 

 of this species. It produces abundant seed ; the seeds are small, 

 light and amply winged, and easily carried by the wind; the 

 seeds germinate well ; the tree endures much shade and the 

 seedlings do well under heavy cover. The trees have great long- 

 evity and bear seeds for an extended period of time. On the 

 other hand the Engelmann spruce grows slowly even under the 

 best of conditions; it demands much moisture in the soil; its 

 temperature limits while they may be wide for brief periods 

 (-60 to 108) are more especially favorable toward the cooler end 

 of the scale. 



The chief determining factors in the distribution of Engel- 

 mann spriK e are Avater and temperature. It has been held (27) 

 that moisture alone is the dominant influence and that low tem- 

 peratures operate mainly in increasing relative humidity. This 

 seems to be true and the fact that in regions where the species 

 abounds it grows to large size in narrow canyons but may rarely 

 appear in the broader open stretches of river bottoms in the same 

 locality and at the same altitude where conditions are unfavor- 

 able for an equal degree of relative humidity. The effect of 

 high relative humidity, however, must not be regarded merely 

 as insuring a greater amount of soil moisture, but, as in the 

 case of the yew and other species, seems to operate directly on 

 some phase of the transpiratory function. Places where soil 

 waters are near the surface and never lacking are seldom if 

 ever occupied by Engelmann spruce under conditions where 

 the weather is continuously warm during several weeks of sum- 

 mer and the relative humidity low. While day temperatures 

 may be occasionally high, in the favored habitat of this species, 



