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the timber-line is higher up than on isolated mountains. This 
observation seems to hold good in the Rockies. In the isolated 
Belt Mountains and Crazy Mountains of Montana, the timber- 
line is lower than in the main Rockies. In the Wasatch and 
La Salle Mountains of Utah, the timber- line j is lower than in the 
Rockies of Colorado. Even in the Rockies, the timber-line is 
ey where the mountains are more massed. For instance at 
— it is higher than on Pikes Peak, Sierra saat or 
ee o adequate explanation has been made and D 
Kobe ae no suggestions to make. 
8. Exposure to or protection from direct sunlight h identl 
diss: great influence upon the altitude of the dimberhae. 
9. Physiographical Barriers. To these may be counted snow- 
drifts and the glaciers already Senne Besides fee are the 
sees cliffs and rock-slides. Meeting one of these barriers 
the timber ceases to grow sometimes a thousand feet lower 
ie the physiological timber-line. Alpine plants are often 
found in the crevices of the cliffs and among the rocks in such 
places. 
10. Oecological Barriers. Sometimes an oecological timber- 
line is mentioned, 7. e., w the bacteria in the soil and other 
ganisms necessary for am growth of trees cease to exist. 
Theses it is ) to see that such a timber-line may exist, 
but practically there is no information regarding its existen 
in the Rocky eae ag from the merely phy: ree 
ones. 
conomic timber-line. In Switzerland there exists an 
economic timber-line. The alpine meadows are used for summer 
a for sheep and goats. These animals make ees 
on the new trees and hinder the spreading of the mee but i 
many ae the subalpine forests are actually cut down by 
man to make room for more pastures. Such economic hie 
line cannot be spoken of in the Rockies 
After having discussed the causes oi the timber-line, it is 
easier to define what an alpine plant is. In short, it is a plant 
that can endure the climate of the mountains above the timber 
ine. It is, therefore, a plant that requires less temperature 
