682 RYDBERG: PHYTOGEOGRAPHICAL NOTES 
instance, there is no timber line at all on the southern and western 
sides, for no timber is growing between the semi-arid cedar-pinyon 
belt and the top of the mountains. The whole southern and western 
slopes of the mountains proper are covered by a semi-arid grass 
formation. The highest peaks (altitude about 11,000 feet) just 
reach the timber line on the eastern and northern sides, only one 
or two hundred feet belonging to the alpine region. The desic- 
cating effects of the winds are increased by the thinness of the 
atmosphere. 
DEEP SNOW 
Deep snow is also a factor. As the desiccating wind lowers the 
altitude on the wind-swept ridges so does the snow in the heads 
of the valleys. I have already mentioned that the great snow- 
drifts or glaciers here shorten the growing season. But the snow- 
drifts have also a direct mechanical influence on the timber line 
in the way of smothering the tree vegetation. Herbs and low 
shrubs can withstand being covered by snow much better than a 
tree, for their growing season does not begin before the snow is 
practically off the ground, while the tops of the trees may be above 
the snow and exposed to the summer heat months before the snow 
cover of their roots and lower branches has melted. The lower 
portion of the tree is cut off from the air while the upper portion 
is already in vital activity. It is easy to distinguish trees stunted 
by the action of the wind from those stunted by the smothering 
snow. In the former the lower branches are enormously developed 
compared with the upper, and often creeping along the ground, 
while in the latter the lower branches are dead and covered by 
fungi or their mycelia. 
The usual condition in the Rockies is, that wherever there is 2 
large valley head, where the snow has a chance to lodge, this is 
always devoid of trees, except in places of higher ground, where 
the snow-drift has not been so deep and has had time to melt 
earlier in the summer. On such higher places there are often 
groves or isolated trees. The absence of trees in such a valley head 
is due less to the shortness of the season, produced by the snow, 
than to the smothering of the tree vegetation. 
