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 a 

 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. 



