Botany in Colorado 33 



many cases devoid of bark a condition produced in all probability 

 by blowing sand and ice particles during winter gales. The 

 Engelmann spruce is also a common tree at timber-line in the 

 canons. Here this species also forms low brush-like vegetation 

 and straggling trees are found here and there with prostrate 

 trunks and spreading branches which form a huge woody mat 

 often several feet in diameter. 



One who has never been in the mountains is inclined to 

 think that "timber-line" is at all places a perfectly regular and 

 clearly defined line of demarcation between the forest and the 

 alpine heights. But this is rarely true. Local conditions 

 modify the altitude to which trees ascend, and in fact "wind- 

 timber" runs up in widely isolated tongues to various altitudes. 

 Above such areas there are oftimes smaller areas of dwarfed 

 and gnarled trees isolated from the main body below. Timber- 

 line is usually much "higher" in canons and on north slopes 

 than on ridges and south slopes. 



Besides the conifers at timber-line there may be found 

 dwarf birches and willows forming rather extensive areas of 

 brush about two or three feet in height. This brush may pass 

 well up into the Alpine Desert Zone. Bearberry, Arctostaphylos 

 uva-ursi, is a very common prostrate creeping woody plant 

 forming extensive carpets of thick glossy leaves on the open hill- 

 sides in the lower stretches of Subalpine Forest Zone. With it 

 are many other species of thick-leaved xerophytes. 



Ponds and boggy places are quite common on flats along 

 stream courses. Such places are always characterized by a 

 brush formation composed of birch, aspen, and various species 

 f willow. The zonation about the ponds or lakes is in many 

 places well denned. Usually the first zone or the zone nearest 

 the water is composed of species of sedges extending all the way 

 around the lake; back of this occurs a shrub zone composed 

 mostly of birch and willow which may give way at once to the 

 conifi-rous forest or to a zone of aspen which is in turn surrounded 

 by t e conifers. The great controlling factor in such radial 

 zonation is of course water content. The percentage of water 

 in the soil differs considerably for the various zones, in general 

 dimi lishing outward from the edge of the lake. 



5. The Alpine Desert Zone. 



I have called this a desert zone because in all its essential 

 features it is a desert and the plants found in the zone are almost 

 without exception extreme xerophytes. The zone is of little 

 economic importance, but nevertheless it is of great scientific 



