276 BOTANICAL GAZETTE [APRIL 
recognizing timber line to be a climatic division line, has shown that 
in the Long’s Peak region, Engelmann spruce has not reached the 
point where tree growth is climatically excluded, and that the present — 
timber line as represented by Engelmann spruce is not the true 
climatic timber line. In this region, however, limber pine forms a 
true climatic timber line. 
The average height of timber line in Colorado is near 11,500 feet. 
It is higher on the ridges than in the gulches, on south and west 
exposures than on north and east exposures. Engelmann spruce, 
subalpine fir, and limber pine are the principal timber line trees. 
ALPINE ZONE 
Climatic conditions for this zone in Colorado are not well known. 
The lower limit of the alpine zone is timber line. Although precipita- 
tion is ample and the relative humidity frequently high, tree growth 
is absent partly because of the thin soil, steep slope which cannot 
retain sufficient moisture, low air and soil temperatures, high winds, 
snow,'® rapid transpiration, and liability to frosts. The lower por- 
tions of the zone are covered with grasses and low alpine mat forms. 
The upper parts are usually rock fields supporting little except @ 
lichen vegetation. 
Some of the most common alpine plants of Colorado are Des- 
champsia caespitosa (L.) Beauv., Phleum alpinum L., Trisetum sub- 
spicatum (L.) Beauv., Festuca brachyphylla Schultes, Carex atrata L., 
Polygonum viviparum L., Silene acaulis L., Caltha leptosepala Hook.; 
Dryas octopetala L., Cienantsin rhodantha (A. Gray) Rose, Sieversia 
turbinata (Rydb.) Greene, Castilleja occidentalis Torr., ‘Campanula 
petiolata DC, Mertensia alpina (Torr.) Don., Rydbergia grandiflora 
(T. and G.) Greene, Artemisia scopulorum A. Gray, Eritrichium 
argenteum Wight, Trifolium dasyphyllum Torr., T. Parryi A. Gtays 
and Tetraneuris lanata (Nutt.) Greene.?° 
19 SHAw, C. H., Causes of timber line on mountains. Plant World 12:169-181- 
1909. 
2° For a list of the alpine plants of Colorado with statistics of their geographical 
distribution, see — xs CocKERELL, T. D. A., Alpine flora of Colorado. Amet- 
Nat. 40:861-873. 1 
