1910] ROBBINS—VEGETATION IN COLORADO 275 
not form so extensive forests as it does in the northern part of the state. 
East and west of the continental divide the white fir (Abies concolor 
Lindl.) occurs in the montane zone of southern Colorado, but not in 
the northern part. Aspen (Populus tremuloides Michx.) is common 
throughout the montane zone in all parts of the state, but particularly 
So on the western slope. There it covers extensive areas and grows 
to a considerable size. In northwestern Colorado it often dom‘nates 
extensive areas, or may be mixed with lodgepole pine. Here oak 
shrubbery is often abundant in the lower parts of the zone. In south- 
western Colorado the montane zone extends between the altitudes 
9000 feet and 10,500 feet and is chiefly aspen-covered. 
SUBALPINE ZONE 
Precipitation: mean annual above 20 inches; often 35 or 40 in some localities; 
maximum amount in the summer; daily afternoon showers common. Tem 
perature: mean annual below 35°; mean summer below 55°; mean annual and 
diurnal ranges low. Relative humidity: fluctuating between high and low but 
generally high. ; 
Tn all parts of the state, except the extreme southern part, this 
zone extends from an average altitude of 10,000 feet to timber line; 
it is the highest timbered zone. The dominant and characteristic 
tree in all parts of the state is the Englemann spruce. It is nearly 
always accompanied by limber pine and subalpine fir. Other com- 
mon trees and shrubs are Salix chlorophylla Anders., S. glaucops 
Anders., S. Bebbiana Sarg., S. pseudolapponum Seem., Betula glandu- 
losa Michx., Ribes parvulum (A. Gray) Rydb., R. lentum (Jones) 
Coville & Rose, Sambucus microbotrys Rydb., and Lonicera involu- 
craia Banks. Vaccinium oreophilum Rydb. and V. caespitosum 
Michx. are common shrubs growing on the forest floor. Meadows 
and swampy areas are abundant throughout the zone. 
TIMBER LINE.—Timber line is the upper limit of tree growth. In 
Colorado it usually marks the limit above which trees are climatically 
excluded. Harvey?’ has pointed out that timber line on Mt. 
Ktaadn in Maine is not climatic but physical. Cooper,'® although 
7 Harvey, L. H., A study of the physiographic ecology of Mt. Ktaadn, Maine. 
Univ. Maine Studies. 1903. 
8 Cooper, W. S., Alpine vegetation in the vicinity of Long’s Peak, Colorado. 
Bor. Gazerre 45 3190-337. 1908. 
