5° UNIVERSITY OF COLORADO STUDIES 
country, with an altitude of from 4,900 to 5,500 ft. ‘To the west is a 
region of foothills, dissected plateaus and alpine peaks. The peaks 
vary in height from 12,500 ft. (Bald Mt.) to over 14,000 ft. (Long’s 
Peak). The foot of the main range, or continental divide, is 10,000 to 
10,500 feet in altitude and the lowest passes from 1,000 to 2,000 ft. 
higher. Boulder County is in north central Colorado, the city of Boulder 
being about 25 miles northwest of Denver. 
Altitudinal distribution.—The limits of altitude printed in the 
systematic list are based not only on collections, but also on field notes 
taken at various times when no collections were made. It is believe 
that they are substantially correct. However, occasional plants of 
nearly every species may be found somewhat above or below the limits 
given. This is naturally the case since a number of edaphic factors 
may unite in places to overbalance the climatic factor of temperature 
which is the chief one introduced by altitude. It is worthy of note 
that most of the trees of the plains region ascend only to 6,000 or 6,500 
ft.; a few of the trees and a number of shrubs reach 8,000 or 9,000 ft. 
Above 9,000 ft. there is considerable change in the flora and at an alti- 
tude of 10,000 ft. practically all of the woody plants are true mountain 
species. If, then, one wishes to go from the plains at Boulder (or 
Denver) to where there is an entirely different flora it will be necessary 
to reach an altitude of about 10,000 ft. The chart on page 52 
shows the altitudinal distribution for a number of our most character- 
istic and easily identified species of woody plants. 
Zones of plant life——The following zones of plant life in Boulder 
County may be distinguished.t (1) Plains zone, altitude up to 5,800 
ft—a grassland formation; trees and shrubs only along streams and on 
rock ridges. (2) Foothill zone, altitude 5,800 to 8,000 ft.—generally a 
rather open forest of rock pine on hillsides with a mixture of Douglas 
spruce on north slopes and some deciduous trees in the canyons. (3) 
Montane zone, altitude 8,000 to 10,000 ft.—closer forest of lodgepole 
pine with admixtures of rock pine and limber pine at various places; 
Engelmann spruce in the higher parts. (4) Sub-alpine zone, altitude 
10,000 to 11,500 ft.—characterized by forests of Engelmann spruce 
x See a note on this subject by the present writer in Science, Vol. XXVI, p. 642, 1907. 
