274 BOTANICAL GAZETTE [APRIL 
30°-40°; diurnal range decreases with an increase of altitude. Average date of 
last spring frost May 30 to June 31; average length of growing season 3 months, 
21 days, to 3 months, 6 days. Relative humidity: annual amount greater than 
on plains and mesas. 
This area extends the full length of the state along the eastern 
slope. It is between the altitudes of 6000 and 8000 feet. It is 
dominated by a forest of Rocky Mountain yellow pine and Douglas 
fir. The upper limit of the zone is coextensive with the upper limit 
of abundant growth of Rocky Mountain yellow pine. South Park, 
although mostly above 8000 feet, has a foothill vegetation and 
should be classed as upper foothill country. It is much drier than 
localities at the same altitudes on the eastern slope. It is an upland 
country supporting a xerophytic vegetation.*® 
MONTANE ZONE 
Precipitation: mean annual 15-20 inches; many localities have greater 
amounts; July and August are the wettest months; snow abundant. Tem- 
perature: mean annual 35°-40°; mean summer 55°-60°; mean annual and 
diurnal ranges of temperature generally small but varying greatly with exposure. 
No data for average date of last spring frost and average length of growing 
season. Relative humidity: fluctuating between high and low; annual amount 
increases with increase of altitude. 
This zone roughly includes those portions of the state with an 
altitude between 8000 feet and 10,000 feet. On the western slope 
and on the minor mountain ranges and plateaus of the western part 
of the state, the montane zone often extends to higher altitudes, which 
is due chiefly to the generally drier condition of this slope. 
The vegetational composition and appearance of the montane 
zone varies in different sections of Colorado. In the northeasterD 
_ part of the state it is dominated by lodgepole pine, being adjoined by 
the Rocky Mountain yellow pine forests of the foothills and the 
Engelmann spruce forests of the subalpine regions. Other less 
abundant conifers of this zone are the bristle cone pine (Pinus 
aristata Engelm.), limber pine (Pinus flexilis James), Colorado blue 
spruce (Picea Parryana [Andree] Sarg.), and subalpine fir (Abies 
lasiocarpa [Hook.] Nutt.). In southern Colorado lodgepole pine does 
16 RMALEY, FRANCIS, Plants of the Florissant region in Colorado. Univ. Colo 
Studies 3:178-185. 1906. 
