1910] ROBBINS—VEGETATION IN COLORADO 267 
in explanation of the cause of the treeless condition. For example, 
the Rocky Mountain yellow pine will grow on the plains without 
irrigation if it is tided through the seedling stage. The seedling can- 
not tolerate great extremes of temperature. 
DECREASE OF TEMPERATURE WITH INCREASE OF ALTITUDE.5—The 
rate of decrease varies with the topography, altitude, and season. It 
* Jan. Feb. Mar. Apr May June July Aug. Sept. Oct.” Nov. Dec. 
DegF 
70 ae) 
60 Pa 
VY \ 
Ze a 
— 
Ay 
AA 
Y= ee 22 
20 Pd 
ee 
Z 
7 
10 ell 
> 
kes Peq 
Fic. 7.—Mean annual march of temperatures: Grand Junction (4594 feet) in 
the western part of Colorado and Pueblo (4672 feet) on the plains have a much smaller 
range of temperature than do the mountain stations Idaho Springs (7453 feet) and 
Pike’s Peak (14,108 feet). 
is less at high altitudes than at lower and less in winter than in summer. 
able V gives the decrease in temperature for an increase in altitude 
for several Colorado stations. 
Colorado Springs is at the base of Pike’s Peak; Lake Moraine is 
at an elevation of 10,265 feet on its eastern slope. It will be seen 
from the table that the decrease of temperature between Lake Moraine 
and Colorado Springs is less than that between Lake Moraine and 
Pike’s Peak. This is contrary to the general rule, and is probably due 
to the inversion of temperature which occurs at the base of the moun- 
5 Hany, I. c., p. 243. 
