Native Vegetation 
i i 
a measure for the comparatively short growing seasons, and cool 
nights. Clements* secured the following results at Minnehaha 
(altitude 8,400 feet) on Pike’s Peak. The air temperature was 
75 . 2 °, the surface of the gravel soil 104 °, and the surfaces of the 
plant leaves as follows. Parmelia (lichen), 104 °; Eriogonum, 
101 . 48 °; Arctostaphylos (kinnikinick), 95 0 ; Thlaspi (wild candy¬ 
tuft), 89 . 24 ° ; and Senecio, 87 . 8 °. As is the case with native plants, 
the temperature of crop plants at high elevations is greater than 
that of the air about them, the difference depending somewhat upon 
the transpiring power of the plant considered. 
Effect of Exposure upon Air and Surface Temperatures .—By 
exposure is meant direction of slope. A north exposure, for ex¬ 
ample, faces north. The effect of exposure at high altitudes is 
much more marked than at low elevations. This greater effect is 
a direct result of the increased rate of radiation at high altitudes. 
The intensity of sunlight is distinctly affected by exposure and 
also by degree of slope. A given area of soil or plant surface that 
is at right angles to the direction of the rays of light will receive 
much more heat than one upon which the sun’s rays fall obliquely, 
for under the latter condition the rays are spread out over a larger 
area than when they fall perpendicularly. If we assume the in¬ 
tensity of sunlight to be 100 when it strikes a surface at right 
angles, its intensity when striking that surface at an angle of 70° 
will be approximately 98.5 ; at an angle of 60°, 96.5 ; and at an angle 
of 10°, 33.4. Light intensity has its effect upon both air and sur¬ 
face temperatures, which indirectly affect the amount of moisture 
in the soil, and the relative humidity over the soil. The differ¬ 
ences between the native vegetation on adjacnt north and south ex¬ 
posures is so conspicuous in the mountainous sections as to attract 
the attention of the most inobservant person. In a valley that 
trends east and west the slope exposed to the south has a much 
greater total effective heat during the year than the northerly ex¬ 
posure across the valley. The greater light intensity on the south 
exposure not only results in a warmer, but a drier, habitat than 
occurs on the neighboring north exposure. A south exposure re¬ 
ceives the greatest total heat during the day, the east the next 
greatest, then the west, and the north exposure least of all. There 
are numerous examples illustrating the differences between the 
native vegetation of north and south exposures. In the lower 
mountain valleys of the north and central parts of Colorado, the 
south exposures support a semi-arid type of vegetation, composed 
of cedars, scattered yellow pine ( Finns scopulorum) , yucca, Colo- 
♦Research Methods in Ecology, page 71. 
