684 RYDBERG: PHYTOGEOGRAPHICAL NOTES 
is much lower than in the main Rockies, as for instance in the 
Yellowstone Park. The Belt Mountains would not be high enough 
to have a timber line if they were in the Rockies. So also in the 
Wahsatch and La Sal Mountains, the timber line is much lower 
than in the Rockies of Colorado. Even in the Colorado Rockies 
themselves, the timber line seems to be higher in places where the 
mountains are more massed. So for instance is it higher on 
Mount Massive and other mountains around Leadville than on 
the more isolated Pikes Peak, Sierra Blanca, or Longs Peak. I 
understand that on the isolated Mount Shasta, the timber line 
is much lower down than on the peaks of the Sierra Nevada, but 
here it may depend upon the proximity of the ocean, the greater 
moisture, and the consequently larger snowcap on Mount Shasta. 
EXPOSURE TO SUNLIGHT 
Exposure to the direct sunlight and protection from it evi- 
dently also have influence on the altitude of the timber line, though 
perhaps not so much as one might expect. The insolation on the 
mountain tops in direct sunlight is very great. Schroeter esti- 
mates that on the top of Mont Blanc it is 26 per cent stronger 
than in Paris. The Rockies of Colorado have about the same 
height as Mont Blanc and are situated from 5° to 8° farther south, 
and the insolation is fully as great. The amount of light and heat 
which can be absorbed by the plant is therefore much greater on 
the mountain tops than on the plains. The radiation is also 
very great in the higher altitudes so that the temperature in the 
shadow is much lower. According to Schroeter the timber line 
lies 100-200 meters higher on the southern side than on the 
northern, and DeCandolle claims that the limiting line of vege- 
tation, of plants in general, is at an average of 200-300 meters 
higher on the equatorial side. These statements cannot be veri- 
fied in the Rockies. The timber line in Colorado is, perhaps, 
higher on the northern side, but this is probably due to other 
conditions. The timber line trees of Colorado are mainly Picea 
Engelmannii, Abies subalpina, and Pinus aristata. The first 
two are trees that need a great deal of moisture, and their seedlings 
require shade. These two trees are therefore more confined to 
the more shady and wetter northern slopes, where they also 
