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 Pinns 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 



