Rydberg: Phytogeographical notes 683 



Form of precipitation 



Another important factor influencing the timber line is the 

 form of precipitation. In high altitudes the air is too rare to hold 

 much moisture and the rain falls at the least lowering of the tem- 

 perature. The rain falls therefore either in the form of mists or in 

 light showers, which only wet the surface of the ground. It may 

 be sufficient to keep alive the low rosettes or cushions of the alpine 

 vegetation, but it is not sufficient for the deep-rooted trees. 

 Furthermore, if a little heavier rain should come, the water would 

 rush down the steep slopes of the mountains, not having time to 

 sink down into the ground. The tops of the mountains are there- 

 fore arid, because the air is too rare to hold much moisture and 

 quickly gives it up in light showers. Nowhere in the Rockies proper 

 is the moisture very great. In the foot-hill regions and on the 

 surrounding plains the temperature is too high in the summer to 

 allow any precipitation. These zones are therefore also arid. 

 It is at middle elevations that the precipitation is the greatest. 

 The air here is dense enough to hold more moisture and the tem- 

 perature low enough to allow precipitation. It is also at middle 

 altitudes that we find the forest areas in the Rocky Mountain 

 region. 



Large mountain masses 



In the Swiss Alps, observations have been made that in regions 

 of large mountain masses, as for instance in the Monte Rosa 

 region and the Engadine and others, the timber line is higher 

 than on isolated mountains. I have not seen any satisfactory 

 explanation of this fact. It may be due partly to the fact that 

 the central mountains of such massed groups are more or less pro- 

 tected from the desiccating winds. It may be due also to the 

 circumstances that in the winter more snow lodges between the 

 mountains, the melting of it is more retarded, and the water is 

 more arrested in its downward course by the trees and their roots. 

 The air in the summer time would be therefore, from the evapo- 

 ration, more loaded with moisture, which would naturally also 

 benefit the mountain tops. Whatever the real cause may be, it 

 seems as if the observations made in Switzerland hold good in the 

 Rockies. From my own experience, I know that the timber line 

 in the isolated Belt Mountains and Crazy Mountains in Montana 



