Rydberg: Phytogeographical notes 679 



include all open spaces of the transition zone as these have a 

 flora alpine in character. In treating the subalpine region, how- 

 ever, I would place the boundary at the absolute timber line, so 

 as to include the groves, isolated trees, and krumholz as well. 

 The groves, if of any size, contain not only the trees themselves, 

 but also wood plants and underbrush belonging to the subalpine 

 region. They are either encroaching on the region above or are 

 themselves remnants of a former forest. So are also the isolated 

 trees and krumholz, although not associated with other plants of 

 the forest. 



In a mountain region extending through twenty degrees of 

 latitude, from lat. 35 to lat. 55 (the Rockies north of 55 have 

 not been considered in my work), the altitude of the timber line 

 necessarily varies greatly. In Colorado the lower timber line is 

 found at an altitude of between 3,200 and 3,400 m. and the upper 

 at 3,400 to 3,500 or rarely 3,600 m. In Montana the lower one 

 is at 2,200-2,500 m. and the upper 2,500-2,700 m. In the Cana- 

 dian Rockies they are even lower. 



Factors governing the timber line 

 The conditions that have been given as causing or modifying 

 the timber line are : 



1. A decreased temperature during the growing season. 



2. Too short a growing season. 



3. Late frost on account of lack of protection from snow. 



4. Strong desiccating winds. 



5. Deep snow. 



6. Form of precipitation. 



7. Large mountain masses. 



8. Exposure to and protection from direct sunlight. 



9. Physiographical barriers. 



10. Ecological barriers. 



11. Economic timber line. 



Low temperature 

 It is natural that too low a temperature should be one of the 

 important factors causing the disappearance of the forest. The 

 temperature during the winter has, however, very little influence 



