RYDBERG: Puy APHICAL 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 toinclude 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. 
Late frost on account of lack of protection from snow. 
Strong desiccating winds. 
Deep snow. 
Form of precipitation. 
Large mountain masses. 
Exposure to and protection from direct sunlight. 
9. Physiographical barriers. 
10, Ecological barriers. 
11. Economic timber line. 
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PI AKL SG 
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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 
