RYDBERG: PHYTOGEOGRAPHICAL NOTES 685 
extend higher up. Pinus aristata is a tree that stands much more 
drought and is found more on the southern slopes, but it is a tree of 
little value as a forest tree, growing scatteringly only. Tome 
it appears to be a species which has passed its best wadaondd oizalcty 
and is in process of dying out. 
PHYSIOGRAPHICAL BARRIERS 
One of the conditions modifying the altitude of the timber 
line is to be found in physiographical barriers. Among these may 
be counted snowdrifts and glaciers, but these have been already 
mentioned. Besides these, the most important are precipitous cliffs 
and rock-slides. Very little needs to be said about these barriers. 
Neither gives the forest trees a chance to grow. Meeting oneof 
these barriers, the timber may cease to grow thousands of feet below’ 
the physiological timber line. Wherever a steep cliff arrests the 
forest, many of the alpine plants will be found growing in the 
crevices, hundreds or even a thousand feet lower than usual, and 
there are a few plants characteristic of the rock-slides. These may 
be best included in the alpine vegetation. 
ECOLOGICAL TIMBER LINE 
Sometimes an ecological timber line is mentioned, i. e., where 
bacteria in the soil and other organisms necessary for the growth 
of trees cease to exist. Theoretically, I can easily see that such a 
timber line may exist, but practically I have no information that 
such a one is found in the Rocky Mountains, distinct from the 
merely physiological one. No investigation in this line has been 
made. 
ECONOMIC TIMBER LINE 
In Switzerland there exists also an economic timber line. The 
alpine meadows are there used as summer pastures for sheep and 
goats. These animals make depredations on the young trees 
and hinder the spreading of the forest, but in many places the 
subalpine forest is actually cut down by men to make room for more 
pastures. In either case the alpine conditions will be brought 
further down the mountains and the timber line lowered. Such 
an economic timber line cannot be said to exist in the Rockies. 
