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. To me 

 it appears to be a species which has passed its best development 

 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 one of 

 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. 



