582 RELATION TO ENVIRONMENT. 



ated where they are protected from the wind, and the slope of 

 the ground is such that the sun's rays are nearly perpendicular 

 to the surface. The ground is thawed out to a much greater 

 depth and the ground-water thus becomes much warmer than in 

 other places. This encourages a more luxuriant growth and the 

 protection from wind also favors a taller vegetation. The result 

 is that the vegetation in these places stands out in marked con- 

 trast to the usual vegetation type because of the height and lux- 

 uriance of the growth. 



II. Alpine Plant Societies. 



1072. Alpine plant societies occur on mountain heights 



where the change in temperature and other conditions of environ- 

 ment are so great as to produce a decided change in the vegetation 

 type. Schimper treats of three zones of vegetation in the 

 mountain regions as follows: 



1. Basal region, where the vegetation is more hygrophile, and 

 similar to that of the lower elevations of neighboring lowlands 

 because of the increase in precipitation at the lower mountain 

 elevation. 



2. Montane region, where the vegetation is more hygrophile 

 and less thermophile than in neighboring lowland, comparable 

 to the vegetation of higher elevations of lowlands because of the 

 increase in precipitation induced by the cool mountain region. 



3. Alpine region, where the vegetation is influenced by an 

 alpine climate, without analogy in lowlands. At this height 

 precipitation has rapidly diminished and is less than in the 

 lowlands. This alpine region corresponds to the alpine zone 

 of the forest region, according to Merriam. 



In passing from the base of mountains to the lofty summits, 

 one passes rapidly through the different life zones corresponding 

 to the change in climate, the forest formations showing changes 

 similar to those seen in passing from the austral region north- 

 ward to the arctic zone. As one passes upward beyond the 

 tree-line or woodland formation, there appears a grass-land 



