Life Zones 175 



permanently frozen soil) prevents the penetration of tree roots, and, 

 although trees will grow in an unfrozen stratum over permafrost, its 

 presence usually increases the severity of solifluction. Ravip ( 1951 ) 

 regards timber line in Alaska as occurring at the point where frost 

 action prevents the white spruce from surviving in numbers on the 

 uplands. Isolated stands of spruce trees are found growing hundreds 

 of miles north of the recognized tree line in areas where local drain- 

 age conditions result in little or no solifluction. 



Fortunately, the student of ecology may observe the profound 

 effects of the tree line without the necessity of visiting Alaska or the 

 vicinity of Hudson Bay. In the United States many of the accessible 

 mountains both in the east and in the west rise to sufficient heights 

 to display altitudinal tree lines. At the upper edges of the coniferous 

 forest, the spruce, fir, and birch become progressively reduced in size 

 until they culminate at tree line in a dense scrub growth or dwarfed 

 individual specimens. The actual altitude at which tree line occurs 

 differs widely according to local circumstances. The harmful effects 

 of strong wind, including especially excessive evaporation, are be- 

 lieved to restrict tree line to lower altitudes on exposed mountain 

 peaks than would be the case if temperature were acting alone 

 (Griggs, 1946). Physical influence of ice formations, avalanches, 

 landslides, and solifluction may exert a further control locally (Fig. 

 5.17). Thus on different slopes of Mt. Washington, New Hampshire, 

 described as having "the worst weather in the world," trees give way to 

 alpine tundra at altitudes ranging from about 1200 m to 1740 m, 

 whereas in the Rocky Mountains forests may extend to 3300 m (Fig. 

 5.18). Tree line occurs at sea level in parts of Belle Isle Strait but 

 at an altitude of about 300 m at Bay of Islands, Newfoundland. 



Life Zones. The progressive change in temperature and other 

 climatic factors from the equator toward the poles and from the low- 

 lands to the mountain peaks controls the distribution of certain major 

 vegetation types and these in turn are accompanied by characteristic 

 sets of subordinate plants and associated animal species. As a result, 

 we may recognize a series of life zones extending across the land from 

 the tropics to the polar regions, and, on a much smaller scale, extend- 

 ing up mountain slopes from the warm lowlands to the alpine condi- 

 tions of high elevations. Although many irregularities exist and 

 although the special circumstances influencing the distribution of 

 each individual species must be kept in mind, the general sequence 

 of the latitudinal and altitudinal life zones is clear, and the two series 

 have recognizable counterparts (Fig. 5.19). The continental life 

 zones are formed by the roughly latitudinal arrangement of certain of 



