plants and ground animals renew their growth and 

 activity (Sorensen 1941 ). Below the surface, ground 

 temperature rise lags behind rising air temperature. 

 Temperature just below the surface is higher than 

 at greater depths in the summer, but lower in the 

 winter, the turnover coming in April or May in the 

 spring and September or October in the autumn 

 (Beckel 1957). 



Photoperiods north of the Arctic Circle vary 

 from zero hours during mid-winter to twenty-four 

 hours during mid-summer. Elsewhere, the length of 

 periods depends on latitude. Even in the summer, 

 however, light intensity is low compared with trop- 

 ical latitudes. 



In contrast to the arctic plains, alpine habitats 

 on the higher mountain slopes are on rugged, often 

 precipitous, terrain. Each mountain top is an island 

 isolated from other mountain tops by intervening 

 forested lowlands. High plateaus may have more or 

 less level surfaces, but the size of such tundra-covered 

 areas, except for the Tibetan plateau of Asia, is gen- 

 erally very limited. These alpine habitats also lack a 

 permafrost in the subsoil, except in the far North, 

 and the extreme change in photoperiod during the 

 year that arctic regions experience. Soils are thin 

 and unstable except in small pockets on the slopes, 

 in valleys, or on protected flat surfaces. Average tem- 

 perature is low, but the range between daily maxi- 

 mum and minimum during the summer is sometimes 

 as great as 32 °C (58°F). North slopes are colder 

 than south slopes. The length of the growing season 

 between killing frosts is similar to what it is in the 

 arctic. Precipitation and humidity are commonly 

 high, and the mountain tops are frequently shrouded 

 in fog. Snowfall in some areas, as in the Cascades, 

 may reach 18 m (60 ft) and is generally much 

 greater than in the arctic. Water runoff is rapid 

 because of the severe topographic relief. Winds are 

 strong. On clear days, light intensity, notably ultra- 

 violet, and evaporation may be high because of the 

 thin air. A unique characteristic of the alpine habitat 

 not shared by the arctic is low barometric pressure 

 and o.xygen concentration, which probably does not 

 affect plants as much as it does some animals. Alto- 

 gether, the alpine environment imposes greater se- 

 verities on plant development than does the arctic 

 environment (Bliss 1956), and this is doubtless true 

 for animal activities also. 



VEGETATION 



Tundra has the appearance of short-grass 

 plains, but differs in that the vegetation consists of 

 sedges, rushes, lichens, mosses, ericaceous or de- 

 cumbent shrubs, and flowering herbs as well as 

 grasses. The plants are generally of small size. 



stunted growth, and compact structure adopted to 

 resist desiccation and mechanical abrasion from wind, 

 snow, and sand. Germination of seeds is poor, and 

 most species require several years to produce the first 

 flowering. Most tundra plants are therefore peren- 

 nials, and vegetative reproduction is important. Serai 

 vegetation varies in composition depending on 

 whether it develops around ponds, in low wet places, 

 or on clay, sand, gravel, or rock. Flowering herbs 

 are often abundant with different species coming into 

 bloom progressively during the year (Sorensen 1941, 

 McClure 1943). 



Succession has been studied in only a few areas, 

 and the true nature of the climax is unknown over 

 much of the arctic. There is doubt as to whether or 

 not a stable climax, as understood for southern lati- 

 tudes, actually develops (Raup 1951, Sigafoos 1951, 

 Britton 1957). This is due to the instability of the 

 soil, varying depths to which the soil thaws out in 

 the summer, depth and duration of the snow cover, 

 exposure to wind, and grazing and trampling by 

 animals. Although tundra associations are not recog- 

 nized in this book, we do make a primary division of 

 the biome into arctic tundra and alpine tundra. 



Antic tundra 



The so-called barren grounds of the far North 

 are divisible into four significant types in respect to 

 animal distribution. Bush or mat tundra contains 

 dwarf trees, decumbent shrubs, or heath, usually 

 mixed with mosses and lichens. Near Churchill in 

 northern Manitoba, much of the area is muskeg ; but 

 climax vegetation is interpreted as a mixture of low 

 Ericaceae heath and Cladonia lichen growing in a 

 mat of sphagnum and other organic material 7 to 10 

 cm (3-4 in.) thick. This climax develops on wet 

 ground, clay, sand, and on gravel and rock ridges 

 (Shelford and Twomey 1941). A variety of dwarf 

 shrub-lichen-grass-sedge types have been described 

 in Alaska (Hanson 1953, Churchill 1955) and in the 

 eastern arctic (Polunin 1934-35, 1948, Holttum 

 1922). 



Grass tundra is largely limited to deeper mineral 

 and organic soils. The soils are more fertile, and in 

 places thaw out in the summer to a depth of one 

 meter. Different species of grasses and sedges are 

 dominant in recognizable serai and climax stages 

 (Hanson 1951). Lichen-moss barrens (Tanner 1944, 

 Hanson 1953) have been called desert tundra or rock 

 desert by various investigators. The soil is thin, and 

 there is much exposed rock. Vegetation is scant 

 and consists of crustose and foliose lichens, mosses, 

 and scattered short herbs or very small shrubs. In the 

 low arctic, this may be a serai stage, but in the high 

 arctic it is often the only vegetation able to tolerate 



316 Geographic distribution of communities 



