Arctic tundra occurs in the northern most parts of the Northern Hemisphere, intergrading across the latitudinal tree-line with the boreal forest to the south. In North America, arctic tundra occurs on all non-glaciated regions of the islands of northern Canada and on Greenland, as a northern fringe of Alaska and continental Canada, and penetrating to relatively southern latitudes in the vicinity of Hudson Bay, which acts as a climate-influencing extension of the cold Arctic Ocean. (An ecological analogue of arctic tundra occurs on the southernmost islands of the Southern Ocean and as a narrow fringe of parts of Antarctica. However, compared with arctic tundra, antarctic tundras are less well developed and have few species of plants.) The distribution of arctic tundra is determined by climate, generally occurring where the annual precipitation is less than about 2 in (50 cm) each year, and the average annual temperature is colder than 23oF (-5oC). Much of the arctic tundra covers permanently frozen ground, or permafrost, above which only the active layer—comprised of the surface 1.6 ft (0.5 m) or so—thaws during the growing season. Arctic tundra experiences continuous inputs of solar radiation during much of the growing season, a condition that can last for more than two months, depending on latitude. The incessant insolation during this time allows the vegetation to be relatively productive, as long as the availabilities of moisture and nutrients are not excessively constrained. During the arctic winter, when plants are dormant but some animals remain active, there is continuous night for several months, and extremely cold conditions.
There are several categories of arctic tundra vegetation. The low-arctic tundra occurs in southern areas. On wet sites, the low-arctic tundra of North America develops as relatively productive wet meadows dominated by a tussock-forming cottongrass (Eriophorum vaginatum), while better-drained sites are dominated by shrubs such as willow (e.g, Salix glauca) and birch (e.g., Betula glandulosa), growing to about 1.6 ft (0.5 m) in height. These sites also support herbaceous plants, such as arctic lupin (Lupinus arcticus).
The high-arctic tundra is less productive and the plants are of lower stature. In North America, poorly drained wet meadows are dominated by graminoid plants, especially sedges (e.g., Carex membranacea and C. stans) and cottongrass (Eriophorum angustifolium). Drier sites are typically dominated by dwarf shrubs such as arctic willow (Salix arctica), arctic heather (Cassiope tetragona), mountain cranberry (Vaccinium vitis-idaea), and arctic bilberry (V. uliginosum), along with herbaceous plants such as lousewort (Pedicularis spp.), grasses (such as Arctagrostis latifolia), mosses, and lichens.
The polar desert is a very sparse tundra that occurs where climatic extremes of temperature and moisture availability allow only an incomplete cover of vegetation to develop. Such sites typically have a sparse cover of lichens and a few species of vascular plants, including cushion plants such as arctic avens (Dryas integrifolia) and purple saxifrage (Saxifraga oppositifolia), along with a few grasses and herbs. However, within the polar desert landscape there are occasional places that, because of their topography and drainage patterns, are relatively warm and moist throughout the growing season. These more moderate places are called high-arctic oases. They sustain a relatively lush growth of vegetation and, if the oasis is large enough, relatively large populations of animals.
The arctic tundra sustains only a few species of resident animals that remain active throughout the year. In the high-arctic tundra of North America, resident birds include the raven (Corvus corax) and rock ptarmigan (Lagopus mutus). Resident mammals include Peary caribou (Rangifer tarandus pearyi), muskox (Ovibos moschatus), arctic fox (Alopex lagopus), collared lemming (Dicrostonyx torquatus groenlandicus), and arctic hare (Lepus arcticus). However, there is a much larger number of seasonally abundant animals. These include insects, some of which can be very abundant during the growing season, including dense populations of mosquitoes. Migratory species of birds include finches such as snow bunting (Plectrophenax nivalis), Lapland longspur (Calcarius lapponicus), and hoary redpoll (Acanthis hornemanni); shorebirds such as Baird's sandpiper (Calidris bairdii) and red knot (Calidris canutus); waterfowl such as oldsquaw duck (Clangula hyemalis) and greater snow goose (Chen caerulescens atlantica); and larids such as arctic tern (Sterna paradisaea), glaucous gull (Larus hyperboreus), and parasitic jaeger (Stercorarius parasiticus).