Tundra

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Tundra

Tundra is a generic name for a low-growing ecosystem found in climatically stressed environments with short and cool growing seasons . Latitudinal tundra occurs in the Arctic and to a much lesser extent in the Antarctic, where the environments are characterized by cool, short growing seasons. Altitudinal tundra occurs under a similar climatic regime, but at the tops of mountains .

After temperature , the second most-important environmental factor affecting most tundra communities is moisture. Under wet conditions, sedge and grass-dominated meadow communities develop, while moist conditions favor a vegetation dominated by dwarf shrubs and herbaceous species , and dry sites have cushion plants and lichens . The vegetation of arctic and alpine tundras share many structural characteristics, most genera, and some species. However, there are important environmental differences between these two tundras, with the alpine type being subject to much larger variations of daily temperature during the growing season, as well as more intense inputs of solar radiation during the day. In contrast, arctic tundra can experience continuous exposure to the sun's rays, with 24-hour days for an extended period during the growing season.

Arctic tundra

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).

Alpine tundra

Alpine tundra occurs in climatically stressed environments at high mountainous altitudes. Alpine tundra can even occur on mountaintops at tropical latitudes, although this vegetation type is much more common in temperate regions. Compared with the arctic tundra, alpine environments have much larger daily variations of temperature and solar radiation during the growing season. Because of the thinness of the atmosphere at high altitude, alpine tundra is also subject to large inputs of ultraviolet radiation, which can be an important biological stressor. Because the skies are often clear at high altitude, the surface cools very quickly at night, so that frost can be a daily occurrence during the growing season.

In general, alpine tundra is considerably richer in plant species than arctic tundra. At temperate latitudes, this occurs because alpine environments were not regionally obliterated by glacial ice during the most recent glaciations, so the component species were able to endure this period of intense climatic stress. This survival was made possible by the occurrence of non-glaciated refugia where plants could survive on some mountaintops (these are called nunataks). In addition, as the climate deteriorated, alpine tundra could migrate to lower-altitude, non-glaciated parts of mountainous regions, as the tree-line moved downwards. In contrast, almost all of the arctic tundra was destroyed by the extensive continental glaciers that covered northern regions during the most recent ice ages . It is believed that after deglaciation the arctic tundra was re-established by a northward migration of some of the plant species of alpine tundra. However, because only some species were capable of undertaking the extensive migrations that were necessary, the arctic tundra is relatively poor in species, compared with temperate alpine tundras.

There are relatively few animals that only occur in alpine tundra and not in other types of ecosystems. In North America some of the characteristic mammals of alpine tundra include a small relative of rabbits called the pika (Ochotona princeps), and the hoary marmot (Marmota caligata). The gray-crowned rosy finch (Leucostichte tephrocotis), horned lark (Eremophila alpestris), and water pipit (Anthus spinoletta) all breed in alpine tundra of North America, but also in arctic tundra (the lark also breeds in other open habitats, such as prairie and fields).