Tundra

I

Introduction

Tundra, biome in the Northern hemisphere lying above the zone of northern coniferous forests (taiga) and below the polar oceans and terrestrial regions of permanent ice and snow. Alpine tundra is a climatically similar type of environment found above the altitudinal limits of tree growth in temperate regions. The Arctic tundra stretches in an unbroken strip across the northern latitudes of Eurasia and North America and encompasses Greenland, Iceland, the islands of the Canadian archipelago, the Pribilof Islands, Nunivak, St Matthew and St Lawrence islands, Wrangel Island, the New Siberian Islands, Severnaya Zemlya, Novaya Zemlya, Franz Josef Land, Svalbard, and numerous smaller islands. In North America, most of the tundra lies above 60^o N, while in Eurasia it tends to occur above 70^o N. The more northerly extent of taiga in Eurasia is a reflection of warmer summers in the vast Eurasian continental interior.

Alternate freezing and thawing of the ground on a seasonal cycle and the presence of a permanently frozen subsoil layer, the permafrost, are the unique features that distinguish Arctic tundra from polar regions to the north and taiga to the south. Freeze-thaw cycles often give rise to ground characterized by cracks, polygons, hummocks, knolls, frost boils, and earth stripes resulting from differential movement of soil, stones, and rocks, combined with a steady downward creep (solifluction) of the soil mantle.

II

Climate

The tundra climate is characterized by low average temperatures, harsh winters, and a short period of thaw during the summer when most plant growth occurs. Winter temperatures can be extremely low, typically averaging -20^o C to -30^o C (-4° F to –22° F) between November and February, and sometimes falling below -50^o C (-58° F). The summer lasts for three to four months but at the highest latitudes, average summer monthly temperatures may not climb above freezing. Typical average monthly temperatures recorded on the tundra around Fort Yukon, Alaska, are -30^o C (-22° F) in January, -5^o C (23° F) in April, 10^o C (50° F) in July, -10^o C (14° F) in October, and -30^o C (-22° F) again in December. The tree line marking the southern boundary of the Arctic tundra corresponds fairly closely to the 10^o C (50° F) July isotherm, so tundra and higher polar regions are often considered to be those where the average temperature in the warmest month does not exceed 10^o C (50° F).

High atmospheric pressure over the tundra means that the cold air is relatively dry. Annual precipitation is generally less than 380 mm (15 in) over most of the Arctic tundra. In some regions, drought conditions may prevail for periods of up to two months during the summer when temperatures are at their maximum. However, evaporation of moisture from the ground surface and upper soil horizons is slow in most areas because of the low ambient temperatures, so there is usually sufficient moisture on low-lying ground to support a continuous ground cover of plants. Indeed, the combination of frozen soil at depth and flat terrain can impede drainage so that the ground, when not frozen solid, becomes marshy and dotted with pools. These boggy areas support rich communities of plants and an abundance of insects which are, in turn, consumed by many species of bird which migrate north to the tundra each year to breed.

III

Flora

The Arctic tundra is, by definition, treeless. However, although the tree line in atlases is depicted as a firm line between green conifers and brown tundra, in practice it is often difficult to locate the boundary to within 200 km (124 mi) in the field. Deciding what constitutes a tree is not as straightforward as it might seem, for species which occur as tall specimens in the taiga may occur as stunted, shrub-like, and dwarfed forms within the forest/tundra transition zone. Trees are arbitrarily defined as those tall enough to stand proud of the winter snows, a convention which allows boundaries to be plotted for each of the dozen or so species typically found at the forest edge, and an average tree line to be calculated. Even so, tongues and isolated rafts of trees penetrate far into the tundra in sheltered valleys and hollows, while elevated or swampy ground isolates islands of tundra vegetation within the forest edge.

The number of species of plant on the tundra proper is usually small compared with temperate regions. Arctic tundra is characterized by tough, low-growing dwarf shrubs, including heather, birch, willow, bilberry, and crowberry, and a carpet of sedges, grasses, rushes, mosses, and lichens. Sedges and mosses tend to dominate between hummocks whereas, on slightly elevated sites, often only 15 cm to 60 cm (6 in to 24 in) above the waterlogged soils, willows, grasses, and rushes are more common. Low temperatures combined with a short growing season limit annual primary (plant) productivity to 0.1-0.4 kg/sq m, a level similar to that of many deserts.

Alpine tundra communities consist of mat-forming and cushion-forming plants adapted to heavy snows, gusting winds, and widely fluctuating temperatures. In higher mountain areas, mosses and lichens manage to grow on exposed rock surfaces. Vascular plants usually die out at or just below the line of permanent snow. Annual primary productivity varies from 0.05 kg/sq m in windswept habitats, to 0.15 kg/sq m in alpine meadows, and 0.3 kg/sq m in dwarf shrub communities.

IV

Fauna

The same or closely related species of animal tend to be found in tundra environments around the world. Musk oxen, caribou, and reindeer are the main large grazers feeding on grasses, sedges, willows, and lichens. Hares and lemmings are common consumers of grasses and sedges, while predators include polar bears, wolves, lynxes, foxes, eagles, hawks, falcons, and owls. Many migratory birds breed on the tundra during the brief summer, feeding on the flush of seeds and berries and/or the rapidly emerging populations of mosquitoes, black flies, blow flies, springtails, weevils, beetles, and spiders. Alpine tundra supports mountain goats, bighorn sheep, ibex, chamois, foxes, wildcats, pikas, marmots, ground squirrels, rabbits, voles, many summer-visiting birds, and the resident ptarmigan, a high-altitude version of the red or willow grouse that feeds mainly on willow buds and the succulent parts of other dwarf shrubs. Flies are scarce on alpine tundra, but butterflies, beetles, and grasshoppers are often abundant.

Animals have evolved a number of ecological strategies and physiological adaptations to cope with the harsh tundra environment. The most obvious strategy is to migrate to the tundra when the climate warms in the summer and when food supplies are at their maximum, and leave when temperatures begin to fall. Barren-ground caribou, for example, migrate in large herds from the fringes of the taiga to feed on the flush of plants hurriedly flowering and setting seed during the short summer growing season. Dwarf willows, blueberries (bilberries), cranberries, grasses, sedges, and carpets of lichen all develop rapidly at this time providing fodder for grazers and berries and seeds for other mammals and birds. Predatory wolves track the herds on their northward migration, while foxes clean up carrion and the sickly.

The migration of close to 100 species of breeding bird to the tundra coincides with the spring flush of vegetation and the emergence of insects from the soil and from beneath the regenerating carpet of plants. The herbivorous willow grouse makes only a short journey to the tundra from the evergreen forests to the south, but Arctic terns travel 40,000 km (25,000 mi) twice a year from the Arctic to the Antarctic and back again. Visiting birds include grouse, terns, ducks, geese, and waders that, along with rapidly multiplying populations of small rodents, draw in predatory eagles, peregrines, merlins, and owls.

The emergence of insects on which many summer visiting birds and their chicks rely is triggered by the springtime melting of ice at the soil surface that is unable to drain away because of the impermeable barrier of underlying permafrost. This creates boggy conditions ideal for invertebrate larvae, which hatch from eggs deposited in the previous year. Each small patch of tundra may be home to millions of herbivorous springtails, plant-sucking weevils, carnivorous beetles and spiders, and detritus-eating blow flies, dung beetles, and burying beetles. The most notorious insects of the tundra, however, are the mosquitoes and black flies, which rise in summer from the surface of pools and marshy areas like clouds of smoke. Female mosquitoes possess the piercing mouthparts used to suck blood from their victims. They can prove such an irritation to caribou that some individuals are driven to distraction.

A few animals are hardy enough to stay on the tundra all year round. The largest resident grazers are musk oxen, which survive the bitter Arctic winter by insulating themselves with thick layers of fur and fat. Young musk oxen are born with huge deposits of heat-producing (thermogenic) fat in their bodies and regularly hide beneath their mothers’ shaggy skirts of fur for extra protection from the penetrating winds. Adult musk oxen also huddle together in groups for warmth and, when threatened by predators, will often form a protective circle with the young calves at the centre.

Arctic foxes are also resident on the tundra, feeding mostly on the lemming population, which finds refuge in the winter beneath the insulating mantle of snow. Relative to their size, Arctic foxes have the thickest pelts in the animal kingdom and are stockier than their temperate counterparts with snub noses and tiny ears to reduce the surface area of skin in contact with the cold air. Like many other cold-climate animals, Arctic foxes have evolved an ingenious method of preventing heat escaping from their paws into the freezing snow. A fine network of blood vessels at the top of the leg called the rete mirabile (“wonderful net”) carries cool blood back to the body from the feet and warm blood in the opposite direction. The blood vessels come into such close contact in the rete that heat flows from the leg-bound blood, cooling it before it reaches the feet, while at the same time warming the ascending blood before it re-enters the torso. In this way, the paws of an Arctic fox are kept constantly at around 0^o C (32° F) or slightly higher, while blood in the animal’s body remains at 38^o C (100° F). Because there is little difference in temperature between paws and ground (that is, a negligible thermal gradient) little heat escapes from the animal’s legs into the environment. Caribou have similar heat-exchangers and a special type of fat in their lower legs which, unlike the fat in their bodies, remains pliable at low temperatures. Inuit have long appreciated the difference and used the marrow-fat at the top of Caribou legs as a solid food and the fat extracted from the feet as a fluid lubricant.