Tundra is the treeless biome of the coldest lands on Earth, where summers are too short and cool for trees to grow and the ground beneath is often permanently frozen. It comes in three forms: Arctic tundra across northern Russia, Canada, Alaska, Greenland and Scandinavia; Antarctic tundra on the ice-free coasts of Antarctica and the subantarctic islands; and alpine tundra on high mountains at any latitude. Climatologists classify it as Köppen type ET, meaning no month averages above 10 °C but at least one averages above 0 °C. Mosses, lichens, sedges, grasses and knee-high shrubs make up the vegetation. In the Arctic the soil sits on permafrost, which underlies about 18 million km² of land worldwide and is now thawing as the climate warms.
The Tree Line, Permafrost and the 10 °C Rule
The word itself came into English in 1824, spelled “toundra”, from the Russian тундра. Its deeper origin is uncertain: one theory traces it to a Sámi word, tundar, for a fell or elevated wasteland, and some sources point to the Finnish tunturi, “treeless plain”.
What makes a place tundra is the absence of trees, and three things work together to keep them out. The first is summer warmth. Wladimir Köppen noticed that the polar tree lines follow the 10 °C isotherm for the warmest month: where July (or January in the south) averages less than that, forests cannot survive. More recent research puts the global tree line where the growing-season mean temperature is about 6 °C, with a minimum of 94 days warm enough for growth. Tree-line conifers are frost-hardy for most of the year but can be damaged by just one or two degrees of frost in midsummer, which is exactly what tundra summers deliver.
The second is permafrost, ground that stays at or below 0 °C for at least two years in a row. Around 15% of the exposed land of the Northern Hemisphere sits on it. Above the permafrost lies the active layer, which thaws each summer; as of 2018 it averaged about 145 cm thick across the Northern Hemisphere, but in north-eastern Siberia, Alaska and Greenland it is often less than 50 cm. Tree roots cannot anchor in so thin a layer, and a shallow-rooted tree is easily toppled by wind. The third is the wind itself: Arctic tundra regularly sees winds of 50 to 100 km/h, which dry and scour anything that stands above the snow.
Forest rarely gives way to tundra cleanly. In north-western Canada the forest-tundra transition averages about 145 km wide, with stunted, wind-shaped trees (krummholz) giving way to open lichen woodland and finally to bare tundra. Trees push furthest north where continental summers are warmest: the northernmost forests on Earth grow at Ary-Mas, at 72°28′N in the Novaya river valley on the Taymyr Peninsula, and in the Lukunsky grove at 72°31′N. Our guide to the subarctic climate covers the taiga belt that lies just south of that boundary.
Arctic Tundra from the Yamal Peninsula to Alaska’s North Slope
Arctic tundra runs in a broad belt north of the taiga across Eurasia and North America. Its winters are long, dark and windy, averaging around −28 °C and sometimes reaching −50 °C, although the very lowest temperatures in Russia, Canada and Alaska have been recorded in the forests further south, where cold air pools in sheltered valleys. Summer daytime highs reach about 12 °C but can fall back to 3 °C or below freezing.
Utqiaġvik (formerly Barrow), Alaska’s northernmost town, about 530 km north of the Arctic Circle, shows the pattern in figures. Its 1991–2020 normals give a January mean of −24.2 °C, a July mean of just 5.4 °C and an annual mean of −10.2 °C. The daily high reaches 0 °C or more on only about 136 days a year, and annual precipitation is about 137 mm, less than many hot deserts. The Arctic tundra as a whole receives only 150 to 250 mm a year, which is why it is often described as a polar desert. Even so, summer turns it soggy: the permafrost stops meltwater draining away, so the surface fills with ponds, bogs and lakes.
Few species manage here: the Arctic tundra holds about 1,700 species of vascular plants and only 48 species of land mammals. Typical plants include crowberry, bog blueberry, lingonberry, Labrador tea and reindeer lichen (Cladonia rangiferina), the grey-white lichen that feeds reindeer through the winter. Animals include reindeer and caribou, musk oxen, Arctic hares, Arctic foxes, lemmings, snowy owls and ptarmigan, with polar bears along the coast. Millions of migratory birds arrive each summer to breed in the marshes, along with clouds of mosquitoes. Frogs and lizards are almost entirely absent. Our guide to Arctic wildlife follows these species through the year.
People have lived on the Arctic tundra for thousands of years, many of them as nomadic reindeer herders, including the Nenets and Nganasan of northern Russia and the Sámi of northern Scandinavia. The tundra also holds oil, gas and uranium. The Yamalo-Nenets Autonomous Okrug, which covers the Yamal Peninsula and its neighbouring tundra in western Siberia, produces about 90% of Russia’s natural gas, and Alaska’s North Slope oilfields lie on tundra too.
Antarctic Tundra: Two Flowering Plants and No Land Mammals
Most of Antarctica is too cold and dry for tundra; it is ice sheet or cold desert. Tundra survives only where rock and soil are exposed for part of the year, chiefly on the northern and western Antarctic Peninsula and on islands such as the South Orkneys, the South Shetlands, South Georgia and the Kerguelen Islands. The flora here is mostly non-flowering: about 300 to 400 species of lichen, around 100 mosses, 25 liverworts and some 700 species of terrestrial and freshwater algae.
Only two flowering plants are native to Antarctica: Antarctic hair grass (Deschampsia antarctica) and Antarctic pearlwort (Colobanthus quitensis). Both grow in low clumps. Guinness World Records lists the hair grass as the southernmost flowering plant, after a specimen was found on the Refuge Islands off the Antarctic Peninsula at 68°21′S in 1981. Since 2009 both species have been spreading rapidly, which studies link to rising air temperatures and fewer fur seals trampling the ground.
Unlike the Arctic, the Antarctic tundra has no native land mammals, mostly because the Southern Ocean has isolated it from other continents. Seals and seabirds such as penguins live along the shore, while rabbits and cats were introduced by people to some subantarctic islands. The main subantarctic islands, including South Georgia, Prince Edward, Crozet, Kerguelen, Heard and Macquarie, lie between 46.4° and 54.6°S, and none of them has trees, because the winds of the Roaring Forties and Furious Fifties keep woody plants flat. South of New Zealand, the Bounty, Auckland, Antipodes and Campbell islands, with Macquarie, form the Antipodes Subantarctic Islands tundra ecoregion, although stunted southern rata forest survives in sheltered valleys on the Auckland Islands. The ecoregion is home to the only two subantarctic orchids, the royal penguin and the Antipodean albatross. Everything south of 60°S is protected under the Antarctic Treaty System.
Alpine Tundra Above the Timberline, from Mount Washington to Kilimanjaro
Alpine tundra has no permafrost requirement and occurs wherever a mountain rises above its tree line. The cause is altitude rather than latitude: air cools by about 10 °C per kilometre as it rises, so climbing 100 m is roughly equivalent to moving 80 km towards the pole. The tree line therefore rises as you approach the equator, by about 75 m per degree of latitude between 70°N and 50°N, and by about 130 m per degree from 50°N to 30°N, before levelling off at roughly 3,500 to 4,000 m in the tropics.
Local conditions bend those averages. On Mount Washington in New Hampshire, wind holds the tree line down to about 1,340 m (4,400 ft), nearly 600 m below the 1,917 m summit. The summit station has a Köppen ET tundra climate, hurricane-force gusts on about 110 days a year, and a 1934 wind record of 372 km/h. The mountain carries about 65 acres of alpine tundra, the most of any peak east of Colorado, including the Alpine Gardens plateau at about 1,600 m with plants otherwise found far to the north in the Arctic. In Colorado’s Rocky Mountain National Park the tree line climbs to about 3,550 m on warm slopes, and Trail Ridge Road, completed in 1938, crosses its alpine tundra.
Alpine growing seasons last only 45 to 90 days, with summer averages near 10 °C and frost possible on any night. Alpine soils are better drained than Arctic ones, so the landscape is drier: perennial grasses, sedges, cushion plants and prostrate shrubs that store most of their mass in roots, where it survives the winter. Because each mountain range is isolated, no animal is common to all alpine tundra. The kea lives in New Zealand’s Southern Alps, the chinchilla in the Andes, the yak, Himalayan tahr and snow leopard in the Himalaya, and marmots, pikas, mountain goats and bighorn sheep in the Rockies.
Thawing Permafrost and the Brooks Range Fires
Tundra soils are a vast carbon store. Dead plants that never fully decayed have piled up in the permafrost for thousands of years; the oldest permafrost has stayed frozen for around 700,000 years. As it thaws, microbes break that material down and release carbon dioxide and methane, which add to the warming that caused the thaw. The IPCC’s Sixth Assessment Report estimates the release at the equivalent of 14 to 175 billion tonnes of carbon dioxide per 1 °C of warming. For comparison, human emissions of carbon dioxide alone were about 40 billion tonnes in 2019. Permafrost emissions are expected to stay smaller than human ones.
Thaw also changes the land itself. Ground rich in ice slumps or collapses when it melts. The Batagaika crater in Russia’s Sakha Republic, the largest known permafrost crater, began to sink in the 1960s after forest was cleared above it; it is now a gash about 1 km long and up to 100 m deep, and its headwall has retreated by more than 10 m a year. Nearly 70% of the roads, pipelines and buildings on permafrost could be at risk by 2050. Fire is a growing threat too: in 2007 a single fire burned 1,039 km² of tundra on the north slope of Alaska’s Brooks Range, in a landscape where fires normally return only every 150 to 200 years. Trees are also moving uphill: in North Cascades National Park in Washington State the tree line has risen more than 120 m (400 ft) in 50 years, squeezing alpine tundra from below.
The tundra’s future is tied to what happens underground, and our explainer on what permafrost is goes deeper into the frozen layer that holds this biome together.