Boreal Forests (Taiga) and Tundra Environments

Boreal forests, or taiga, are cold-climate forests of conifer trees such as spruce, pine, fir and larch that stretch across northern North America and Eurasia, while tundra is a treeless biome of low plants such as mosses, lichens and dwarf shrubs found beyond the tree line in the Arctic and on high mountains. In brief: the taiga has long, very cold winters, short mild summers and about 300–900 mm of precipitation, much of it snow; the tundra is even colder and drier, with a growing season of only about 50 to 60 days and ground that stays frozen as permafrost. Both biomes store large amounts of carbon.

These northern biomes are sparsely populated and play an important role in the global climate. This guide explains the climate, plants, animals and soils of the boreal forest and tundra, compares them in a table, works through some calculations, outlines how they were explored and how they are changing, and corrects common mistakes.

What is the climate of the boreal forest?

The boreal forest lies between about 50° and 65° north, in Canada, Alaska, Scandinavia, Russia (where it is called the taiga) and northern parts of Asia. It is the largest land biome after the polar and desert regions and covers roughly a third of the world’s forest area. Winters can last 6 months or more, with temperatures that fall below −30 °C in the coldest continental areas such as Siberia. Summers are short and cool to warm, and the average temperature in the warmest month is around 10 to 20 °C. The annual range of temperature in Siberia is among the largest in the world.

How do boreal plants and animals adapt?

Most trees are evergreen conifers whose features suit the cold: a conical shape that sheds heavy snow, narrow needle-like leaves with a waxy coat that limit water loss, and dark needles that absorb the weak sunlight, so photosynthesis can begin as soon as temperatures rise. Larch is an exception, as it sheds its needles in winter. Birch and aspen are the main broadleaved trees.

Soils are thin, acidic and low in nutrients. Needle litter decomposes slowly in the cold, and acids from the litter leach minerals downward, forming podzols with a pale, ash-gray layer. Animals include moose, lynx, wolves, brown bears, snowshoe hares and many birds, and some, such as bears, hibernate. Insects can be abundant in summer. Fire is a natural part of the system, clearing old growth and allowing new regrowth. Lakes, peatlands and wetlands are common.

What is the tundra?

Arctic tundra lies north of the tree line, around the Arctic Ocean, in northern Canada, Alaska, Greenland, Scandinavia and Russia. Alpine tundra occurs above the tree line on high mountains at any latitude. Average annual temperatures are often below 0 °C, winter temperatures can drop below −30 °C or much lower, and summers are cool, usually reaching only 5 to 10 °C in the warmest month. Precipitation is low, typically under 250 mm per year, so it is classed as a cold desert in some respects, but evaporation is slow and the ground is often waterlogged in summer.

The key feature is permafrost: ground that remains frozen for at least two years, often for thousands. In summer, only the top layer, the active layer, thaws, perhaps 0.3 to 1 m deep. Because meltwater cannot drain through the frozen layer below, the surface forms bogs, ponds and marshy ground. Freezing and thawing also create patterned ground and small mounds.

How do tundra plants and animals adapt?

Plants grow low to avoid wind and to stay in the warmer air layer near the ground. Mosses, lichens, sedges, grasses and dwarf willows are typical, and many have hairy or waxy leaves and grow slowly. Roots are shallow because of the permafrost, and many plants flower quickly during the short season. Animals such as caribou (reindeer) and musk ox, Arctic foxes, snowy owls and lemmings are found there. They are adapted by thick fur, layers of fat, white winter coats (Arctic fox, Arctic hare) and migration. Many birds visit only in summer to breed and feed on abundant insects. The tundra is fragile: damaged vegetation takes decades to recover.

Boreal forest and tundra compared

Worked example: seasons and permafrost

1. Growing season as a share of the year: a tundra growing season of 60 days is 60 ÷ 365 = 16.4% of the year. A taiga season of 120 days is 120 ÷ 365 = 32.9%.
2. Active layer: if the active layer is 0.5 m deep and thaws over about 60 days, it deepens by an average of 0.5 ÷ 60 = 0.0083 m, or about 8 mm per day. A thicker active layer of 0.8 m would reflect warmer summers.
3. Temperature range: a Siberian town has a January mean of −40 °C and a July mean of 18 °C, giving an annual range of 18 − (−40) = 58 °C.
4. Carbon (hypothetical figure): if permafrost soil holds 30 kg of carbon per square meter, 1 km² (1,000,000 m²) holds 30 × 1,000,000 = 30,000,000 kg = 30,000 tonnes of carbon. Thawing allows microbes to decompose it and release carbon dioxide and methane.
5. Snow depth and water: snow with a density of about one tenth that of water yields roughly 1 cm of water for each 10 cm of snow, so a 120 cm snowpack holds about 12 cm (120 mm) of water, a large share of the annual precipitation.

How are these biomes studied and changing?

Arctic explorers in the 19th century described the tundra, and in the 20th century scientists mapped permafrost, which was a major challenge for building the Trans-Siberian railway, pipelines and roads. The Alaska pipeline of the 1970s was raised on supports in places to avoid melting the frozen ground. The Arctic is warming faster than the global average, so permafrost is thawing, shrubs are spreading north and the tree line is moving in some regions. Fires and insect outbreaks in the boreal forest have become more frequent in recent decades. Research focuses on the carbon stored in these soils and on how their changes may affect the global climate.

Which mistakes are common?

Mistake 1: Thinking that the tundra is covered in ice all year

The surface thaws in summer and is covered with plants. It is the layer below that remains frozen.

Mistake 2: Confusing taiga and tundra

The taiga is a forest of conifers. The tundra begins where trees can no longer grow.

Mistake 3: Assuming that cold biomes are lifeless

Both support many specialized species and play a major role in migration, for example of birds and caribou.

Mistake 4: Considering permafrost unimportant

Permafrost holds a large store of carbon, and its thaw can damage buildings, roads and pipelines as well as release greenhouse gases.