Boreal forest and tundra

In the Arctic tundra the average winter temperature is −34 °C, and plants get only about 10 to 12 weeks to grow each year.

The boreal forest

The boreal forest, also called taiga or coniferous forest, lies south of the Arctic Circle across most of Canada, Alaska, Russia and northern Europe. Winters are long, cold and dry; summers are short, cool and wet. Annual precipitation of 40 cm to 100 cm falls mostly as snow, and little of it evaporates because it is so cold. The forest is dominated by cone-bearing evergreen trees such as pine, spruce and fir, which keep their needle-shaped leaves all year.

Why needles win in the north

OpenStax gives two reasons. First, a needle needs less energy from the sun to warm up than a broad leaf, so evergreens can start photosynthesising earlier in spring; in the boreal forest they grow faster than deciduous trees. Second, boreal soils tend to be acidic with little available nitrogen. Leaves are rich in nitrogen, and a deciduous tree must build a whole new set every year, while a conifer keeps its needles and the nitrogen in them. Needles also decompose slowly once they drop, so fewer nutrients return to the soil, which keeps the soil poor. The trees grow slowly but live long, so the forest builds up a large standing biomass, even though its net primary productivity is lower than in temperate or tropical forests. Its structure is simple, often just a tree layer and a ground layer.

The tundra

North of the boreal forest lies the Arctic tundra. OpenStax gives an average winter temperature of −34 °C and average summer temperatures of 3 °C to 12 °C. The growing season lasts only about 10 to 12 weeks, but in that time the sun shines for almost 24 hours a day and plants grow fast. Precipitation is very low and changes little through the year, and as in the boreal forest, little evaporates.

Below the surface there can be ground that remains at 0 °C or colder for at least two consecutive years: permafrost. This permafrost stops roots from going deep and slows the decay of dead plants, which holds back the release of nutrients. Tundra plants are therefore low to the ground: shrub willows, sedges, mosses and lichens. Diversity, productivity and above-ground biomass are all low, yet in the short summer the ground can be completely covered with plants or lichens. The 24-hour summer daylight here is the same effect you can see in the seasons model: north of the Arctic Circle, the whole circle of latitude stays in sunlight around the June solstice.

A cold kind of dry

The tundra gets very little precipitation, usually under 25 cm a year, and National Geographic Education notes that by this measure it also counts as a desert. Yet it does not dry out the way a hot desert does, and the reason is evaporation. In a hot desert the sun and dry air pull water out of the ground faster than rain replaces it. In the boreal forest and the tundra the cold keeps evaporation low, so the little water that falls stays, much of it as snow and ice. OpenStax also notes that the fewest species live near the poles, where it is colder and drier and life has had less time to settle since the last glaciations.