Boreal Forests

Taiga (; rus, тайга́, p=tɐjˈɡa; relates to Mongolic and Turkic languages), generally referred to in North America as a boreal forest or snow forest, is a biome characterized by coniferous forests consisting mostly of pines, spruces, and larches.

The taiga or boreal forest has been called the world's largest land biome. In North America, it covers most of inland Canada, Alaska, and parts of the northern contiguous United States. In Eurasia, it covers most of Sweden, Finland, much of Russia from Karelia in the west to the Pacific Ocean (including much of Siberia), much of Norway and Estonia, some of the Scottish Highlands, some lowland/coastal areas of Iceland, and areas of northern Kazakhstan, northern Mongolia, and northern Japan (on the island of Hokkaidō).

The main tree species, depending on the length of the growing season and summer temperatures, vary across the world. The taiga of North America is mostly spruce, Scandinavian and Finnish taiga consists of a mix of spruce, pines and birch, Russian taiga has spruces, pines and larches depending on the region, while the Eastern Siberian taiga is a vast larch forest.

Taiga in its current form is a relatively recent phenomenon, having only existed for the last 12,000 years since the beginning of the Holocene epoch, covering land that had been mammoth steppe or under the Scandinavian Ice Sheet in Eurasia and under the Laurentide Ice Sheet in North America during the Late Pleistocene.

Although at high elevations taiga grades into alpine tundra through Krummholz, it is not exclusively an alpine biome, and unlike subalpine forest, much of taiga is lowlands.

The term "taiga" is not used consistently by all cultures. In the English language, "boreal forest" is used in the United States and Canada in referring to more southerly regions, while "taiga" is used to describe the more northern, barren areas approaching the tree line and the tundra. Hoffman (1958) discusses the origin of this differential use in North America and how this differentiation distorts established Russian usage.

Climate change is a threat to taiga, and how the carbon dioxide absorbed or emitted should be treated by carbon accounting is controversial.

Climate and geography

Taiga covers or 11.5% of the Earth's land area, second only to deserts and xeric shrublands. The largest areas are located in Russia and Canada. In Sweden taiga is associated with the Norrland terrain.

Temperature

After the permanent ice caps and tundra, taiga is the terrestrial biome with the lowest annual average temperatures, with mean annual temperature generally varying from . Extreme winter minimums in the northern taiga are typically lower than those of the tundra. There are taiga areas of eastern Siberia and interior Alaska-Yukon where the mean annual reaches down to , and the lowest reliably recorded temperatures in the Northern Hemisphere were recorded in the taiga of northeastern Russia.

Taiga has a subarctic climate with very large temperature range between seasons. would be a typical winter day temperature and an average summer day, but the long, cold winter is the dominant feature. This climate is classified as ''Dfc'', ''Dwc'', ''Dsc'', ''Dfd'' and ''Dwd'' in the Köppen climate classification scheme, meaning that the short summers (24 h average or more), although generally warm and humid, only last 1–4 months, while winters, with average temperatures below freezing, last 5–7 months.

In Siberian taiga the average temperature of the coldest month is between and .''Encyclopedia Universalis'' édition 1976 Vol. 2 ASIE – Géographie physique, p. 568 There are also some much smaller areas grading towards the oceanic ''Cfc'' climate with milder winters, whilst the extreme south and (in Eurasia) west of the taiga reaches into humid continental climates (''Dfb'', ''Dwb'') with longer summers.

According to some sources, the boreal forest grades into a temperate mixed forest when mean annual temperature reaches about . Discontinuous permafrost is found in areas with mean annual temperature below freezing, whilst in the ''Dfd'' and ''Dwd'' climate zones continuous permafrost occurs and restricts growth to very shallow-rooted trees like Siberian larch.

Growing season

The growing season, when the vegetation in the taiga comes alive, is usually slightly longer than the climatic definition of summer as the plants of the boreal biome have a lower temperature threshold to trigger growth than other plants. Some sources claim 130 days growing season as typical for the taiga.

In Canada and Scandinavia, the growing season is often estimated by using the period of the year when the 24-hour average temperature is or more. For the Taiga Plains in Canada, growing season varies from 80 to 150 days, and in the Taiga Shield from 100 to 140 days.

Other sources define growing season by frost-free days. Data for locations in southwest Yukon gives 80–120 frost-free days. The closed canopy boreal forest in Kenozersky National Park near Plesetsk, Arkhangelsk Province, Russia, on average has 108 frost-free days.

The longest growing season is found in the smaller areas with oceanic influences; in coastal areas of Scandinavia and Finland, the growing season of the closed boreal forest can be 145–180 days. The shortest growing season is found at the northern taiga–tundra ecotone, where the northern taiga forest no longer can grow and the tundra dominates the landscape when the growing season is down to 50–70 days, and the 24-hr average of the warmest month of the year usually is or less.

High latitudes mean that the sun does not rise far above the horizon, and less solar energy is received than further south. But the high latitude also ensures very long summer days, as the sun stays above the horizon nearly 20 hours each day, or up to 24 hours, with only around 6 hours of daylight, or none, occurring in the dark winters, depending on latitude. The areas of the taiga inside the Arctic Circle have midnight sun in mid-summer and polar night in mid-winter.

Precipitation

The taiga experiences relatively low precipitation throughout the year (generally annually, in some areas), primarily as rain during the summer months, but also as snow or fog. Snow may remain on the ground for as long as nine months in the northernmost extensions of the taiga biome.

The fog, especially predominant in low-lying areas during and after the thawing of frozen Arctic seas, stops sunshine from getting through to plants even during the long summer days. As evaporation is consequently low for most of the year, annual precipitation exceeds evaporation, and is sufficient to sustain the dense vegetation growth including large trees. This explains the striking difference in biomass per square metre between the Taiga and the Steppe biomes, (in warmer climates), where evapotranspiration exceeds precipitation, restricting vegetation to mostly grasses.

In general, taiga grows to the south of the July isotherm, occasionally as far north as the July isotherm, with the southern limit more variable. Depending on rainfall, and taiga may be replaced by forest steppe south of the July isotherm where rainfall is very low, but more typically extends south to the July isotherm, and locally where rainfall is higher, such as in eastern Siberia and adjacent Outer Manchuria, south to the July isotherm. In these warmer areas the taiga has higher species diversity, with more warmth-loving species such as Korean pine, Jezo spruce, and Manchurian fir, and merges gradually into mixed temperate forest or, more locally (on the Pacific Ocean coasts of North America and Asia), into coniferous temperate rainforests where oak and hornbeam appear and join the conifers, birch and Populus tremula.

Glaciation

The area currently classified as taiga in Europe and North America (except Alaska) was recently glaciated. As the glaciers receded they left depressions in the topography that have since filled with water, creating lakes and bogs (especially muskeg soil) found throughout the taiga.

Soils

Taiga soil tends to be young and poor in nutrients, lacking the deep, organically enriched profile present in temperate deciduous forests.Sayre, 19. The colder climate hinders development of soil, and the ease with which plants can use its nutrients. The relative lack of deciduous trees, which drop huge volumes of leaves annually, and grazing animals, which contribute significant manure, are also factors. The diversity of soil organisms in the boreal forest is high, comparable to the tropical rainforest.

Fallen leaves and moss can remain on the forest floor for a long time in the cool, moist climate, which limits their organic contribution to the soil. Acids from evergreen needles further leach the soil, creating spodosol, also known as podzol, and the acidic forest floor often has only lichens