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Taiga (; ; from Turkic〔"taiga." ''Dictionary.com Unabridged (v 1.1)''. Random House, Inc. 12 Mar. 2008. (''web link'' )〕) also known as boreal forest or snow forest, is a biome characterized by coniferous forests consisting mostly of pines, spruces and larches. The taiga is the world's largest terrestrial biome. In North America it covers most of inland Canada and Alaska as well as parts of the extreme northern continental United States (northern Minnesota through the Upper Peninsula of Michigan to Upstate New York and northern New England), where it is known as the Northwoods. In Eurasia, it covers most of Sweden, Finland, much of Norway, some lowland/coastal areas of Iceland, much of Russia from Karelia in the west to the Pacific Ocean (including much of Siberia), and areas of northern Kazakhstan, northern Mongolia, and northern Japan (on the island of Hokkaidō). However, the main tree species, the length of the growing season and summer temperatures vary. For example, the taiga of North America mostly consists of spruces; 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. A different use of the term taiga is often encountered in the English language, with "boreal forest" used in the United States and Canada to refer to only the more southerly part of the biome, while "taiga" is used to describe the more barren areas of the northernmost part of the biome approaching the tree line and the tundra biome. Hoffman (1958) discusses the origin of this differential use in North America and why it is an inappropriate differentiation of the Russian term. Although at high elevations taiga grades into alpine tundra through Krummholz, it is not an alpine biome only like subalpine forest, and much of taiga is lowlands. ==Climate and geography== Taiga is the world's largest land biome, and makes up 29% of the world's forest cover;〔(【引用サイトリンク】title=Taiga biological station: FAQ )〕 the largest areas are located in Russia and Canada. The taiga is the terrestrial biome with the lowest annual average temperatures after the tundra and permanent ice caps. Extreme winter minimums in the northern taiga are typically lower than those of the tundra. The lowest reliably recorded temperatures in the Northern Hemisphere were recorded in the taiga of northeastern Russia. The taiga or boreal forest has a subarctic climate with very large temperature range between seasons, but the long and cold winter is the dominant feature. This climate is classified as ''Dfc'', ''Dwc'', ''Dsc'', ''Dfd'' and ''Dwd'' in the Köppen climate classification scheme,〔(【引用サイトリンク】title=radford:Taiga climate )〕 meaning that the short summer (24-hr average 10 °C (50 °F) or more) lasts 1–3 months and always less than 4 months. In Siberian taiga the average temperature of the coldest month is between and . 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. The mean annual temperature generally varies from -5 °C to 5 °C, (23 °F to 41 °F)〔(【引用サイトリンク】title=Marietta the Taiga and Boreal forest )〕 but there are taiga areas in eastern Siberia and interior Alaska-Yukon where the mean annual reaches down to -10 °C. (14 °F) According to some sources, the boreal forest grades into a temperate mixed forest when mean annual temperature reaches about 3 °C (37 °F). Discontinuous permafrost is found in areas with mean annual temperature below 0 °C, whilst in the ''Dfd'' and ''Dwd'' climate zones continuous permafrost occurs and restricts growth to very shallow-rooted trees like Siberian larch. The winters, with average temperatures below freezing, last five to seven months. Temperatures vary from −54 °C to 30 °C (-65 °F to 86 °F) throughout the whole year. The summers, while short, are generally warm and humid. In much of the taiga, -20 °C (-4 °F) would be a typical winter day temperature and 18 °C (64 °F) an average summer day. 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 threshold to trigger growth. In Canada, Scandinavia and Finland, the growing season is often estimated by using the period of the year when the 24-hour average temperature is or more.〔(Canada: Taiga Shield reference )〕 For the Taiga Plains in Canada, growing season varies from 80 to 150 days, and in the Taiga Shield from 100 to 140 days.〔(【引用サイトリンク】title=Climate of Canadian ecozones )〕 Some sources claim 130 days growing season as typical for the taiga.〔(【引用サイトリンク】title=Berkley: about biomes )〕 Other sources mention that 50–100 frost-free days are characteristic.〔(【引用サイトリンク】title=Taiga )〕 Data for locations in southwest Yukon gives 80–120 frost-free days.〔(【引用サイトリンク】title=Southwest Yukon:Frost-free days )〕 The closed canopy boreal forest in Kenozersky National Park near Plesetsk, Arkhangelsk Province, Russia, on average has 108 frost-free days.〔(【引用サイトリンク】title=Kenozersky National Park )〕 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.〔(【引用サイトリンク】title=University of Helsinki: Carabid diversity in Finnish taiga )〕 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,〔(【引用サイトリンク】title=Tundra )〕〔(【引用サイトリンク】title=NatureWorks:Tundra )〕 and the 24-hr average of the warmest month of the year usually is 10 °C (50 °F) or less.〔(【引用サイトリンク】title=The Arctic )〕 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, with only around 6 hours of daylight 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. The taiga experiences relatively low precipitation throughout the year (generally 200–750 mm annually, 1,000 mm in some areas), primarily as rain during the summer months, but also as fog and snow. This fog, especially predominant in low-lying areas during and after the thawing of frozen Arctic seas, means that sunshine is not abundant in the taiga even during the long summer days. As evaporation is consequently low for most of the year, precipitation exceeds evaporation, and is sufficient to sustain the dense vegetation growth. Snow may remain on the ground for as long as nine months in the northernmost extensions of the taiga ecozone.〔A.P. Sayre, ''Taiga'', (New York: Twenty-First Century Books, 1994) 16.〕 In general, taiga grows to the south of the 10 °C July isotherm, but occasionally as far north as the 9 °C (48 °F) July isotherm.〔Arno & Hammerly 1984, Arno ''et al.'' 1995〕 Rich in spruces, Scots pines in the western Siberian plain, the taiga is dominated by larch in Eastern Siberia, before returning to its original floristic richness on the Pacific shores. Two deciduous conifers mingle throughout southern Siberia: birch and populus tremula.〔 The southern limit is more variable, depending on rainfall; taiga may be replaced by forest steppe south of the 15 °C (59 °F) July isotherm where rainfall is very low, but more typically extends south to the 18 °C (64 °F) July isotherm, and locally where rainfall is higher (notably in eastern Siberia and adjacent Outer Manchuria) south to the 20 °C (68 °F) 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. 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. In Sweden the taiga is associated with the Norrland terrain. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「taiga」の詳細全文を読む スポンサード リンク
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