Taiga (Biodiversity)
Contents
Taiga
Western Canadian mid-summer taiga landscape. @ C.Michael Hogan
Taiga is a high latitude northern hemisphere biome with the overarching ecological feature of conifer forest. Floristically the taiga can be viewed as some of the most northern elements of the Circumboreal Province. Temperature regimes within some of the taiga are among the lowest on Earth, since there is a continental aspect to the interior portions of the taiga, making it colder than many locations in the polar desert to the north. Compared to other biome types, taiga manifests low biodiversity.
Because Asia and North America were earlier unified via the Bering (Bering Strait) land bridge, many of the same flora and fauna species colonised both continents and are distributed throughout the taiga. Speciation occurred within the genera, permitting diversity among different world regions of the taiga.
Location
Example taiga distribution (in yellow) for Scandinavia. WWF By area ranking, taiga ecoregions comprise a more extensive land area than any other terrestrial biome, constituting almost thirty percent of the Earth’s entire forests. In Asia taiga covers a considerable portion of Russia, notably Siberia, but also northern Kazakhstan, the north part of Mongolia as well as the island of Hokkaid? within Japan. For Europe, taiga covers a band across most of Sweden (Ecoregions of Sweden), much of Finland and northern Norway. Within North America, taiga constitutes the majority of interior Canada, along with the extreme northern elements of the USA including much of Alaska. In the southern portions of North American occurrences this biome is often termed boreal forest.
Example ecoregions
The following are examples of specific taiga ecoregions:
- East Siberian taiga
- Northeast Siberian taiga
- Scandinavian and Russian taiga
- Southern Hudson Bay taiga
- Alaska Peninsula montane taiga
- Northwest Territories taiga
- Cook Inlet taiga
- Copper Plateau taiga
- Eastern Canadian Shield taiga
- Northern Canadian Shield taiga
Climatic aspects
Only the polar deserts and tundra biomes offer lower temperatures compared to taiga. Extreme minimums in the taiga are actually typically lower than those of the tundra. The Köppen climate classification system designates taiga as a region of short summers and protracted very cold winters, with high amplitude temperature swings betwixt the two extreme seasons. Relatively small portions of this biome are classified under Köppen as Cfc (milder winters), and in the extrema of the southern part of the biome there are some very small areas classified as Dwb or Dfb, (areas of longer summer and humid continental climate). Mean annual temperatures are almost always within minus five to plus five degrees Celsius, save for parts of east Siberia and interior elements of the Yukon, for which mean annual temperatures may extend as low as eleven degrees Celsius.
Discontinuous permafrost is found in areas with mean annual temperature below freezing; however, in the Dfd and Dwd Köppen climate zones continuous permafrost is typically found, limiting tree species to trees with extremely shallow root zones, such as Siberian larch. The winters, with average temperatures below freezing, last five to seven months. Temperatures vary from −54 °C to 30 °C throughout the entire annual cycle. The summers, while brief, are characteristically warm and humid. In much of the taiga, -20 °C would be a typical winter daytime temperature and 18 °C that of a typical summer day. The river valley taiga near Verkhoyansk, Russia, at 67°N, experiences the coldest winter temperatures in the northern hemisphere; however, the extreme interior continental nature of the climate yields an average thermal maximum of 22 °C in July
Taiga is exposed to low precipitation throughout the year (typically 200 to 750 millimeters (mm) annually, and up to 1000 mm), 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 sunlight is not abundant in the taiga even during the long summer days. Thus, since evapotranspiration is low most of the year, precipitation exceeds evaporation, and is sufficient to sustain the dense vegetation growth.
Snow may blanket the surface for as much as nine months in the northernmost extensions of the taiga ecozone In general, taiga thrives to the south of the 10 °C July contour line isotherm, but occasionally as far north as the 9 °C July isotherm. Since surface soils freeze in winter, restricting root tissue from taking up water, desiccation can be a significant issue in late winter for conifers.
The southern taiga limit is somewhat variable, depending on rainfall; taiga may be replaced by forest steppe south of the 15°C July isotherm where rainfall is very low, but more typically extends south to the 18°C July isotherm, and locally where rainfall is higher (notably in eastern Siberia and adjacent northern Manchuria) south to the 20°C July isotherm.
Geology and soils
Taiga soil is geologically young and generally lacking in nutrients. In contrast, the deep, organically enriched soil profile present in temperate deciduous forests is absent in taiga zones. The thinness of the taiga soil is attributed chiefly to the cold climate, which impedes the development of soil and the facility with which plantlife can extract soil nutrients. 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 conifer needles further leach the soil, creating spodosol. Since the soil is acidic due to the fallen pine needle detritus, the forest floor flora consists chiefly of lichens and mosses. The area currently classified as taiga in Europe and North America (except Alaska) was glaciated during the Wisconsin period. As the glaciers receded they left kettle depressions in the topography that have since filled with water, creating lakes and bogs (especially muskeg soil) found throughout the taiga.
Specific soil types found in the Boreal frozen taiga soils are: Frozen taiga (taiga ferruginous), frozen bog and frozen solonetz (frozen soluds or frozen gleyey soluds). Specific soil types found in the boreal taiga and forest are: Taiga forest (podzolics), sod taiga (sod gleyey) and hydromorphic bog soils.
Tangent biomes
To the north of taiga lie tundra and polar desert ecoregions, both of which are more floristically depauperate, particularly with respect to plantlife of any significant stature. The southern elements of taiga grade to temperate mixed forest ecoregions, where edaphic conditions are favorable and mean annual temperatures are in the plus three to plus five degrees Celsius (C) mean annual temperature range.
Ecology
Abiotic factors of meteorology and soils are chief drivers of the plant and animal associations of taiga. 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 and Scandinavia, the growing season is often estimated by using the time interval of the year when the 24-hour average temperature is five degrees C or more. In Canada's taiga plains, the growing season varies from 80 to 150 days, and in the Taiga Shield, from 105 to 140 days. Locations in southwest Yukon offer 80 to 120 frost-free days.
The closed canopy boreal forest in Kenozyorsky National Park near Plesetsk, Arkhangelsk Oblast, 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 to 70 days, and the 24-hour average of the warmest month of the year usually is ten degrees C or less. High latitudes mean that the sun does not rise far above the horizon, and less sunlight impinges than further south. But the high latitude also ensures very long summer days, as the sun stays above the horizon almost twenty hours per day, with only around six 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.
In the warmer areas of taiga occurrence, there is a higher species diversity, with more warmth-loving species such as Korean pine, Jezo spruce, and Manchurian fir; this southern zone merges gradually into temperate broadleaf and mixed forests or, in Pacific coastal zones of North America and Asia, into coniferous temperate rainforests.
Trees of the taiga trees generally exhibit shallow rooting systems to take advantage of the thin soils; moreover, many tree species seasonally alter their metabolic biochemistry, making them more resistant to freezing. The narrow conical shape of northern conifers, and their downward-drooping limbs, also enhance their ability to slough-off snow. Because sunlight is restricted during most of an annual cycle, photosynthetic activity is impeded. Pine, spruce and fir do not shed leaves autumnally, and can thus engage in photosynthesis, employing old leaf tissue in late winter and spring, when light is plentiful, but temperatures are still too low for new growth to commence. The adaptation of evergreen needles limits the water lost due to transpiration and their dark green color increases their absorption of sunlight.
Periodic wildfires constitute disturbance regimes that decimate canopies (with return times ranging from decades to centuries), allowing sunlight to generate new flora propagation and growth on the forest floor. For some taxa, taiga wildfires are an essential element of the life cycle; some species such as Jack Pine exhibit cone characteristics, which only open only upon fire or heat stimulus.
Vegetative associations
Dwarfed and ice-pruned black spruce stand, Churchill, Canada. @ C.Michael Hogan There are two major varieties of taiga. The southern reach is a closed canopy forest, comprised by many closely spaced trees and mossy ground cover. In forest clearings, many species of shrubs and forbs are present. The other taiga type is the lichen woodland or sparse taiga, having trees that are spaced further apart and a lichen ground cover; the latter taiga is characteristic in the northernmost taiga, where trees are often dwarfed in growth form; moreover, ice pruned asymmetric Black Spruce (in North America) are often seen, with diminished foliage on the windward side.
In Canada, Scandinavia and Finland, the boreal forest is usually divided into three subzones: The high boreal (north boreal) or taiga zone; the middle boreal (closed forest); and the southern boreal, a closed canopy boreal forest with scattered temperate deciduous trees such as maple, elm and oak. This southern boreal forest has the longest and warmest growing season of the biome, and in some regions (including Scandinavia, Finland and western Russia) this subzone is commonly used for agriculture. The boreal forest is home to many types of berries; some are confined to the southern and middle closed boreal forest (e.g. raspberry), others grow in most areas of the taiga (such as cranberry and cloudberry), and some can grow in both the taiga and the southern arctic tundra (e.g. bilberry, lingonberry).
Relatively few species in four main genera are found in the coniferous dominant taiga: spruce, fir, and pine, and the deciduous larch. Across Scandinavia and western Russia, the Scots pine is a common component of the taiga, while taiga of far eastern Asia, including Mongolia, is dominated by larch.
Faunal associations
Polar bear migrating to Hudson Bay pack ice, Churchill, Canada. @ C.Michael Hogan
A broad gamut of fauna is found in taiga. The North American boreal forests alone exhibit over 85 mammalian species, 130 different freshwater fish taxa, and more than 32,000 species of insects. The last group occupy a vital role as prey, pollinators and decomposers. Long cold winters make taiga a difficult environment for reptiles and amphibians, who depend on ambient meteorlogical conditions to regulate body temperature. Some hibernate underground or in snow dens to survive the winter. Some megafaunal herbivores such as moose and caribou inhabit parts of the taiga. Portions of the southern closed boreal forest support other cervid species such as elk and roe deer. There is also a gamut of rodentia such as squirrel, beaver, vole, mountain hare and snowshoe hare. These mammals have adapted to deal with severe winters in their northern ranges. Some larger mammals, such as polar bear, eat fulsomely at sea in summers to increase body mass, before hibernating. Other animals have adapted layers of fat, pelage or feathers to provide heavy insulation from the cold.
Image by Dmitry A. Mottl
References
- S.F.Arno and R.P.Hammerly R. P. 1984. Timberline Mountain and Arctic Forest Frontiers. The Mountaineers, Seattle. ISBN 0-89886-085-7
- S.F.Arno, J.Worral and C.E.Carlson. 1995. Larix lyallii: Colonist of tree line and talus sites. in W.C.Schmidt and K.J.McDonald, K. J., eds., Ecology and Management of Larix Forests: A Look Ahead. USDA Forest Service General Technical Report GTR-INT-319.
- S.W.Buol. 2003. Soil genesis and classification. Wiley-Blackwell. 494 pages
- C. Michael Hogan. 2008. Black Spruce: Picea mariana, GlobalTwitcher.com, ed. Nicklas Stromberg
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- Thomas Kurkowski. 2008. Relative Importance of Different Secondary Successional Pathways in an Alaskan Boreal Forest.Canadian Journal of Forest Research. 38. 1911-1923.
- Serge Payette. 2000. Origin of the lichen woodland at its southern range limit in eastern Canada: the catastrophic impact of insect defoliators and fire on the spruce-moss forest. Canadian journal of forest research. 30.2: 288-305.
- E.A.Johnson. 1981. Vegetation Organization and Dynamics of Lichen Woodland Communities in the Northwest Territories. Ecology. 62.1: 200-215.
- J.P.Jasinski J.P. 2005. The Creation of Alternative Stable States in Southern Boreal Forest: Quebec, Canada. Ecological Monographs. 75.4: 561-583.
- A.P. Sayre. 1994. Taiga. New York: Twenty-First Century Books
