Taiga
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 ...
Taiga
From Wikipedia, the free encyclopedia
| Taiga |
|
The taiga is found throughout the high northern latitudes, between the tundra, and the temperate forest, from about 50°N to 70°N, but with considerable regional variation. |
| Ecology |
| Biome |
Terrestrial subarctic, humid |
| Geography |
| Countries |
Russia, Mongolia, Japan, Norway, Sweden, Iceland, Finland, United States, Canada, Scotland |
| Climate type |
Dfc, Dwc, Dsc |
Taiga (
;
Russian:
тайга́;
IPA: [tɐjˈɡa]; from
Turkic[1]) 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.
[2] 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;
[3]
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,
[4]
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 −6 °C (21 °F) and
−50 °C (−58 °F).
[5] 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)
[6] but there are taiga areas in eastern Siberia and interior Alaska-
Yukon where the mean annual reaches down to -10 °C. (14 °F)
[7][8]
According to some sources, the boreal forest grades into a temperate
mixed forest when mean annual temperature reaches about 3 °C (37 °F).
[9] 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 taiga in the river valley near
Verkhoyansk,
Russia,
at 67°N, must deal with the coldest winter temperatures in the northern
hemisphere, but the extreme continentality of the climate gives an
average daily high of 22 °C (72 °F) in July.
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 +5 °C
(41 °F) or more.
[10] For the Taiga Plains in Canada, growing season varies from 80 to 150 days, and in the Taiga Shield from 100 to 140 days.
[11] Some sources claim 130 days growing season as typical for the taiga.
[12] Other sources mention that 50–100 frost-free days are characteristic.
[13] Data for locations in southwest Yukon gives 80–120 frost-free days.
[14] The closed canopy boreal forest in
Kenozersky National Park near
Plesetsk,
Arkhangelsk Province, Russia, on average has 108 frost-free days.
[15]
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.
[16] 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,
[17][18] and the 24-hr average of the warmest month of the year usually is 10 °C (50 °F) or less.
[19] 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.
Lakes and other water bodies are common in the taiga. The
Helvetinjärvi National Park, Finland, situated in the closed canopy taiga (mid-boreal to south-boreal)
[20] with mean annual temperature of 4 °C (39 °F).
[21]
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.
[22]
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.
[23]
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 trees
mingle throughout southern Siberia: birch and populus tremula.
[5]
Late September in the
fjords near
Narvik,
Norway. This oceanic part of the forest can see more than 1,000 mm
precipitation annually and has warmer winters than the vast inland taiga
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.
[24]
Soils
Taiga
soil tends to be young and poor in nutrients. It lacks the deep, organically enriched
profile present in temperate deciduous forests.
[25]
The thinness of the soil is due largely to the cold, which hinders the
development of soil and the ease with which plants can use its
nutrients.
[25] 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.
[26] Since the soil is acidic due to the falling pine needles, the forest floor has only
lichens
and some mosses growing on it. In clearings in the forest and in areas
with more boreal deciduous trees, there are more herbs and berries
growing. Diversity of soil organisms in the boreal forest is high,
comparable to the
tropical rainforest.
[27]
Flora
Since
North America and
Asia used to be connected by the
Bering land bridge, a number of animal and plant
species (more animals than plants) were able to colonize both continents and are distributed throughout the taiga biome (see
Circumboreal Region). Others differ regionally, typically with each
genus having several distinct species, each occupying different regions of the taiga. Taigas also have some small-leaved
deciduous trees like
birch,
alder,
willow, and
poplar; mostly in areas escaping the most extreme winter cold. However, the
Dahurian larch
tolerates the coldest winters in the Northern Hemisphere in eastern
Siberia. The very southernmost parts of the taiga may have trees such as
oak,
maple,
elm and
lime scattered among the conifers, and there is usually a gradual transition into a temperate mixed forest, such as the
eastern forest-boreal transition
of eastern Canada. In the interior of the continents with the driest
climate, the boreal forests might grade into temperate grassland.
There are two major types of taiga. The southern part is the
closed canopy forest,
consisting of many closely spaced trees with mossy ground cover. In
clearings in the forest, shrubs and wildflowers are common, such as the
fireweed. The other type is the
lichen woodland or
sparse taiga, with trees that are farther-spaced and
lichen ground cover; the latter is common in the northernmost taiga.
[28] In the northernmost taiga the forest cover is not only more sparse, but often stunted in growth form; moreover,
ice pruned asymmetric black spruce (in North America) are often seen, with diminished foliage on the windward side.
[29] 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 some scattered temperate deciduous trees among the conifers,
[30]
such as maple, elm and oak. This southern boreal forest experiences 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 agricultural purposes. The boreal forest is home to
many types of
berries; some are confined to the southern and middle closed boreal forest (such as
wild strawberry and
partridgeberry); others grow in most areas of the taiga (such as
cranberry and
cloudberry), and some can grow in both the taiga and the low arctic (southern part of) tundra (such as
bilberry,
bunchberry and
lingonberry).
The forests of the taiga are largely
coniferous, dominated by
larch,
spruce,
fir and
pine. The woodland mix varies according to geography and climate so for example the
Eastern Canadian forests ecoregion of the higher elevations of the
Laurentian Mountains and the northern
Appalachian Mountains in Canada is dominated by balsam fir
Abies balsamea, while further north the
Eastern Canadian Shield taiga of northern Quebec and Labrador is notably black spruce
Picea mariana and tamarack larch
Larix laricina.
Evergreen
species in the taiga (spruce, fir, and pine) have a number of
adaptations specifically for survival in harsh taiga winters, although
larch, the most cold-tolerant of all trees,
[citation needed] is
deciduous. Taiga trees tend to have shallow roots to take advantage of the thin soils, while many of them seasonally alter their
biochemistry to make them more resistant to freezing, called "hardening".
[31] The narrow conical shape of northern conifers, and their downward-drooping limbs, also help them shed snow.
[31]
Because the sun is low in the horizon for most of the year, it is difficult for plants to generate energy from
photosynthesis.
Pine, spruce and fir do not lose their leaves seasonally and are able
to photosynthesize with their older leaves in late winter and spring
when light is good 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.
Although precipitation is not a limiting factor, the ground freezes
during the winter months and plant roots are unable to absorb water, so
desiccation can be a severe problem in late winter for evergreens.
Although the taiga is dominated by coniferous forests, some
broadleaf trees also occur, notably
birch,
aspen,
willow, and
rowan. Many smaller
herbaceous plants, such as
ferns and occasionally
ramps grow closer to the ground. Periodic stand-replacing
wildfires
(with return times of between 20–200 years) clear out the tree
canopies, allowing sunlight to invigorate new growth on the forest
floor. For some species, wildfires are a necessary part of the life
cycle in the taiga; some, e.g.
jack pine
have cones which only open to release their seed after a fire,
dispersing their seeds onto the newly cleared ground; certain species of
fungi (such as
morels) are also known to do this.
Grasses grow wherever they can find a patch of sun, and
mosses and
lichens
thrive on the damp ground and on the sides of tree trunks. In
comparison with other biomes, however, the taiga has low biological
diversity.
Jack pine cones and morels after fire in a boreal forest.
Coniferous trees are the dominant plants of the taiga biome. A very
few species in four main genera are found: the evergreen spruce, fir and
pine, and the deciduous larch. In North America, one or two species of
fir and one or two species of spruce are dominant. Across Scandinavia
and western Russia, the
Scots pine is a common component of the taiga, while taiga of the
Russian Far East and
Mongolia is dominated by
larch.
Fauna
The boreal forest, or taiga, supports a relatively small range of
animals due to the harshness of the climate. Canada's boreal forest
includes 85 species of
mammals, 130 species of fish, and an estimated 32,000 species of
insects.
[32] Insects play a critical role as
pollinators,
decomposers,
and as a part of the food web. Many nesting birds rely on them for food
in the summer months. The cold winters and short summers make the taiga
a challenging biome for
reptiles and
amphibians,
which depend on environmental conditions to regulate their body
temperatures, and there are only a few species in the boreal forest
including
red-sided garter snake,
common European adder,
blue-spotted salamander,
northern two-lined salamander,
Siberian salamander,
wood frog,
northern leopard frog,
boreal chorus frog,
American toad, and
Canadian toad.
Most hibernate underground in winter. Fish of the taiga must be able to
withstand cold water conditions and be able to adapt to life under
ice-covered water. Species in the taiga include
Alaska blackfish,
northern pike,
walleye,
longnose sucker,
white sucker, various species of
cisco,
lake whitefish,
round whitefish,
pygmy whitefish,
Arctic lamprey, various
grayling species,
brook trout (including sea-run brook trout in the Hudson Bay area),
chum salmon,
Siberian taimen,
lenok and
lake chub.
The taiga is home to a number of large
herbivorous mammals, such as
moose and
reindeer/
caribou. Some areas of the more southern closed boreal forest also have populations of other deer species such as the
elk (wapiti) and
roe deer.
[33][34] The largest animal in the taiga is the
wood bison, found in northern Canada, Alaska and has been newly introduced into the Russian far-east.
[35] Small mammals of the Taiga biome include
rodent species including
beaver,
squirrel,
North American porcupine and
vole, as well as a small number of
lagomorph species such as
snowshoe hare and
mountain hare. These species have adapted to survive the harsh winters in their native ranges. Some larger mammals, such as
bears, eat heartily during the summer in order to gain weight, and then go into
hibernation
during the winter. Other animals have adapted layers of fur or feathers
to insulate them from the cold. Predatory mammals of the taiga must be
adapted to travel long distances in search of scattered prey or be able
to supplement their diet with vegetation or other forms of food (such as
raccoons). Mammalian predators of the taiga include
Canada lynx,
Eurasian lynx,
stoat,
Siberian weasel,
least weasel,
sable,
American marten,
North American river otter,
European otter,
American mink,
wolverine,
Asian badger,
fisher,
gray wolf,
coyote,
red fox,
brown bear,
American black bear,
Asiatic black bear,
polar bear (only small areas at the taiga - tundra ecotone) and
Siberian tiger.
More than 300 species of
birds have their
nesting grounds in the taiga.
[36] Siberian thrush,
white-throated sparrow, and
black-throated green warbler migrate to this
habitat to take advantage of the long summer days and abundance of
insects found around the numerous bogs and lakes. Of the 300 species of birds that summer in the taiga only 30 stay for the winter.
[37] These are either
carrion-feeding or large
raptors that can take live mammal prey, including
golden eagle,
rough-legged buzzard (also known as the rough-legged hawk), and
raven, or else seed-eating birds, including several species of
grouse and
crossbills.
Fire
Fire has been one of the most important factors shaping the composition and development of boreal forest stands (Rowe 1955);
[38] it is the dominant stand-renewing disturbance through much of the Canadian boreal forest (Amiro et al. 2001).
[39] The fire history that characterizes an
ecosystem is its
fire regime,
which has 3 elements: (1) fire type and intensity (e.g., crown fires,
severe surface fires, and light surface fires), (2) size of typical
fires of significance, and (3) frequency or return intervals for
specific land units (Heinselman 1981).
[40] The average time within a fire regime to burn an area equivalent to the total area of an ecosystem is its
fire rotation (Heinselman 1973)
[41] or
fire cycle (Van Wagner 1978).
[42] However, as Heinselman (1981)
[40]
noted, each physiographic site tends to have its own return interval,
so that some areas are skipped for long periods, while others might burn
two-times or more often during a nominal fire rotation.
The dominant fire regime in the boreal forest is high-intensity crown
fires or severe surface fires of very large size, often more than
10,000 ha, and sometimes more than 400,000 ha (Heinselman 1981).
[40]
Such fires kill entire stands. Fire rotations in the drier regions of
western Canada and Alaska average 50–100 years, shorter than in the
moister climates of eastern Canada, where they may average 200 years or
more. Fire cycles also tend to be long near the tree line in the
subarctic spruce-lichen woodlands. The longest cycles, possibly 300
years, probably occur in the western boreal in floodplain white spruce
(Heinselman 1981).
[40]
Amiro et al. (2001)
[39]
calculated the mean fire cycle for the period 1980 to 1999 in the
Canadian boreal forest (including taiga) at 126 years. Increased fire
activity has been predicted for western Canada, but parts of eastern
Canada may experience less fire in future because of greater
precipitation in a warmer climate (Flannigan et al. 1998).
[43]
The mature boreal forest pattern in the south shows
balsam fir dominant on well-drained sites in eastern Canada changing centrally and westward to a prominence of
white spruce, with
black spruce and
tamarack
forming the forests on peats, and with jack pine usually present on dry
sites except in the extreme east, where it is absent (Rowe and Scotter
1973).
[44]
The effects of fires are inextricably woven into the patterns of
vegetation on the landscape, which in the east favour black spruce,
paper birch, and jack pine over balsam fir, and in the west give the
advantage to aspen, jack pine, black spruce, and birch over white
spruce. Many investigators have reported the ubiquity of charcoal under
the forest floor and in the upper soil profile, e.g., La Roi (1967).
[45] Charcoal in
soils provided Bryson et al. (1965)
[46]
with clues about the forest history of an area 280 km north of the then
current tree line at Ennadai Lake, District Keewatin, Northwest
Territories.
Two lines of evidence support the thesis that fire has always been an
integral factor in the boreal forest: (1) direct, eye-witness accounts
and forest-fire statistics, and (2) indirect, circumstantial evidence
based on the effects of fire, as well as on persisting indicators (Rowe
and Scotter 1973).
[44]
The patchwork mosaic of forest stands in the boreal forest, typically
with abrupt, irregular boundaries circumscribing homogenous stands, is
indirect but compelling testimony to the role of fire in shaping the
forest. The fact is that most boreal forest stands are less than 100
years old, and only in the rather few areas that have escaped burning
are there stands of white spruce older than 250 years (Rowe and Scotter
1973).
[44]
The prevalence of fire-adaptive morphologic and reproductive
characteristics of many boreal plant species is further evidence
pointing to a long and intimate association with fire. Seven of the ten
commonest trees in the boreal forest—
jack pine,
lodgepole pine,
aspen,
balsam poplar (
Populus balsamifera),
paper birch,
tamarack,
black spruce—can
be classed as pioneers in their adaptations for rapid invasion of open
areas. White spruce shows some pioneering abilities, too, but is less
able than black spruce and the pines to disperse seed at all seasons.
Only balsam fir and alpine fir seem to be poorly adapted to reproduce
after fire, as their cones disintegrate at maturity, leaving no seed in
the crowns.
The oldest forests in the northwest boreal region, some older than
300 years, are of white spruce occurring as pure stands on moist
floodplains (Rowe 1970).
[47]
Here, the frequency of fire is much less than on adjacent uplands
dominated by pine, black spruce and aspen. In contrast, in the
Cordilleran region, fire is most frequent in the valley bottoms,
decreasing upward, as shown by a mosaic of young pioneer pine and
broadleaf stands below, and older spruce–fir on the slopes above (Rowe
and Scotter 1973).
[44]
Without fire, the boreal forest would become more and more homogeneous,
with the long-lived white spruce gradually replacing pine, aspen,
balsam poplar, and birch, and perhaps even black spruce, except on the
peatlands (Raup and Denny 1950).
[48]
Threats
Human activities
Large areas of
Siberia's taiga have been harvested for lumber since the collapse of the
Soviet Union.
Previously, the forest was protected by the restrictions of the Soviet
Forest Ministry, but with the collapse of the Union, the restrictions
regarding trade with Western nations have vanished. Trees are easy to
harvest and sell well, so loggers have begun harvesting Russian taiga
evergreen trees for sale to nations previously forbidden by Soviet law.
[49]
In
Canada,
eight percent of the taiga is protected from development, the
provincial government allows forest management to occur on Crown land
under rigorous constraints.
The main forestry practice in the boreal forest of Canada is
clearcutting,
which involves cutting down most of the trees in a given area, then
replanting the forest as a monocrop (one species of tree) the following
season.
Some of the products from logged boreal forests include
toilet paper,
copy paper, newsprint, and lumber. More than 90% of boreal forest
products from Canada are exported for consumption and processing in the
United States. However, with the recession and fewer US homes being
built, that has changed.
Some of the larger cities situated in this biome are
Murmansk,
[50] Arkhangelsk,
Yakutsk,
Anchorage,
[51] Yellowknife,
Tromsø,
Luleå, and
Oulu.
Most companies that harvest in Canadian forests are certified by an
independent third party agency such as the Forest Stewardship Council
(FSC), Sustainable Forests Initiative (SFI), or the Canadian Standards
Association (CSA). While the certification process differs between these
groups, all of them include forest stewardship, respect for aboriginal
peoples, compliance with local, provincial or national environmental
laws, forest worker safety, education and training, and other
environmental, business, and social requirements. The prompt renewal of
all harvest sites by planting or natural renewal is also required.
Climate change
Seney National Wildlife Refuge
The zone of latitude occupied by the boreal forest has experienced
some of the greatest temperature increases on Earth, especially during
the last quarter of the twentieth century. Winter temperatures have
increased more than summer temperatures. The number of days with
extremely cold temperatures (e.g., −20 to −40 °C (-4 to -40 °F) has
decreased irregularly but systematically in nearly all the boreal
region, allowing better survival for tree-damaging insects. In summer,
the daily low temperature has increased more than the daily high
temperature.
[52]
In Fairbanks, Alaska, the length of the frost-free season has increased
from 60–90 days in the early twentieth century to about 120 days a
century later. Summer warming has been shown to increase water stress
and reduce tree growth in dry areas of the southern boreal forest in
central Alaska, western Canada and portions of far eastern Russia.
Precipitation is relatively abundant in Scandinavia, Finland, northwest
Russia and eastern Canada, where a longer growth season (i.e. the period
when sap flow is not impeded by frozen water) accelerate tree growth.
As a consequence of this warming trend, the warmer parts of the boreal
forests are susceptible to replacement by grassland, parkland or
temperate forest.
[53]
In Siberia, the taiga is converting from predominantly
needle-shedding larch trees to evergreen conifers in response to a
warming climate. This is likely to further accelerate warming, as the
evergreen trees will absorb more of the sun's rays. Given the vast size
of the area, such a change has the potential to affect areas well
outside of the region.
[54]
In much of the boreal forest in Alaska, the growth of white spruce
trees are stunted by unusually warm summers, while trees on some of the
coldest fringes of the forest are experiencing faster growth than
previously.
[55]
Lack of moisture in the warmer summers are also stressing the birch trees of central Alaska.
[56]
Insects
Recent years have seen outbreaks of insect pests in forest-destroying plagues: the spruce-bark beetle (
Dendroctonus rufipennis) in
Yukon and Alaska;
[57] the
mountain pine beetle in
British Columbia; the
aspen-leaf miner; the
larch sawfly; the spruce budworm (
Choristoneura fumiferana);
[58] the spruce coneworm.
[59]
Pollution
The effect of
sulphur dioxide on woody boreal forest species was investigated by Addison et al. (1984),
[60] who exposed plants growing on native soils and tailings to 15.2 μmol/m
3 (0.34 ppm) of SO
2 on CO
2 assimilation rate (NAR). The Canadian maximum acceptable limit for atmospheric SO
2 is 0.34 ppm. Fumigation with SO
2
significantly reduced NAR in all species and produced visible symptoms
of injury in 2–20 days. The decrease in NAR of deciduous species
(trembling aspen [
Populus tremuloides], willow [
Salix], green alder [
Alnus viridis], and white birch [
Betula papyrifera]) was significantly more rapid than of
conifers (white spruce, black spruce [
Picea mariana], and jack pine [
Pinus banksiana])
or an evergreen angiosperm (Labrador tea) growing on a fertilized
Brunisol. These metabolic and visible injury responses seemed to be
related to the differences in S uptake owing in part to higher gas
exchange rates for
deciduous species than for
conifers. Conifers growing in oil sands tailings responded to SO
2
with a significantly more rapid decrease in NAR compared with those
growing in the Brunisol, perhaps because of predisposing toxic material
in the tailings. However,
sulphur uptake and visible symptom development did not differ between conifers growing on the 2 substrates.
Acidification of precipitation by anthropogenic, acid-forming
emissions has been associated with damage to vegetation and reduced
forest productivity, but 2-year-old white spruce that were subjected to
simulated acid rain (at pH 4.6, 3.6, and 2.6) applied weekly for 7 weeks
incurred no statistically significant (P 0.05) reduction in growth
during the experiment compared with the background control (pH 5.6)
(Abouguendia and Baschak 1987).
[61]
However, symptoms of injury were observed in all treatments, the number
of plants and the number of needles affected increased with increasing
rain acidity and with time. Scherbatskoy and Klein (1983)
[62] found no significant effect of
chlorophyll concentration in
white spruce at pH 4.3 and 2.8, but Abouguendia and Baschak (1987)
[61]
found a significant reduction in white spruce at pH 2.6, while the
foliar sulphur content significantly greater at pH 2.6 than any of the
other treatments.
Protection
Peat bog in
Dalarna, Sweden.
Bogs
and peatland are widespread in the taiga. They are home to a unique
flora, and store vast amounts of carbon. In western Eurasia, the
Scots pine is common in the boreal forest.
Many nations are taking direct steps to protect the ecology of the
taiga by prohibiting logging, mining, oil and gas production, and other
forms of development. In February 2010 the Canadian government
established protection for 13,000 square kilometres of boreal forest by
creating a new 10,700-square-kilometre park reserve in the Mealy
Mountains area of eastern Canada and a 3,000-square-kilometre waterway
provincial park that follows alongside the Eagle River from headwaters
to sea.
[63]
Two Canadian provincial governments, Ontario and Quebec, introduced
measures in 2008 that would protect at least half of their northern
boreal forest.
[64][65]
Although both provinces admitted it will take years to plan, work with
Aboriginal and local communities and ultimately map out precise
boundaries of the areas off-limits to development, the measures are
expected to create some of the largest protected areas networks in the
world once completed. Both announcements came the following year after a
letter signed by 1,500 scientists called on political leaders to
protect at least half of the boreal forest.
[66]
The taiga stores enormous quantities of
carbon, more than the world's temperate and tropical forests combined, much of it in
wetlands and
peatland.
[67] In fact, current estimates place boreal forests as storing twice as much carbon per unit area as tropical forests.
[68]
Natural disturbance
One of the biggest areas of research and a topic still full of
unsolved questions is the recurring disturbance of fire and the role it
plays in propagating the lichen woodland.
[69]
The phenomenon of wildfire by lightning strike is the primary
determinant of understory vegetation and because of this, it is
considered to be the predominant force behind community and ecosystem
properties in the lichen woodland.
[70]
The significance of fire is clearly evident when one considers that
understory vegetation influences tree seedling germination in the short
term and decomposition of biomass and nutrient availability in the long
term.
[70] The recurrent cycle of large, damaging fire occurs approximately every 70 to 100 years.
[71]
Understanding the dynamics of this ecosystem is entangled with
discovering the successional paths that the vegetation exhibits after a
fire. Trees, shrubs, and lichens all recover from fire-induced damage
through vegetative reproduction as well as invasion by propagules.
[72]
Seeds that have fallen and become buried provide little help in
re-establishment of a species. The reappearance of lichens is reasoned
to occur because of varying conditions and light/nutrient availability
in each different microstate.
[72]
Several different studies have been done that have led to the formation
of the theory that post-fire development can be propagated by any of
four pathways: self replacement, species-dominance relay, species
replacement, or gap-phase self replacement.
[69]
Self replacement is simply the re-establishment of the pre-fire
dominant species. Species-dominance relay is a sequential attempt of
tree species to establish dominance in the canopy. Species replacement
is when fires occur in sufficient frequency to interrupt species
dominance relay. Gap-Phase Self-Replacement is the least common and so
far has only been documented in Western Canada. It is a self replacement
of the surviving species into the canopy gaps after a fire kills
another species. The particular pathway taken after a fire disturbance
depends on how the landscape is able to support trees as well as fire
frequency.
[73] Fire frequency has a large role in shaping the original inception of the lower forest line of the lichen woodland taiga.
It has been hypothesized by Serge Payette that the spruce-moss forest
ecosystem was changed into the lichen woodland biome due to the
initiation of two compounded strong disturbances: large fire and the
appearance and attack of the spruce budworm.
[74]
The spruce budworm is a deadly insect to the spruce populations in the
southern regions of the taiga. J.P. Jasinski confirmed this theory five
years later stating “Their [lichen woodlands] persistence, along with
their previous moss forest histories and current occurrence adjacent to
closed moss forests, indicate that they are an
alternative stable state to the spruce–moss forests”.
[75]
Taiga ecoregions
See also
References
White spruce taiga, Denali Highway, Alaska Range, Alaska
The taiga in the river valley near Verkhoyansk, Russia, at 67°N, must deal with the coldest winter temperatures in the northern hemisphere, but the extreme continentality of the climate gives an average daily high of 22 °C (72 °F) in July.
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 +5 °C (41 °F) or more.[10] For the Taiga Plains in Canada, growing season varies from 80 to 150 days, and in the Taiga Shield from 100 to 140 days.[11] Some sources claim 130 days growing season as typical for the taiga.[12] Other sources mention that 50–100 frost-free days are characteristic.[13] Data for locations in southwest Yukon gives 80–120 frost-free days.[14] The closed canopy boreal forest in Kenozersky National Park near Plesetsk, Arkhangelsk Province, Russia, on average has 108 frost-free days.[15] 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.[16] 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,[17][18] and the 24-hr average of the warmest month of the year usually is 10 °C (50 °F) or less.[19] 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.Boreal forest near Shovel Point in Tettegouche State Park, along the northern shore of Lake Superior in Minnesota.
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.[22]Lakes and other water bodies are common in the taiga. The Helvetinjärvi National Park, Finland, situated in the closed canopy taiga (mid-boreal to south-boreal)[20] with mean annual temperature of 4 °C (39 °F).[21]
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.
In Sweden the taiga is associated with the Norrland terrain.[24]
Since North America and Asia used to be connected by the Bering land bridge, a number of animal and plant species (more animals than plants) were able to colonize both continents and are distributed throughout the taiga biome (see Circumboreal Region). Others differ regionally, typically with each genus having several distinct species, each occupying different regions of the taiga. Taigas also have some small-leaved deciduous trees like birch, alder, willow, and poplar; mostly in areas escaping the most extreme winter cold. However, the Dahurian larch tolerates the coldest winters in the Northern Hemisphere in eastern Siberia. The very southernmost parts of the taiga may have trees such as oak, maple, elm and lime scattered among the conifers, and there is usually a gradual transition into a temperate mixed forest, such as the eastern forest-boreal transition of eastern Canada. In the interior of the continents with the driest climate, the boreal forests might grade into temperate grassland.Boreal forest near Lake Baikal in Russia
Although the taiga is dominated by coniferous forests, some broadleaf trees also occur, notably birch, aspen, willow, and rowan. Many smaller herbaceous plants, such as ferns and occasionally ramps grow closer to the ground. Periodic stand-replacing wildfires (with return times of between 20–200 years) clear out the tree canopies, allowing sunlight to invigorate new growth on the forest floor. For some species, wildfires are a necessary part of the life cycle in the taiga; some, e.g. jack pine have cones which only open to release their seed after a fire, dispersing their seeds onto the newly cleared ground; certain species of fungi (such as morels) are also known to do this. Grasses grow wherever they can find a patch of sun, and mosses and lichens thrive on the damp ground and on the sides of tree trunks. In comparison with other biomes, however, the taiga has low biological diversity.Moss (Ptilium crista-castrensis) cover on the floor of taiga
Coniferous trees are the dominant plants of the taiga biome. A very few species in four main genera are found: the evergreen spruce, fir and pine, and the deciduous larch. In North America, one or two species of fir and one or two species of spruce are dominant. Across Scandinavia and western Russia, the Scots pine is a common component of the taiga, while taiga of the Russian Far East and Mongolia is dominated by larch.Jack pine cones and morels after fire in a boreal forest.
The boreal forest, or taiga, supports a relatively small range of animals due to the harshness of the climate. Canada's boreal forest includes 85 species of mammals, 130 species of fish, and an estimated 32,000 species of insects.[32] Insects play a critical role as pollinators, decomposers, and as a part of the food web. Many nesting birds rely on them for food in the summer months. The cold winters and short summers make the taiga a challenging biome for reptiles and amphibians, which depend on environmental conditions to regulate their body temperatures, and there are only a few species in the boreal forest including red-sided garter snake, common European adder, blue-spotted salamander, northern two-lined salamander, Siberian salamander, wood frog, northern leopard frog, boreal chorus frog, American toad, and Canadian toad. Most hibernate underground in winter. Fish of the taiga must be able to withstand cold water conditions and be able to adapt to life under ice-covered water. Species in the taiga include Alaska blackfish, northern pike, walleye, longnose sucker, white sucker, various species of cisco, lake whitefish, round whitefish, pygmy whitefish, Arctic lamprey, various grayling species, brook trout (including sea-run brook trout in the Hudson Bay area), chum salmon, Siberian taimen, lenok and lake chub.Brown bear, Kamchatka peninsula. Brown bears are among the largest and most widespread taiga omnivores.
Large areas of Siberia's taiga have been harvested for lumber since the collapse of the Soviet Union. Previously, the forest was protected by the restrictions of the Soviet Forest Ministry, but with the collapse of the Union, the restrictions regarding trade with Western nations have vanished. Trees are easy to harvest and sell well, so loggers have begun harvesting Russian taiga evergreen trees for sale to nations previously forbidden by Soviet law.[49]Plesetsk Cosmodrome is situated in the taiga
The zone of latitude occupied by the boreal forest has experienced some of the greatest temperature increases on Earth, especially during the last quarter of the twentieth century. Winter temperatures have increased more than summer temperatures. The number of days with extremely cold temperatures (e.g., −20 to −40 °C (-4 to -40 °F) has decreased irregularly but systematically in nearly all the boreal region, allowing better survival for tree-damaging insects. In summer, the daily low temperature has increased more than the daily high temperature.[52] In Fairbanks, Alaska, the length of the frost-free season has increased from 60–90 days in the early twentieth century to about 120 days a century later. Summer warming has been shown to increase water stress and reduce tree growth in dry areas of the southern boreal forest in central Alaska, western Canada and portions of far eastern Russia. Precipitation is relatively abundant in Scandinavia, Finland, northwest Russia and eastern Canada, where a longer growth season (i.e. the period when sap flow is not impeded by frozen water) accelerate tree growth. As a consequence of this warming trend, the warmer parts of the boreal forests are susceptible to replacement by grassland, parkland or temperate forest.[53]Seney National Wildlife Refuge
Many nations are taking direct steps to protect the ecology of the taiga by prohibiting logging, mining, oil and gas production, and other forms of development. In February 2010 the Canadian government established protection for 13,000 square kilometres of boreal forest by creating a new 10,700-square-kilometre park reserve in the Mealy Mountains area of eastern Canada and a 3,000-square-kilometre waterway provincial park that follows alongside the Eagle River from headwaters to sea.[63]Peat bog in Dalarna, Sweden. Bogs and peatland are widespread in the taiga. They are home to a unique flora, and store vast amounts of carbon. In western Eurasia, the Scots pine is common in the boreal forest.
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