Encyclopedia
Taiga is a biome characterized by
coniferous forests. Covering most of inland
Alaska,
Canada,
Sweden,
Finland, northern
Kazakhstan and
Russia , as well as parts of the extreme northern continental United States, the taiga is the world's largest terrestrial biome and a major source of
oxygen. In
Canada,
boreal forest is the term used to refer to the southern part of this biome, while "taiga" is used to describe the more barren northern areas south of the
Arctic tree-line.
Since
North America and
Eurasia were recently connected by the
Bering land bridge, a number of animal and plant species were able to colonise both continents and are distributed throughout the taiga biome. Others differ regionally, typically with each genus having several distinct species, each occupying different regions of the taiga.
Climate and geography
The taiga biome has a harsh continental
climate with a very large temperature range between summer and winter, classified as in the Köppen climate classification scheme. Aside from the
tundra and permanent ice caps, it is the coldest biome on Earth. High latitudes mean that, for much of the year the
sun does not rise far above the horizon; winters last at least 6 months, with average temperatures below freezing. Temperatures vary from -50 °C to 30 °C throughout the whole year, with 8 or more months of temperatures averaging below 10 °C. The summers, while short, are generally warm and humid. In general, taiga grows north to the 10 °C July
isotherm, occasionally to the 9 °C July isotherm . The southern limit is more variable, depending on rainfall; taiga may be replaced by open
steppe woodland 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 south to the 20 °C 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 into coniferous
temperate rainforests.
The taiga experiences relatively low precipitation throughout the year , primarily as rain during the summer months, but also as fog and snow; as evaporation is also low for most of the year, precipitation exceeds evaporation and is sufficient for dense vegetation growth. Snow may remain on the ground for as long as nine months in the northernmost extensions of the taiga .
Much of the area currently classified as taiga was
recently glaciated. As the glaciers receded, they left
depressions in the topography that have since filled with water, creating lakes and bogs , found throughout the taiga.
Soils
Taiga
soil tends to be young and nutrient-poor; it lacks the deep, organically-enriched
profile present in temperate deciduous forests . The thinness of the soil is due largely to the cold; it hinders the development of soil, as well as the ease with which plants can use its 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 evergreen needles further leach the soil, creating
spodosol .
Flora
There are two major types of taiga,
closed forest, consisting of many closely-spaced trees with mossy ground cover, and
lichen woodland, with trees that are farther-spaced and
lichen ground cover; the latter is more common in the northernmost taiga .
The forests of the taiga are largely
coniferous, dominated by
larch,
spruce,
fir, and
pine.
Evergreen species in the taiga have a number of adaptations specifically for survival in harsh taiga winters, though larch, the most cold-tolerant of all trees, 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" . The narrow conical shape of northern conifers, and their downward-drooping limbs, also help them shed snow .
Because the sun is low in the horizon for most of the year, it is difficult for plants to generate energy from
photosynthesis. Pine and spruce 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 grow closer to the ground. Periodic stand-replacing
wildfires 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.
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 a low biological diversity.
Fauna
The taiga is home to a number of large
herbivorous mammals and smaller
rodents. These animals have also adapted to survive the harsh climate. Many of the larger mammals, such as black bears, eat 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.
Due to the climate,
carnivorous diets are an inefficient means of obtaining energy; energy is limited, and most energy is lost between trophic levels. However,
predatory birds and other smaller carnivores, including
foxes and
weasels, feed on the rodents. Larger carnivores, such as
lynxes and
wolves, prey on the larger animals.
Omnivores, such as
bears and
raccoons are fairly common, sometimes picking through human garbage.
A considerable number of
birds such as Siberian Thrush,
White's Thrush and
Dark-throated Thrush,
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 perhaps 300 species of birds that summer in the taiga, only 30 stay for the winter . These are either
carrion-feeding or large raptors that can take live mammal prey, including
Golden Eagle,
Rough-legged Buzzard, and
Raven, or else seed-eating birds, including several species of
grouse and
crossbills.
Fire
Fire is the dominant natural disturbance in the taiga, as well as being an important disturbance mechanism in many other forest types, such as temperate, sub-alpine and chaparral forests. Large, stand-replacing fires, particularly in the taiga, determine the age distribution and spatial age mosaic of the forested landscape.
Fire suppression
In North America, the belief that fire suppression has substantially reduced the average annual area burned is widely held by resource managers and is often thought to be self-evident. Direct empirical evidence however is essentially limited to just two studies by Stocks and Ward and Tithecott , that use
Ontario government fire records to make comparisons of average annual area burned between areas with and without aggressive fire suppression policies. Numerous subsequent studies have presented the same information, often in a different format . The proponents of these studies argue that areas without aggressive fire suppression policies have larger average fire sizes and greater average annual area burned and a longer interval between fires and that this is evidence of the effect of fire suppression.
However, the idea that fire suppression can effectively reduce the average annual area burned is the focus of a vocal debate in the scientific literature. In particular, several recent papers have argued against this idea . These papers claim that statistically rigorous techniques for estimating the average annual area burned, called the fire cycle, do not show changes in the fire cycle associated with fire suppression and that the evidence used to support the effect of fire suppression is biased and has been presented in a way that is flawed. Note that none of these papers criticize fire management agencies for being anything less than completely committed to their mandate. Nor do they suggest that fire personnel are not well trained, efficiently deployed or well managed. Instead, these papers simply suggest that despite the resources employed, fire management agencies are simply unable to effectively reduce the average annual are burned.
The impact that effective fire suppression may have on the average annual area burned is important for many reasons, but in particular, its impact is key to the current paradigm of sustainable forest management in many jurisdictions. One of the core aspects of SFM in many jurisdictions is the use of wood supply models to determine sustainable harvest levels. This determination of sustainable harvest levels often assumes that fire suppression has been effective at reducing the average annual area burned. Thus, if current assumptions about the effect of fire suppression are wrong, the impact on SFM could be substantial.
Evidence that fire suppression has been effective
For the most part, studies that support the effects of fire suppression compare either the number of fires or the average fire size between areas with and without aggressive fire suppression policies. Typically, these studies use the same or similar data from provincial fire records for
Ontario covering a span of about 20 years.
Proponents of these studies have argued that, firstly, fires are, on average, much larger in areas without aggressive fire suppression policies than in areas with aggressive fire suppression policies because these fires are allowed to spread freely. Secondly, the proponents have argued that far more lightning caused fires are detected in areas with aggressive fire suppression and yet, the average annual area burned is much higher in areas without aggressive fire suppression. It is implied that fire suppression must, therefore, be reducing the area burned by lightning fires.
Recently, Cumming used novel approaches to analyze multiple components of fire management activity in greater detail than previously done, and confirmed the effectiveness of fire suppression.
Evidence that fire suppression has not been effective
On the other side of the debate, the evidence that fire suppression has not been effective at reducing the average annual area burned has come primarily in two forms. Firstly, in the form of time-since-fire studies, which, it has been argued, do not show detectable changes in the fire cycle that can be associated with fire suppression. Secondly, in the form of criticism of the way that provincial fire records have been used to support the effect of fire suppression.
Numerous time-since fire studies have been done in the temperate, boreal and sub-alpine forests across Canada and the U.S. . The techniques used in these studies are felt to be well founded in statistical theory and recent improvements offer a way to statistically compare different periods of time to see if they possess significantly different fire cycles . While these studies often show a change in the fire cycle at the beginning of the 20th century, this change is usually associated with large-scale climatic factors, such as the end of the little ice age , and not fire suppression. In particular, around Ontario there have been at least six time-since-fire studies that show there has been no change in the fire cycle since 1920 . In
Ontario, active fire suppression activities began sometime in the late 1910's, but these suppression activities are generally thought to be minimal compared with post 1950 when fire suppression began in earnest and technological advances made fire fighting much more effective .
Comparisons of the average annual area burned between areas with and without aggressive fire suppression policies, it is argued, are biased by the fact that small fires are virtually unreported in areas without aggressive fire suppression policies, where detection often relies on reports from settlements or commercial aircraft. Critics have argued that the number of lightning caused fires in areas with and without aggressive fire suppression policies are in fact quite similar and that the smaller average fire size, and the lower proportion of fires in larger size classes in areas with aggressive fire suppression is clearly a consequence of this bias .
Critics have also argued that despite suppression attempts the actual number of large fires in both areas is quite similar . It has been argued that if fire suppression cannot impact the large fires, then it cannot impact the average annual area burned since almost all of the area is burned by only a few large fires.
Finally, studies that compare areas are also often based on averages of annual area burned made over periods of 12 to 17 years . Some have argued that this is too short a time period because the extreme year to year variation in area burned makes such averages highly variable and difficult to interpret .
Reasons why fire suppression may not have affected the fire cycle
Several people have explored reasons why fire suppression may not have affected the fire cycle. In general, they feel that in closed-canopied forests, like the boreal, as little as 3% of the lightning caused fires account for up to 95% of the area burned . Most fires remain small, but a few occur under conditions that allow them to increase rapidly in size. It is this small proportion of large lightning caused fires which has the most influence on the area burned and the fire cycle.
In years with a large area burned, fires in these closed-canopied forests characteristically have high intensities, high rates of spread and high duff consumption. In these years, extreme fire behaviour is preceded by a persistent anomalous high pressure system which produces prolonged periods of above normal temperatures and below normal precipitation , and leads to the severe drying of both medium and heavy fuels. Under these extreme conditions, fire behaviour exhibits little difference between aspect, elevation and vegetation type . In years with only a small area burned, differences in aspect, slope, elevation and vegetation composition can have a significant effect on the fire behaviour , however, the area burned in these years is insignificant.
The extreme fire behaviour associated with persistent high pressure systems results in large areas burned. It has been argued that during these years, it is unlikely that fire suppression can significantly influence the total area burned because under these conditions fire management agencies are quickly overwhelmed .
See also
- Boreal Forest Conservation Framework
References
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External links
- Biomes of the World: Coniferous Forest. Videocassette. Schlessinger media .
- Coniferous Forest. Earth Observatory. NASA. .
- Dixon, D. Franklin Watts Picture Atlas Forests. London: Franklin Watts.
- A network of NGOs, indigenous peoples or individuals that works to protect the boreal forests.
- The Nature Conservancy and its partners work to protect
