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1236
Section: Biology
There’s No Forest without Fire: Wildfires in the Taiga as a Natural Factor

There’s No Forest without Fire: Wildfires in the Taiga as a Natural Factor

Forests have always burnt and will continue to burn ...A strong proof in favor of this statement was provided last summer, when forest fires spread over vast areas in Russia. Enormous material damage, fire-devastated woodlands and habitations, drastically deteriorated ecology that negatively affected people’s health and living conditions – these are just a few of the catastrophic consequences of the phenomenon people themselves are often to be blamed for. In boreal forests, however, wildfire outbreaks happened long before humans appeared there. Provoked by natural causes like thunderstorms, wildfires are an essential factor of the dynamics of natural plant communities

The largest part of the vast Siberian territory is situated in the permafrost zone. And the largest part of the permafrost zone is covered with larch forests. Larch is one of the most cold-resistant and undemanding coniferous tree species.

Nothern larch forms regularly low crown closure stands. As a result, fires occurring in larch forests, with rare exceptions, are on-ground (fires spread at a low height rather than jump over from crown to crown). Under permafrost conditions, the root systems of the trees are located in the thin surface layer overgrown with lichens and mosses – so when the layer burns out, the roots get damaged and the forest perishes.

What affects the fire return intervals in larch forests? Does the wildfire periodicity change with time? What are fire risk factors? To find answers to these and other questions, scientists of the Krasnoyarsk-based Sukachev Forest Institute SB RAS sent expeditions to the larch domination zone (the Central Siberian and Anabar Plateaus) and to its southern boundary, where other forest-forming species (Siberian pine, spruce, fir, pine, birch, and aspen) grow. Very helpful for tracing burns is satellite imagery, which is applied in operations planning.

Catastrophic fires are depicted in the historical chronicles. For example, the Nikon (Patriarch’s) Chronicle reads: “Many pine forests, as well as the bogs, took fire on their own” (1094); “The ground and smoke rising over the ground” (1364); “It was a Great Drought and remarkable heat and sultriness so that one could not see at a sazhen, and many men fell their faces down… and the birds… fell from the air onto the ground… the beasts were blinded and were wandering about the villages and towns; bears, wolves and foxes got mixed with men.” (1371) 
The Novgorod chronicle of 1430 reads: “That autumn there was very little water, and the ground and forests were on fire and there was smoke everywhere.” 
In 1735, Empress Anna Ioanovna wrote from Saint-Petersburg to General Ushakov: “Andrey Ivanovich, there is so much smoke here that it is not possible to open a window… the forest is burning, just like it was last year.. and this is not the first year… please send people to extinguish the fire.” Devastating fires raged in Russia in 1867, when military regimens had to be sent to fight them.
In 1915 in Siberia, forest fires spread to 12.5 million hectares. During the drought in the Volga region in 1921, the forests were on fire from spring to late autumn and destroyed 300,000 hectares of forests and 60 settlements. 
In 1972, forest and peat fires covered 1.8 million hectares. In 1998, fires began in March and continued until November. All in all, 23,000 forest wildfires were recorded; the fire swept over more than 2.4 million hectares of forestland. 
In 2002, there were about 38,000 forest wildfires and the area of fire-affected forests was over 1 million hectares.
The fires of the summer of 2010 do not need any reminder...

In accordance with the accepted investigation procedures, test plots were established on the fire-sites. Within the bounds of these plots, trees whose boles had the so-called fire scars were sawn down. The tree rings analysis was used to date the fires and determine fire return intervals. Along with the survived trees, the trees that had perished from former fires were examined to extend the fire chronology.

Fire turnover

One of the questions to be answered was how the frequency of fires had changed over the past two centuries, when human activity grew sharply?

In the 19th century, a typical forest in Siberia had 101 ± 12 years to grow before it burned down, while in the 20th century the fire return interval shortened to 65 ± 6 years. In the mixed taiga of the Yenisei range, the fire return interval almost halved, from 97 ± 22 to 50 ± 14 years. This tendency is partially due to the anthropogenic effect; however, natural factors such as climatic change should not be ignored. In the 20th century, a rise in the “fire turnover” occurred on the backdrop of climate warming. Cross-correlation analysis confirmed that regional anomalies in fire incidence were associated with air temperature anomalies.

The greatest damage is done by the fires extending to vast territories (over 200 hectares). Such fires account for only 10 % of total fire occurrences but cause up to 90 % of total damage. 
In forest preserves (about 2/3 of the total forested area), from 12,000 to 36,000 forest fires are recorded every year with the total coverage of up to 5 million hectares. Satellite imagery yields even higher numbers. The photographs made immediately after a fire clearly show the area affected. However, direct assessment of the damaged area based on these “flash” data appears to be overstated because vegetation is destroyed in full only on one-fourth of the fire-affected territory.
In addition to forest destruction, the fires increase emissions of greenhouse gases, mainly carbon dioxide. 
Carbon dioxide emissions consist of the fire and post-fire emissions. Direct fire emissions are estimated at 40 to 250 million tons of carbon a year (IIASA). Similar estimates were obtained for post-fire emissions due to the decay of destroyed tree stands

Landscape is of no small importance either. Humidification depends on slope exposition and steepness. Windward slopes receive more precipitation, but if they are steep, the water runs down and accumulates in hollows.

The data obtained indicate that north-eastern slopes, as well as peat bogs, have the longest fire return intervals, while on south-western slopes ignitions occur the most frequently because the slopes are the best warmed by the sun and inflammable forest materials dry faster. The altitude-related climate gradient also influences fire risks.

In the future, the duration of the fire-prone season is expected to increase globally as a result of observed and projected global warming. Thunderstorm activity, which is the cause of natural ignition, is expected to grow too.

From the north to the south

Does fire frequency depend on latitude? The return times appear to increase in the northward direction, from 80 years in the south of Evenkia to 300 years on the Anabar Plateau, situated in close vicinity to the northern boundary of larch forests.

The intra-annual fire distribution in the southern taiga is known to have a bimodal (two-hump) shape with a higher peak in late spring and a lower one in late fall. In the northern latitudes, the distribution becomes almost unimodal with the only peak in early summer. Here, the annual fire-prone season is shorter by almost two-thirds: 80 days in the northern forests (72°n.l.) against 210 days in the southern forests.

Northern summers often lack warmth needed to dry fuel materials and make them prone to lightening discharges or campfires that have not been extinguished. Also, northern areas are under a lower anthropogenic pressure. As compared with the middle-latitude taiga, where 80 % of ignitions are caused by the “human factor,” in the north, almost 90 % of fires are started by lightening discharges.

Attempts to fully exclude wildfires from forest life are not always a good thing. For example, fire suppression in the south of Alaska made tree stands overripe – they grew too old and lost resistance to pests. As a result, the forests rescued by people from fires perished from bark beetles. 
To prevent catastrophic fires, pyrologists have suggested periodic “controlled” burnings. Such low-intensity on-ground fires clear the forests from “combustion conductors,” keeping them from accumulating in critical amounts, which would invite disaster

A high “accuracy” of thunder-bolts in the cryolite zone is due to a drop of electric conductivity on the border with the permafrost layer: the discharge energy is liberated in a thin (less than 30 cm) root-inhabited layer.

On the contrary, day-to-day activity of local population, whose density in Evenkiya is very low (0.03 person per km2), is not a frequent cause of fires because indigenous people have known since ancient times how to behave in the forest. Old Believers, who also live in these parts, cultivate an attitude of care towards the taiga.

On the other side of the damage

Undoubtedly, fires cause an enormous damage to wood resources. However, the ecological conditions of burned areas are better: the depth of seasonal frost retreat increases, the soil is enriched with biogenic elements, the drainage and lighting regimes improve. As a result, over the first 20 to 30 years after the fire, the trees that have survived demonstrate higher growth increments.

With time, as the heat-insulating lichen-moss cover thickens, the depth of the seasonal permafrost retreat starts decreasing at an average rate of 0.5—1.0 cm/year. This leads to a “compression” of the root-inhabited zone (to 30 cm or less) and to a drop in annual growth. Forest stands become “drowsy” in expectation of the next wildfire, which is provoked by the growing lichen-moss “cushion,” which, when dried, turns into an excellent fuel.

A consequence of the shorter return times in the cryolite zone forests is the broadening biodiversity attributed to the penetration of “southern” woody species to the larch dominated territories.

The mechanism of penetration is as follows: under improved ecological conditions, fire-sites are kind of launching pads for the migration of “evergreen conifers” (spruce, Siberian pine, fir, and pine) to the zones where larch dominates because of its remarkable cold resistance. Currently, an understory of Siberian pine and spruces is forming beneath the larch canopy on the southern boundary of larch forests. If these trends in climatic change persist, the invading species are likely to form the upper canopy and become dominant.

In the larch and mixed taiga ecotones, fires provoke the development of birch and aspen stands, which rapidly conquer the vacated territories. On the fire-sites, the population of birch undergrowth may reach 1 million sprouts per hectare, that is, as many as 100 (!) sprouts per square meter. Larch, in its turn, develops denser stands and moves further to the tundra zone.

For the “southern” species, shorter return times could have both positive and negative consequences. The survey of fire-sites revegetation has showed that over a number of years after the fire the Siberian pine undergrowth population always increases. This is explained by the Siberian pine’s ability to get rooted in the moss layer whereas “hovering” larch roots fail to reach the soil through the moss.

That is why frequent fires help the larch to keep its dominant position as a pyrophyle (i. e. “fire-loving”) species in the creolyte zone. Larch is excellently protected from fire by thick bark, so a certain part of larch trees normally survive the fire. Moreover, fires encourage larch reproduction because larch germs root better in a well mineralized soil.

How will the climate change and higher fire frequency influence the northern forests? Will the larch forests continue playing their important biospheric role in carbon sequestration?

On the one hand, global warming facilitates the productivity of northern woodlands and migration of larch to the tundra. As a result, the absorption of carbon dioxide from the air increases, which will lead to a more moderate anthropogenic impact on the biosphere. On the other hand, warming results in a rise in fire frequency involving emissions of carbon dioxide to the atmosphere, which may bring to naught the total amount of carbon sequestrated by larch forests. Warming is also expected to bring about a rise in the emissions of greenhouse gases from the permafrost retreat layer.

By comparing these opposite tendencies, most ecological models predict the transformation of larch forests into carbon-emitting territories (IPCC, 2007). This, however, is not the only possible scenario because a rise in seasonal frost retreat and improved drainage may lead to a sharp (by times) increase in the annual growth of larch forests, which is not taken into account by the current models.

Therefore, it cannot be excluded that a rise in the productivity of larch forests will make more important the contribution of boreal forests to carbon sequestration and, consequently, to the mitigation of the “greenhouse effect.” To test the scenarios of fire impact on the taiga under the changing climatic conditions, more expeditions to high latitudes should be organized.

References
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Forests and forestry of Russia. IIASA FOR. Version 1.0. 2007.
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Kharuk V. I., Im S. T., Ranson K. J., Naurzbaev M.M. Time dynamics of larch in forest tundra ecotone // Reports of RAS, 2004. N 398(3). P. 404—408.
Kharuk V., Ranson K., and Dvinskaya M. Forest fires dynamic in the larch dominance zone// Geophys. Res. Let., 2008. V. 35. N 1.
Kharuk V., Ranson K., Dvinskaya M. Evidence of Evergreen Conifer Invasion into Larch Dominated Forests During Recent Decades in Central Siberia // Eurasian Journ. of Forest Res. 2007.
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Korovin G. N., Zukert N. V. Influence of climatic change on forest fires in Russia // Klimaticheskiye izmeneniya: vzgliad iz Rossii [Climate changes: A view from Russia]. Ed. by Danilov-Danilian. M.: TEIS. 2003. P. 69—98.
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The work was supported by the Russian Foundation of Fundamental Research (Project No. 09-05-98008)

Photographs the courtesy of V. Kharuk

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