Characteristics Of Boreal Forest Fires

Boreal forest fires may be classified, based on their physical fire behaviour characteristics, into three general categories (Van Wagner 1983): smoldering fires in deep organic layers with frontal fire intensity levels <10 kW/m, surface fires with intensities ranging between 200 and 15,000 kW/m, and crown fires with intensities from 8,000 to > 100,000 kW/m (frontal fire intensity is the product of a fire's rate of spread, the amount of fuel consumed in the flaming front, and the latent heat of combustion). Crown fires can be either intermittent (trees torching individually) or active (with solid flame development in the crowns), with active crown fires being by far the most common. Crown fire development depends on a number of interacting factors: the height of the crown layer above he ground, the bulk density of crown foliage, the crown foliage moisture content, and the initial surface fire intensity. In general, surface fires must generate sufficient intensity to involve the crown layer, resulting in ready access to the ambient wind field which largely determines the rate of spread of the fire. The surface and crown phases of the fire advance as a linked unit dependent on each other. The fast-spreading active crown fires that dominate the boreal landscape are primarily the result of strong winds, and are aided by both short- and longrange spotting of firebrands ahead of the flame front.

The frequency of fires in a given area depends on both the climate and the rate at which potential fuels accumulate following each fire. The fire frequency must be in long-term equilibrium with the longevity of the primary tree species and their reproductive ages. The natural fire cycle averages 50-200 years in the boreal forest (Heinselman 1981). However, human use/protection of the boreal zone has created a much wider gap in fire return intervals than would be the case under natural conditions. Stocks et al (1996), based on 1980s data for Canada, showed mean fire return intervals ranging from <100 years in remote, modestly-protected regions of the northern boreal to >500 years in heavily protected boreal zones.

Fire-adapted forests can generally be divided into two categories (Van Wagner 1983): those species able to regenerate although all trees have been killed over a large area, and those species of which some individuals must remain alive to provide seed for the next generation. Species of the first type are either conifers that store seed in insulated serotinous cones that require heat to open, or hardwoods that regenerate through suckering from the root layer following fire. Species of the second type are conifers that release seed every year when the cones mature. Canadian and Alaskan boreal forests are dominated by species (e.g. Pinus banksiana (jack pine) and Picea mariana (black spruce)) that bear serotinous cones and require lethal fire to regenerate, and the boreal landscape in North America reflects this, consisting almost entirely of large tracts of pure, even-aged stands of fire-origin species resulting from high-intensity, active crown fires. Alternatively, Eurasian boreal forests are dominated by conifer species not generally considered serotinous. Many Eurasian species have adapted to periodic, lower-intensity surface fires (e.g. thicker basal bark), releasing seed annually and creating a much more heterogeneous, uneven-aged forest. It can be assumed then, that active crown fires are far less common in the Eurasian boreal forest, and this is borne out in the Russian fire literature (e.g. Artsybashev 1967) which shows that crown fires account for ~25% of the total area burned in Russia.

Fuel consumption and spread rates can vary considerably, both within and between boreal fires. In general, however, boreal crown fires consume 20-30 tonnes/ha of fuel (Stocks 1991, Stocks and Kauffman 1997) with roughly 2/3 of this total associated with consumption of forest floor (litter, moss, humus layer) and dead woody surface fuels. Crown fuels (needles and fine twigs) account for the remaining 1/3 of the total fuel consumed. Spread rates can vary between ~5 m/min in intermittent (torching) crown fires and >100 m/min in fully-developed crown fires (Stocks and Kauffman 1997). In a recent comparison of the dynamics of boreal and savanna fires, Stocks et al. 1997 showed that boreal fires consume, on average, an order of magnitude more fuel than savanna fires. Despite similar spread rates, this large difference in fuel consumption means boreal fires develop very high energy release rates, and produce towering convection columns that can reach the upper troposphere and lower stratosphere directly. Conversely, savanna fires usually develop less well-defined convection columns, usually only 3-4 kilometres in height. The differing convection column dynamics of boreal and savanna fires are important in terms of the long-range transport of smoke products from biomass burning. Although much larger areas burn in the savannas annually than in the boreal zone (Crutzen and Andreae 1990), smoke transport mechanisms are likely much different. Regionally-generated savanna fire emissions must be transported vertically at the Inter-tropical Convergence Zone (ITCZ) to have a more global impact, whereas boreal fire emissions are injected at much higher atmospheric heights, promoting the likelihood of wider-ranging transport and impacts.

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