Direct Effects of Fire on the Boreal Forest Carbon Budget

Abstract

Past approaches to estimating the amounts of carbon released during fires in boreal forests have depended on two types of data: 1) those collected during prescribed burns; or 2) those collected from a limited number of points in naturally-occurring fires. Neither of these approaches is felt to produce information that is reliable in terms of describing the conditions that exist in natural fires. In this paper, we present the results of the study of two fires located in the boreal forest of interior Alaska, one during the summer of 1990 and the second during the summer of 1994. In this study, we utilized remote sensing imagery for two purposes. First, pre-fire imagery was used to map the distribution of vegetation/forest cover prior to the fire. A field-sampling campaign was then conducted to estimate the levels of above and below-ground biomass levels associated with each vegetation/forest category. Second, approaches were developed to use satellite imagery from the Landsat Thematic Mapper to estimate three levels of fire severity in the burned forests. Ground data were again collected to estimate the percentage of biomass consumed during fires in each of these classes. By combining the two sets of information, it was possible to estimate the amounts of carbon-based greenhouse gases released during each fire. In the two fires, 25 and 38 t C ha-1 were released, values which are two to three times greater than previously estimated for boreal forest ecosystems. These studies show that most (85%) of the carbon released originates from the burning of biomass in the ground layer (e.g., mosses, litter, and organic soil), which contrasts greatly with biomass burning in other biomes, where almost all the biomass consumed is in above-ground vegetation. To estimate the different species of gases released, we developed a fire behaviour model to divide the fires into two classes: flaming and smouldering. We then used published values to estimate the amount of different greenhouses released during these two fire phases. In a relative sense, much lower levels of carbon dioxide and higher levels of methane and carbon monoxide are released during flaming fires in boreal forests because smouldering fires are responsible for burning ground-layer organic matter.