Fire, Global Warming, and the Carbon Balance of Boreal Forests

Abstract

Fire strongly influences carbon cycling and storage in boreal forests. In the near‐term, if global warming occurs, the frequency and intensity of fires in boreal forests are likely to increase significantly. A sensitivity analysis on the relationship between fire and carbon storage in the living‐biomass and ground‐layer compartments of boreal forests was performed to determine how the carbon stocks would be expected to change as a result of global warming. A model was developed to study this sensitivity. The model shows if the annual area burned in boreal forests increases by 50%, as predicted by some studies, then the amount of carbon stored in the ground layer would decrease between 3.5 and 5.6 kg/m2, and the amount of carbon stored in the living biomass would increase by 1.2 kg/m2. There would be a net loss of carbon in boreal forests between 2.3 and 4.4 kg/m2, or 27.1‐51.9 Pg on a global scale. Because the carbon in the ground layer is lost more quickly than carbon is accumulated in living biomass, this could lead to a short‐term release of carbon over the next 50‐100 yr at a rate of 0.33‐0.8 Pg/yr, dependent on the distribution of carbon between organic and mineral soil in the ground layer (which is presently not well‐understood) and the increase in fire frequency caused by global warming.

Soil Carbon stocks and their rates of accumulation and loss in a boreal forest landscape

“…Rates of simulated surface layer accumulation rates are 46 g C m-2yr -• in the scenario where the entire site burned 13 years ago and decrease to 14 g C m-2yr -1 at 120 years after fire (Table 5). Using a uniform stand age of 120 years, the total calculated The fire return interval for Alaskan boreal forests has been estimated to be 150 years [Kasischke et al, 1995], with a range in various North American boreal forest types of 70-500 years [Payette, 1993]. If the fire cycle were about 100 years, then our carbon budget for a year with no fire might suggest that the soil C stock may be roughly at steady state over large areas and long timescales.…”

A second-order impact model for forest fire regimes

“…Fires in savannas are almost periodic surface fires with return times ranging from 1-2 yr in moist areas (Goldammer (1983)) to 5-10 yr in arid areas (Rutherford (1981)). Fires in northern boreal forests are also quite regular, but they prevalently involve crowns (Kasischke et al (1995)) and occur every 50-200yr (Rowe and Scotter (1973);Zackrisson (1977); Engelmark (1984); Payette (1989)). By contrast, in Mediterranean areas, mixed (crown and surface) fires are almost the rule and occur in an apparently random fashion, with highly variable return times (Kruger (1983); Davis and Burrows (1994)).…”

Emissions from smoldering combustion of biomass measured by open‐path Fourier transform infrared spectroscopy

“…Both wildfires and land-management activities expose belowground biomass to fire, and it may account for a significant fraction of the fuel consumption in fires in the boreal forest [Hungerford, 1995] or in tropical forests such as the kerangas or caatinga [dacobs, 1988]. It has been projected that much more of this carbon may bum in global warming scenarios [Kasischke et al, 1995;Keane et al, 1997]. Smoldering combustion should account for essentially all fuel consumption in burning organic soils.…”