Estimating Release of Carbon From Forest Fires in Alaska using Satellite Remote Sensing Data

Estimating Release of Carbon From Forest Fires in Alaska
using Satellite Remote Sensing Data

Eric S. Kasischke^1,2, Nancy F.H. French¹, Laura L. Bourgeau-Chavez¹, Susan L. Ustin³, N.L. Christensen, Jr.² ¹Center for Earth Sciences Environmental Research Institute of Michigan P.O. Box 134001 Ann Arbor, MI 48113 4001 ²School of the Environment+ Duke University Box 90328 Durham, NC 27708-0328 ³Department of Land, Air and Water Resources University of California, Davis Davis, CA 95616

INTRODUCTION

While there is little doubt the fossil fuel burning has led to increases in the atmospheric concentration of CO₂, over the past century, analyses show this increase is significantly less than the total amount of CO₂ released into the atmosphere through this burning. Studies have shown that other human practices (land-clearing and biomass burning) have released significant amounts of carbon into the atmosphere (Houghton 1991). In addition, terrestrial and aquatic biomes act as sources and sinks for atmospheric carbon based upon a complex set of biological, chemical and physical processes. To accurately predict future trends in the rise, in atmospheric CO₂ requires developing a better understanding of the terrestrial and oceanic sources and sinks of carbon.

Approximately 4 Pg C yr^-1 (1 Pg = 10¹⁵ grams) are released into the atmosphere during the burning of biomass. While most of this carbon is reaccumulated in plant biomass through vegetation regrowth on the fire-disturbed areas, in tropical regions Houghton (1991) estimates there is a net release of ~0.7 Pg C yr^-1 to the atmosphere due to burning of biomass in cleared forest lands. When considering biomass burning on a global scale, researchers have discounted the role of fire in boreal forests (see, e.g., Seiler and Crutzen, 1980). However, several oversights were made in these earlier studies. First, the total area burned in boreal forests was significantly underestimated, by as much as an order of magnitude. Second, these initial studies focused on the burning of aboveground biomass, ignoring the large amounts of organic matter in the ground layer that are consumed during fires in boreal forests. Studies have indicated that fires in boreal forests could become a significant source of carbon to the atmosphere over the next half century (Kasischke et. al. 1994a).

Several researchers have demonstrated that low resolution (1 to 4 km) satellite imagery from NOAA's AVHRR system can be used to map and locate fires in Russian and Alaskan boreal forests (Cahoon et. al., 1994; Kasischke et. al., 1993; Kasischke and French, 1994; French et. al. 1994). In this paper, we develop a ground-based model of biomass levels and carbon-release during fires in Alaskan boreal forests to estimate the amounts of carbon released during fire in this region for 1990 and 1991.

METHODS

In this study, we used the location and areal extent of fires throughout Alaska in 1990 and 1991 derived from AVHRR data. Previous approaches used to estimate the amount of carbon released during fires in boreal forests assumed: (1) an average level of biomass throughout the areas burned; and (2) a constant percentage of this biomass is combusted (Cahoon et. al., 1994; Seiler and Crutzen, 1980). In this study, more realistic estimates of carbon release were developed based upon an understanding of the distribution of different land cover types within the study area, the biomass characteristics of the forested areas of these sites, and the degree to which this biomass is consumed during fires.

The potential vegetation map of Kuchler (1970) was digitized and entered into a geographic information system to define the boreal forest region of Alaska. Using the approach of Gabriel and Tande (1983), the physiographic divisions defined by Wahrhaftig (1965) were used to create different boreal forest regions. A digitized version of Wahrhaftig's classification was obtained from the U.S. Geological Survey, and imported into the GIS. By merging Wahrhaftig's and Kuchler's data sets, 64 distinct forest regions were created within the boreal zone of Alaska.

Much of the land area within the boreal forest zone of Alaska is not covered by forest (e.g., rivers and streams, lakes and ponds, non-forested peatlands, sub-alpine tundra, alpine tundra, non-vegetated, rocky surfaces, areas cleared for agricultural purposes, and urban areas). 1° x 3° U.S. Geological Survey topographic maps were used to estimate the non-forested area. The percent areal coverage (to within 5%) was estimated for three different land cover categories for each region: non-vegetated, including water,- glacier and rock covered (areas above 2500 m in elevation usually are devoid of vegetation cover in Alaska); vegetated areas above the beeline (above 1000 m); and wetlands. The remaining area was assumed to be covered by forests. Based upon this approach, it was determined that 50.5 % of Alaska's boreal forest zone is covered by forest (28.5 x 10⁶ ha), 18.3% by peatlands (10.3 x 10⁶ ha), 6.8 by alpine and sub-alpine tundra (3.9 x 10⁶ ha), and 24.3% by non-vegetated surfaces (13.7 x 10⁶ ha).

The approach of Seiler and Crutzen (1980) was used to estimate carbon released (C_r) from wildfires in Alaska:

(1)

where B_t. is the total biomass exposed to burning, b is the fraction of that biomass consumed during fire, and c_f is the fraction of the biomass which is released as gaseous or particulate carbon. For boreal forests, biomass in aboveground vegetation as that stored in the ground-layer as dead organic matter must both be considered

To estimate the amounts of carbon released during fires in the Alaskan boreal forests, the following assumptions were made: (1) the boreal forest is comprised of several different forest types each having different rates of biomass accumulation as a function of stand age; (2) fire frequency is a primary determinant of stand age distribution in boreal forests, and determines the average levels of biomass present; and (3) the amount of biomass consumed during fires is dependent on forest type. To develop biomass estimates for the different boreal forest regions, the following assumptions were used

1. The majority of forest succession in Alaska occurs after disturbance by fire;

2. The boreal forests of Alaska are divided into two forest types,: following Van Cleve et al. (1983). The first forest type consists of a single-species chronosequence, where the dominant canopy trees are black spruce or white spruce. The second forest type follows a chronosequence where a disturbed site is first populated by balsam poplar, trembling aspen, or paper birch. These deciduous species are then succeeded by white spruce

3. The percentage of the landscape occupied by these two forest types is constant throughout the state; and

4. The maximum level of biomass accumulation as a function of time after disturbance is constant.

Based on Yarie (1983) we estimate forest type I comprises 60% of Alaska's boreal forest and forest type 11 comprises the remaining 40%. The maximum biomass levels are 6 kg-m^-2 for forest type I (after 150 yrs) and 18 kg-m^-2 for forest type 11 (after 250 yrs) (Yarie 1983; Van Cleve et. al. 1983). For both forest types, after maximum biomass is reached, the forests become overmature and begin to gradually lose biomass because of mortality of overstory trees. Several factors determine the pattern of biomass accumulation in the ground layer. Cold soils and short growing seasons in boreal forests lead to very low rates of decomposition and accumulation of large amounts of dead-organic matter in the ground layer. Because forest type I is found in areas of lower average soil temperatures (including areas underlain by permafrost) than forest type II, the decomposition rates in these sites are lower, leading higher levels of ground-layer biomass. Fire results in a dramatic increase ID both ground-layer temperatures and rates of decomposition (Viereck, 1983), which leads to the net loss of carbon from the ground layer for the first several years after a fire. Finally, not all ground-layer biomass is consumed during a fire. Measurements by Kasischke a al. (1994b) show that 37% of the total ground-layer biomass was consumed during a fire in a site similar to forest type 1, while in a site similar to forest type 11, 70% of the ground layer biomass was consumed during a fire.

The levels of below-ground biomass for the two different forest types were estimated using data from Van Cleve et al. (19833 and Yarie (1983). The levels at a stand age of 200 years after a fire were assumed to be 20 kg-m^-2 for forest type I and 10 kg-m^-2 for forest type 11. Figure 1 presents the patterns of. biomass accumulation for the two forest types.

Yarie (1981) has shown that where fire is the dominant disturbance factor, the stand-age distribution in boreal forests can be determined by the fire frequency (f). Using a Weibel distribution, the stand age distribution, p(t), can be calculated as:

(2)

where t is the stand age in years, G is the gamma function and c is a shape parameter dependent on the flammability of the forest type (ranging between 0.9 and 2.4).

Given p(t) and the pattern of biomass accumulation as function of time [B(t)], the average level of biomass stored [B_a] can be estimated as

(3)

If we assume a shape factor, c = 1.5, Figure 2 presents a plot of average above and ground-layer biomass as a function of the fire return interval.

To determine the fire frequency for each region used in this' study, data on the locations and sizes of all fires in Alaska since 1954 were obtained (Barry, 1993). his information was imported into the GIS data base, and fire frequency was estimated for each region. For those regions where no fires had occurred over the past 38 years, a fire return interval of 500 years was assumed.

Kasischke et al. (1994b) estimated that for forest type I, 35 % of the aboveground biomass and 37% of the ground-layer biomass are consumed during fire. For forest type 11, 15% of the aboveground biomass and 70% of the ground-layer biomass were consumed during fire. These estimates were derived for forests which were ~ 100 to ~ 150 years old. It is likely that the burn efficiency coefficient, b , is dependent on stand age For this study, however, it is assumed that b is constant.

For this study, it has been assumed that c_f= 0.45.

Historical records and observations clearly show that fires in Alaska are not limited to forested regions. Like forested regions, peatland and tundra-covered areas contain thick mats consisting of dead, undecomposed organic matter. When dry, this ground-layer material will burn quite readily. While the occurrence of fires in tundra and peatlands in boreal regions has been documented (Wein, 1976), virtually no information exists on the amounts of biomass consumed during these fires. Kasischke e' al. (1994b) showed that on average, 6 kg m^-2 of biomass were consumed in the ground layer during a fire in a black spruce forest in Alaska. For this study, it is assumed that on average, 4 kg m^-2 of biomass are consumed in the ground layers of the peatlands and tundra areas in Alaska based on the fact that the soils of areas containing these cover types have higher moisture levels and are less susceptible to burning.

RESULTS

If we use the assumption made by previous researchers that only 2.5 kg-m^-2 of biomass are consumed during fires in boreal' forests (1.125 kg-m^-2 of C), then in 1990/1991 .0014 Pg C was released during fires in Alaskan boreal forests using the AVHRR estimate of fire area or .0022 Pg C using the Alaska Fire Service (AFS) estimate of total area burned). Using the model developed in this study, an average-of 1.77 kg-m^-2 of carbon was released) during fires in Alaskan boreal forests, including those areas where no forests or vegetation were present. Using this statewide, estimate results in a total release of .0023 and .0035 Pg C using the AVHRR and AFS area burned estimates, respectively, Finally, if we use the regional estimates of carbon released during fires developed in this study along with the actual area burned in each area, a total of .0033 and .0054 Pg C were released during fire using the AVHRR and AFS area estimates, respectively. Using this model, an average of 2.46 and 2.72 kg-m^-2 of carbon were released during the fires in Alaska during 1990/1991 (for the AVHRR and AFS fire locations, respectively).

DISCUSSION

Using the biomass accumulation and biomass burning models developed in this study resulted in significantly higher estimates of carbon release in Alaskan boreal forests than models used in other studies (Cahoon et al. 1994). The reason for this higher estimate is that our model used a much higher level of biomass burning than used other models. Note, however, that the regional approach resulted in much higher levels of biomass burning than if a single estimate were used for the entire state. The reason for this is that fires tend to burn in areas where fuel exists. The regions where fires occur had a higher percentage of forested/ vegetated surfaces than the average for the entire state as a whole.

Biomass burning in boreal forests is likely to increase in response to future global warming. To estimate the amounts of greenhouse gases released during this fires will require accurate estimates of the locations and extent of this burns, as well as regional models of biomass burning for the forests found in the various regions.

REFERENCES

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1998, Center for Spatial Technologies and Remote Sensing (CSTARS)
University of California, Davis