Ecosystem carbon emissions from 2015 forest fires in interior Alaska
© The Author(s) 2018
Received: 19 October 2017
Accepted: 22 December 2017
Published: 8 January 2018
In the summer of 2015, hundreds of wildfires burned across the state of Alaska, and consumed more than 1.6 million ha of boreal forest and wetlands in the Yukon–Koyukuk region. Mapping of 113 large wildfires using Landsat satellite images from before and after 2015 indicated that nearly 60% of this area was burned at moderate-to-high severity levels. Field measurements near the town of Tanana on the Yukon River were carried out in July of 2017 in both unburned and 2015 burned forested areas (nearly adjacent to one-another) to visually verify locations of different Landsat burn severity classes (low, moderate, or high; LBS, MBS, HBS).
Field measurements indicated that the loss of surface organic layers in boreal ecosystem fires is a major factor determining post-fire soil temperature changes, depth of thawing, and carbon losses from the mineral topsoil layer. Measurements in forest sites showed that soil temperature profiles to 30 cm depth at burned forest sites were higher by an average of 8–10 °C compared to unburned forest sites. Sampling and laboratory analysis indicated a 65% reduction in soil carbon content and a 58% reduction in soil nitrogen content in severely burned sample sites compared to soil mineral samples from nearby unburned spruce forests.
Combined with nearly unprecedented forest areas severely burned in the Interior region of Alaska in 2015, total ecosystem fire-related losses of carbon to the atmosphere exceeded most previous estimates for the state, owing mainly to inclusion of potential “mass wasting” and decomposition in the mineral soil carbon layer in the 2 years following these forest fires.
The 2015 fire season in Alaska resulted in the second highest acreage burned for the state in a single year. In mid-June 2015, nearly 300 fire starts were reported within 1 week, a consequence of over 61,000 detected lightning strikes during the period . As of mid-September, a total of 2.1 million ha (5 million acres) had burned statewide in over 700 separate wildfires. A relatively low snowpack across southern Alaska, compounded by a warm, dry spring, resulted in extremely burnable fuels . Following one of the wettest summers on record in 2014, Alaska’s intense fire season of 2015 was extreme by most historical standards.
Over the past 50 years, there has been an increase in the frequency and severity of boreal forest wildfires in Alaska . During the 2000s, an average of 767,000 ha per year were burned statewide, 50% higher than in any previous decade since the 1940s. Deeper burning of surface organic layers in black spruce (Picea mariana (Mill.) BSP) forests has occurred during late growing-season fires and on more well-drained sites .
Previous estimates of regional carbon emissions from forest fires in Interior Alaska
The objectives of this study were to (1) conduct field validation and statistical comparisons of the burned index rankings of 2015 wildfire areas near Tanana, Alaska to Landsat burn severity classes mapped (post-fires) in 2015 and 2016, and (2) estimate total ecosystem (live biomass and mineral topsoil) carbon emissions from the 2015 wildfires across the Yukon–Koyukuk forest region. This work was undertaken as a contribution to the NASA Arctic Boreal Vulnerability Experiment (ABoVE) field campaign, chiefly to better understand changes in related hydrologic and biogeochemical mechanisms in the years following boreal forest wildfires. One of the major questions being addressed by ABoVE is “What processes are controlling changes in boreal-arctic land cover properties and what are the impacts of these changes?”.
Forests in the study area are predominately black spruce on wetter soils and white spruce (Picea glauca) on drier soils, described by  as follows: Open black spruce forest description—Total arboreal cover is between 25 and 60%. Paper birch (Betula papyrifera) may be present in small amounts. The trees tend to be small; the largest trees are about 5–10 cm in diameter and 6–10 m tall. A well-developed tall shrub layer dominated by dwarf birch (Betula glandulosa) 1–2 m high often is present. Other tall shrubs locally important on moist sites include Alnus crispa, A. sinuata, Salix spp., and Rosa acicularis. A low shrub layer usually is present and consists primarily of some combination of Vaccinium uliginosum, V. vitis-idaea, Potentilla fruticosa, Arctostaphylos rubra, Empetrum nigrum, and Ledum spp. The moss layer is continuous or nearly so and dominated by a combination of Hylocomium splendens, Pleurozium schreberi, Polytrichum spp., and Dicranum spp. Lichens such as Cladonia spp. are important on some sites.
Closed white spruce forest description—the closed white spruce forest type represents the best developed, most productive forests in Alaska. The over-story canopy cover, usually entirely white spruce but occasionally with either scattered paper birch or balsam poplar (Populus balsamifera) can range from 60 to 100%. On the best sites, trees reach 30 m in height. A well-developed moss layer consisting primarily of the feathermosses Hylocomium splendens, Pleurozium schreberi, and less commonly, Rhytidialdelphus triquetrus is characteristic of these stands. Herbaceous growth is usually sparse but horsetails, primarily Equisetum sylvaticum and E. arvense, may provide as much as 50% cover in flood-plain stands. Other forbs include Pyrola spp., Linnaea borealis, Geocaulon lividum, Mertensia paniculata, and Goodyera repens.
The Soil Survey for the Upper Tanana Area (USDA, 1999) described the soil types most representative of our study sites, namely Goldstream peat on 0–3% slopes, alluvial plains, and moraines. These soils are further characterized in this survey as having an organic surface mat 20–40 cm thick, on top of a dark gray silt loam 15–30 cm deep. These soils are very poorly drained, with permafrost as the root-restricting feature at 25–50 cm depth.
Landsat burn severity classes
Digital maps of burn severity classes at 30-m spatial resolution for wildfires in 2015 across the Yukon–Koyukuk region of Alaska were obtained from the Monitoring Trends in Burn Severity (MTBS) project, which has consistently mapped fires greater than 1000 acres across the United States from 1984 to the present . MTBS is conducted through a partnership between the U.S. Geological Survey (USGS) National Center for Earth Resources Observation and Science (EROS) and the USDA Forest Service.
The normalized burn ratio (NBR) index was first calculated using approximately one-year pre-fire and post-fire images from the near infrared (NIR) and shortwave infrared (SWIR) bands of the Landsat sensors.
Burn index estimation
Soil measurements and sampling
At each sampling site near Tanana, the surface organic layer was excavated in July 2017 to create 30 cm depth soil pits. True color (RGB) and thermal infra-red (TIR) images of all excavated soil pits were collected using a FLIR Series C2 hand-held camera (with an object range of – 10 to 150 °C), recording 320 × 240 pixels per image. All TIR image data was collected over short time window (mid-day hours of 10 a.m. to 2 p.m.) on 5 consecutive days in July 2017 during which air temperature was highly constant and no rainfall events occurred.
At least 500 g of mineral soil sample was collected, starting at 10 cm depth (below the bottom level of the surface organic layer) down to 30 cm mineral soil depth from each pit, sealed in ziplock plastic bags, and shipped to the Oregon State University Crop and Soil Science Central Analytical Laboratory for analysis of carbon and nitrogen content by the total elemental combustion technique. A total of 19 unburned and 19 burned forest soils were excavated to a depth of 30 cm in the soil pits and sampled in this manner.
To verify soil pit TIR imaging patterns with depth, soil temperature was measured using a ThermCo digital thermometer with a 7-cm stainless steel probe inserted into the organic layer ground cover, and at 10 and 30 cm soil depths.
Linear least squares regression was used to test for significant correlation relationships between burn severity attributes. Tests of statistical significance between unburned and burned site attributes were carried out using the two-sample Kolmogorov–Smirnov (K–S) test, a nonparametric method that compares the cumulative distributions of two data sets . The K–S difference test does not assume that data were sampled from Gaussian distributions (nor any other defined distributions), nor can its results be affected by changing data ranks or by numerical (e.g., logarithm) transformations. The K–S test reports the maximum difference between the two cumulative distributions, and calculates a probability (p) value from that difference and the group sample sizes. It tests the null hypothesis that both groups were sampled from populations with identical distributions according to different medians, variances, or outliers. If the K–S p value is small (i.e., < 0.05), it can be concluded that the two groups were sampled from populations with significantly different distributions.
CBI versus RdNBR
Plant growth in HBS areas
At all sites recorded with a CBI value greater than 2.0, there was no observed regrowth in July 2017 of any shrub or tree species that was observed growing in any the unburned spruce forest sites (CBI = 0), as listed in the study area description above. At all HBS locations we surveyed, the substrate layer was comprised of entirely dead (charred blackened) moss and lichen cover. Occasional hummocks 50 cm deep (or deeper) and several meters in length of dead moss layer were encountered in transect crossings of these HBS areas. The low vegetation stratum (< 1-m tall) at all HBS areas visited was comprised of relatively sparse coverage of fireweed (Chamaenerion angustifolium), horsetails, and mixed grasses. Ground cover plant species commonly seen in unburned forest locations, but not seen regrowing in HBS locations in 2017, were bog blueberry (Vaccinium uliginosum) and highbush cranberry (Vibernum edule).
Differences in surface organic layer thickness and temperature
These TIR imaging differences were confirmed by soil probe measurements, which showed that mean soil temperatures recorded at 10 cm depth were significantly greater (p < 0.001) in burned forest sites (CBI > 2, n = 19) at 8.1 °C compared to unburned sites (CBI = 0, n = 19) with a mean value of 3.0 °C. Furthermore, mean soil temperatures recorded at 30 cm depth were significantly greater (p < 0.001) in burned forest sites (CBI > 2) at 6.5 °C compared to unburned sites (CBI = 0) with a mean value of 0 °C.
Differences in mineral soil carbon and nitrogen
Surface mineral soil carbon and nitrogen content from unburned (CBI = 0) and severely burned (CBI > 2) forest sites in 2015 near Tanana Alaska
C (% sample dry weight)
N (% sample dry weight)
Mean CBI = 0
Mean CBI > 2
2SE CBI = 0
2SE CBI > 2
Min CBI = 0
Min CBI > 2
Max CBI = 0
Max CBI > 2
K–S test p
Live moss, surface organic layer, and soil carbon content (to 30 cm depth) estimated for unburned and severely burned (since 2015) forests near Tanana, based on previous bulk density measurements from forest sites across Alaska and percent mineral soil carbon changes from Table 1. Bulk density (g cm−3)
Kg C m−2 unburned
Kg C m−2 burned
Kg C m−2 difference
Landsat burn severity areas for 2015
List of wildfires (greater than 10,000 ha) in the Yukon–Koyukuk region of Alaska in 2015
Middle Yukon Fires
Tanana Area Fires
Ramparts to Ruby
Big Mud River 1
North Fork Kuskokwim River
Lower Innoko River
Lower Tanana River
Ramparts to Ruby
Ramparts to Ruby
North Fork Kuskokwim River
Lower Innoko River
Lower Tanana River
Lower Reindeer Peak
Lower Innoko River
Anvik to Pilot Station
Lower Tanana River
Upper Innoko River
Birch Creek 2
North Fork Kuskokwim River
Sum for all 113 fires
Regional carbon losses from 2015 wildfires
Based on the total MBS and HBS forest areas consumed in 2015 across the Yukon–Koyukuk region (from Table 3), plus the organic layer carbon fractions consumed in MBS and HBS areas from , and the estimated loss of carbon since the 2015 wildfires from the mineral soil layer and the live moss and surface organic layers (from Table 2), it was determined that 154 Tg C were lost following the wildfires in interior Alaska in 2015. Mineral soil losses (and surface organic layer carbon emission totals from combustion) did not include the emission from combustion of aboveground forest biomass, which, based on average area-based carbon losses reported by Tan et al.  of 2.23 kg C m−2, would have added 8.7 Tg C in Alaska wildfire emissions in 2015.
The exceptionally warm and dry conditions leading up to the summer of 2015 were followed by the largest wildfires recorded in decades in interior Alaska. Our estimate of the depth to which MBS and HBS wildfires had burned into and completely consumed surface organic moss layers during the 2015 Tanana fires was between 5 and 10 cm. This burn depth estimate was confirmed using the relationship reported by Harden et al. , that for every centimeter of organic mat thickness in boreal forests, soil temperature under the organic layer remained about 0.5 °C cooler during summer months. The difference (increase) we measured in average temperature at 10 cm soil depth between severely burned and unburned sites was 5 °C, which, according to Harden et al. , would imply a loss of 10 cm in the organic moss layer thickness in severely burned (CBI > 2) forest areas.
In severely burned forest sites, the complete consumption of the living moss organic layer was strongly associated with warming at the soil surface layer. Measurements showed that soil temperature to 30 cm depth was higher by 8–10 °C compared to unburned forest sites. Below 15 cm soil depth, the temperature of unburned sites dropped gradually to sub-zero (°C) levels by 30 cm depth, while soil temperatures at burned sites remained above 5 °C to 30 cm depth. Our results were similar to those reported by Nossov et al.  for fire impacts on forested areas of Yukon Flats and the Yukon-Tanana Uplands—these burns caused a fivefold decrease in surface organic layer thickness, a doubling of water storage in the soil active layer, a doubling of thaw depth, and an increase in soil temperature at the surface (to + 2.1 °C) and at 1 m depth (to + 0.4 °C).
Nearly all of the HBS sites measured during our 2017 field surveys of the Tanana Area Fires had no live surface organic layers remaining. Intense fires during summer of 2015 consumed between 5 and 10 cm of the former live surface organic layer and left behind only a residual dead, charred moss and lichen cover about 3–5 cm deep that had little capacity to insulate the soil layers beneath. We observed that the blackened surface organic layer showed a tendency to be 2–4 °C warmer than the live moss layer under unburned spruce forest strata. These results are consistent with those of Jiang et al.  and Brown et al. , who reported that post-fire thickness of the soil organic layer and its impact on soil thermal conductivity was the most important factor determining post-fire soil temperatures and thaw depth.
Previous estimates of carbon emissions from forest fires in Alaska
Kg C m−2
Tan et al. 
Mack et al. 
Kane and Vogel 
Surface organic layer
Tan et al. 
Turetsky et al. 
French et al. 
Troth et al. 
Kane and Vogel 
Kane and Vogel a
Additional post-fire losses of between 10 and 15 kg C m−2 estimated in our study from thawed mineral soil pools appear to be roughly equivalent to the combined carbon emissions from burned aboveground biomass, live ground cover, and surface organic layers. This potential “mass wasting” and decomposition of the mineral layer (between 10 and 30 cm depth) soil carbon in severely burned areas of the Alaska interior could have occurred at any time between the end of the 2015 fires and the sampling period for this study of July 2017. The soil carbon losses measured in this study were not necessarily emitted during the short 2015 burn period, but instead were likely a consequence of the severe burn conditions that affected these soils following the direct fire emissions of carbon from the nearly complete combustion of aboveground (tree) biomass and in surface organic layers.
When wildfire areas have an overall percentage of MBS plus HBS areas higher than 60%, as in 2015 for Interior Alaska, vast tracts of forest will be burned deeply into the surface organic layer. This sudden thinning or removal of the moss and soil organic layer will raise post-fire soil temperatures and increase thaw depths, leading to large losses of carbon and nitrogen from mineral soils layers that are much wetter and warmer than the unburned forests nearby. Our results from remote sensing and field measurements in unburned and nearby burned forest sites around Tanana were higher by an average of 8–10 °C compared to unburned forest sites. Combined with nearly unprecedented forest areas severely burned in the Yukon–Koyukuk region of Alaska in 2015, updated total ecosystem fire-related losses of carbon to the atmosphere exceeded most previous estimates for the state by a factor of two, due mainly to the inclusion of potential “mass wasting or decomposition” of mineral soil carbon in the 2 years following these forest fires.
This work was supported by NASA Ames Research Center and the NASA ABoVE Logistics Office in Fairbanks, Alaska. Special thanks to Charles Hugny, Sarah Sackett, Cynthia Erickson, Will Putman and the Tanana Chiefs Conference, Gerald Nicholia and Shannon Erhart of the Tanana Tribal Council, all for assistance in access to field sites.
The author declares that he has no competing interests.
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