Christopher Potter · Charles Hugny

Abstract In the summer of 2015, hundreds of forest fires burned across the state of Alaska. Several uncontrolled wildfires near the town of Tanana on the Yukon River were responsible for the largest portion of the area burned statewide. In July 2017, field measurements were carried out in both unburned and burned forested areas nearly adjacent to one another, all within 15 miles of the village of Tanana.These surveys were used to first visually verify locations of different burn severity classes, (low, moderate, or high),estimated in 2016 from Landsat images (collected before and after the 2015 Tanana-area wildfires). Surface and soil profile measurements to 30-cm depth at these same locations were collected for evidence of moss layer and forest biomass burning. Soil temperature and moisture content were measured to 30-cm depth, and depth to permafrost was estimated by excavation wherever necessary. Digital thermal infra-red images of the soil profiles were taken at each site location, and root-zone organic layer samples were extracted for further chemical analysis. Results supported the hypothesis that the loss of surface organic layers is a major factor determining post-fire soil water and temperature changes and the depth of permafrost thawing.In the most severely burned forest sites, complete consumption of the living moss organic layer was strongly associated with both warming at the surface layer and increases in soil water content, relative to unburned forest sites. Soil temperatures at both 10-cm and 30-cm depths at burned forest sites were higher by 8-10 °C compared to unburned sites. Below 15 cm, temperatures of unburned sites dropped gradually to frozen levels by 30 cm, while soil temperatures at burned sites remained above 5 °C to 30-cm depth. The water content measured at 3 cm at burned sites was commonly in excess of 30% by volume,compared to unburned sites. The strong correlation between burn index values and depth to permafrost measured across all sites sampled in July 2017 showed that the new ice-free profile in severely burned forest areas was commonly 50-cm deeper than in unburned soils.

Keywords Alaska · Landsat · Permafrost · RdNBR · Soil temperature · Soil water · Wildfire

Introduction

Over the past 50 years, studies from the ground and satellites have documented an increase in the frequency and severity of boreal forest wildfires in Alaska (Kasischke and Turetsky 2006). During the 2000s, an average of 767,000 ha per year were burned statewide, 50% higher than in any previous decade since the 1940s (Kasischke et al. 2010). Burning deep into surface organic layers in black spruce (Picea mariana (Mill.) BSP) forests has been reported during late growing-season fires and on more well-drained sites (Turetsky et al. 2011).

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 (AICC 2015). As of mid-September, 2.1 million ha (5.1 million acres) had burned statewide in over 700 different wildfires.

Although 2015 was second only to the number of hectares burned in 2004 in Alaska, most of the fire activity was compressed into a much shorter window, with approximately 40% of the season’s fire starts during a single week in June (USFWS 2015). A relatively depleted snow cover across southern Alaska, compounded by a dry, warm spring, caused biomass fuels to be extremely burnable when lightning struck during the long days surrounding the summer solstice (AICC 2015). Following one of the wettest summers on record in 2014, Alaska’s intense fire season of 2015 was extreme by recent historical standards.

Of the hundreds of forest fires that raged in Alaska in 2015, those near the town of Tanana on the Yukon River were responsible for the largest portion of the area burned statewide. The Tanana fires charred at least 200,720 ha(496,000 acres) (InciWeb 2015), an area half the size of Rhode Island. With fires approaching from three sides,many of Tanana’s 300 residents were evacuated in June 2015 (Alaska Public Media 2015). Some were forced to flee by boat because the airstrip was blanketed with smoke according to local news reports (Fairbanks Daily News-Miner 2015).

The objectives of this study were to: (1) conduct field validation and statistical comparisons of the burned index rankings of the 2015 wildfire areas near Tanana to Landsat burn severity classes mapped (post-fires) in 2015 and 2016;and, (2) quantify the changes in soil temperature, depth to permafrost, soil water content, and soil biogeochemistry of forested ecosystems in Interior Alaska from high burn severity (HBS) damage. This work was undertaken as a part of the NASA Arctic Boreal Vulnerability Experiment(ABoVE) field campaign, to better understand changes in related hydrologic and biogeochemical mechanisms in the years following boreal forest wildfires. One of the major research questions being addressed by ABoVE is ‘‘What processes are controlling changes in the distribution and properties of permafrost and what are the impacts of these changes?’’

At a global level, Abbott et al. (2016) reviewed approximately 100 studies from around the pan-permafrost regions of the world and concluded that water balance,vegetation distribution, and permafrost degradation are the most important sources of uncertainty in predicting the timing and magnitude of climate warming feedbacks to the atmosphere and to the carbon cycle. For instance, wildfires or drought have been documented to trigger a transition from coniferous to deciduous tree species dominance,while warming permafrost layers up to 7 °C due to loss of insulating moss and associated surface temperature changes.

Previously published field studies in northern latitudes have documented various attributes of organic soil layers,soil moisture, and the depth to permafrost in coniferous ecosystems, both before and after stand-replacing wildfires.Notably, mature black spruce forests have a thick organic soil layer (10-40 cm in depth) that may insulate the subsoil from warming summer temperatures and facilitate the development of permafrost below the surface (Viereck 1983). Kasischke and Johnstone (2005) reported that soil moisture in forested sites with permafrost was proportional to the depth of the surface organic layer, such that mature sites with deeper organic layers have colder soils, shallower thawed (active) layers, and wetter soils. Harden et al.(2006) found that for every centimeter of organic mat thickness, temperatures at the 5-cm depth stayed about 0.5 °C cooler during the summer months. These researchers speculated that, once permafrost has formed, a positive feedback between cold temperatures, poor drainage, and the chemical recalcitrance of moss layers enhances the preservation of thick organic soil mats by resisting decomposition and combustion.

Most wildfires in the black spruce forests of Alaska have been either crown or ground fires with high enough severity to kill some or all over-story trees (Johnstone et al.2010). Usually some of the organic layer of the forest floor remains after the event, but fires in late summer following extremely dry or windy conditions may consume all of the organic layer, and can expose mineral soil (Viereck 1983).Jiang et al. (2015) and Brown et al. (2016) reported that post-fire thickness of the organic layer and its impact on soil thermal conductivity was a prime factor determining post-fire soil temperatures and ice thaw depth. In burned forest sites, the presence of permafrost can inhibit the loss of the insulating soil organic layer, lower soil drying, and promote surface water pooling.

Materials and methods

Study area

The study area was the boreal forest in various states of low to high burn severity from wildfires surrounding the confluence of the Yukon and Tanana Rivers (near 65°8′N latitude, 152°27′W longitude), about 200 km west of Fairbanks, Alaska. Mean annual temperatures over much of Interior Alaska are well below freezing, which accounts for a permafrost that is commonly continuous, except in the southern portion of the region (Brown 1970; Pastick et al.2015). The climate near Tanana is characterized by mean annual temperature variations between - 27 °C and 22 °C,and a mean annual precipitation total of 29 cm, 11 cm of which falls as snow (www.usclimatedata.com).

Forests in the study area are predominately black spruce on wetter soils and white spruce (Picea glauca Moench Voss) on drier soils, described by Viereck et al. (1992) as follows:

Open black spruce forest description: Total arboreal cover is between 25 and 60%. Paper birch (Betula papyrifera Marsh.) may be present in small numbers. The trees tend to be small; the largest are about 5-10 cm in diameter and 6-10 m tall. A well-developed tall shrub layer, dominated by dwarf birch (Betula glandulosa Michx.) 1-2 m high is often 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 L., Potentilla fruticosa, Arctostaphylos rubra (Rehder and Wilson) Fernald, Empetrum nigrum L. and Ledum spp. The moss layer is continuous or nearly so and dominated by a combination of Hylocomium splendens (Hedw.) Schimp., Pleurozium schreberi, Polytrichum spp., and Dicranum spp. Lichens such as Cladonia spp. are important on some sites.

Closed white spruce forest description: this forest type represents the best developed, most productive forests in Alaska. The over-story canopy cover, usually entirely white spruce but occasionally with scattered paper birch or balsam poplar (Populus balsamifera L.) 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 floodplain stands. Other forbs include Pyrola spp., Linnaea borealis, Geocaulon lividum, Mertensia paniculata, and Goodyera repens (L.) R. Br.

The Soil Survey for the Upper Tanana Area (USDA 1999) describes 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 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 rootrestricting feature at 25-50 cm depth.

Fig. 1 Study area at the confluence of the Yukon and Tanana Rivers(near 0) indicate barren land cover whereas high values of NDVI in boreal Alaska a pre-fire (2013) Landsat normalized difference(near 1.0) indicate dense canopy vegetation cover; b Landsat vegetation index (NDVI) = (NIR - Red)/(NIR + Red) where nearnormalized burn ratio (RdNBR) for 2015 fires based on pre-fire infra-red (NIR) is the reflectance of wavelengths from 0.76 to 0.9 μmimagery from July 2014 and post-fire imagery from September 2015 and Red is the reflectance from 0.63 to 0.69 μm. Low values of NDVI

According to Monitoring Trends in Burn Severity mapping (mtbs.gov; Eidenshink et al. 2007), the Spicer Creek Fire of 2015 burned 71,590 ha (176,904 acres) near Tanana, with 48% consumed at high burn severity (HBS)and another 26% at moderate burn severity (MBS) (Fig. 1).The Bering Creek Fire of 2015 burned 30,874 ha (76,292 acres), with 34% consumed at HBS and another 34% at MBS. By August 1 2015, the main Spicer Creek Fire west of Bear Creek showed few remaining hot spots (InciWeb 2015). The only remaining hot spots showing up on the infrared camera in mid-August were along the north end of the former Distant Early Warning Line radar Site Road.

Burn index estimation

In July 2017, we surveyed burned areas and adjacent unburned forest stands along and within the boundaries of the Spicer Creek Fire and the Blind River-Bering Creek Fires on either bank of the Yukon River near Tanana(Fig. 2). Following the Composite Burn Index (CBI) protocol from Key and Benson (2006), as customized for forests of Alaska (FETG 2007), we made ocular estimates at each soil sampling site of the degree of change caused by the 2015 wildfire within five forest strata: (1) substrate layer, (2) vegetation less than 1-m, (3) tall shrubs/saplings 1-2 m, (4) intermediate trees 2-8 m, and (5) large trees ＞8 m. Within each stratum, four to five variables were scored to generate a CBI ranking between 0 and 3 for the level of burn severity. All live and dead plant species were noted and photographed at each forest site.

Fig. 2 Measurement sites for Composite Burn Index (CBI) estimation and soil attributes within the Spicer Creek Fire’s Landsat normalized burn ratio (RdNBR) classes (burn severity scale same as in Fig. 1b)

Landsat burn severity classes

Digital maps of burn severity classes at 30-m spatial resolution for the 2015 Spicer and Bering Creek Fires were obtained from the MTBS project, which has consistently mapped fires greater than 1000 acres across the United States from 1984 to the present (Eidenshink et al. 2007).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 1-year pre-fire and post-fire images from the NIR and SWIR bands of the Landsat sensors:

Pre- and post-fire NBR images were next differenced for each Landsat scene pair to generate the RdNBR.

RdNBR severity classes of low, moderate, and high potentially cover a range of - 500 to + 1200 over burned land surfaces. Positive RdNBR values represent a decrease in vegetation cover and a higher burn severity, while negative values would represent an increase in live vegetation cover following the fire event.

Soil measurements and sampling

At each sampling site, organic layer and soil pits were excavated to a 30-cm depth and soil temperatures were measured using a ThermCo digital thermometer with a 7-cm stainless steel probe (with an accuracy of ± 1 °C)inserted into the organic layer ground cover, and at 10-cm and 30-cm mineral soil depths. We consistently measured every soil site (unburned and burned sites, N = 19 plots for both), starting at a 10-cm depth (below the surface organic layer) down to 30-cm mineral soil depth. There was no variation in the relative position in soil sampling depth among the all the soil pits excavated.

Percent volumetric water content (VWC) was measured in the surface organic layer and at the same two depths (10-and 30-cm) using a FieldScout 300 Soil Moisture Meter(Spectrum Technologies, Inc.) with 3.8-cm rod probes. The FieldScout 300 is based on time-domain reflectometry(TDR) and has an accuracy of approximately ± 3.0%VWC. The underlying principal of TDR involves measuring the travel time of an electromagnetic wave along a waveguide, in this case composed of the two stainless steel rod probes. The speed of the wave in soil is dependent on the bulk dielectric permittivity of the soil matrix. It should be noted that calibrations for soil VWC sensors are not as accurate when inserted through organic surface layers such as live moss, lichen, or dead fibric layers.

Depth to permafrost was measured at each of the four corners of the square meter sampling area around the soil pit center using a 1-m stainless steel rod (1-cm diameter)inserted at the bottom of each soil pit. 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 °C to 150 °C),recording 320 × 240 pixels per image with a spectral range of 7.5-14 μm and accuracy of ± 2 °C. All TIR image data were collected over a short- time window (midday hours of 10 am to 2 pm) on five consecutive days in July 2017, during which air temperatures were highly constant and no rainfall events occurred.

Statistical analysis

Linear least squares regression was used to test for significant relationships between burn severity and soil attributes, such as depth to permafrost, temperature, and moisture content. 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 (Lehmann 2006). 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.

Results

CBI versus RdNBR

Surveys across a total of 48 unburned and burned (in 2015)forest sites showed that the CBI was significantly correlated (at p ＜0.01; R2regression correlation = 0.85) with the Landsat RdNBR from both 2015 and 2016 post-fire images (Fig. 3a). A CBI value of 3.0, indicating complete consumption of all pre-fire forest (strata) biomass during the 2015 fires, corresponded to a Landsat RdNBR value of about 1000 and the most extreme HBS post-fire conditions.

Plant growth in HBS areas

At all sites recorded with a CBI value greater than 2.0,there were no observed regrowth in July 2017 of any shrub or tree species that was observed growing in 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 dead (charred) moss and lichen cover (Fig. 4). 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 (less than 1-m tall) at all HBS areas visited was comprised of relatively sparse coverage of fireweed (Chamaenerion angustifolium (L.) Scop),horsetails, and mixed grasses.

Fig. 3 Correlations of the Landsat Normalized Burn ratio (RdNBR) with a Composite Burn Index (CBI) estimates and b measured depth to permafrost (DPF) for forest sites surveyed near Tanana in July 2017

Fig. 4 Photos from July 2017 field surveys. Clockwise from uppersurface organic layer of High burn severity (HBS) site with soil probe left: Unburned forest site; burned forest landscape of the Spicer Creekhandle; surface organic layer of unburned forest site with 1-m long Fire; High burn severity (HBS) forest site; eroded roadside on thesoil probe margin of the Spicer Creek Fire; soil pit to 30-cm depth; charred

Ground cover species comparison

Visual identification of all plant species growing in the ground cover layer of unburned forest locations surveyed in 2017 resulted in a list of 26 common plants (Table 1).Comparison of plant species present in all HBS locations surveyed in 2017 showed that only 12 species were regrowing from the mostly charred moss layers, commonly fireweed, (horsetails Equisetum spp.), and liverworts, along with annual grasses (not identified to species level).Ground cover species most commonly seen in unburned forest locations, but not seen re-growing in HBS locations in 2017, were bog blueberry (Vaccinium uliginosum L.)and highbush cranberry (Vibernum edule (Michx.) Raf.).

RdNBR versus depth to permafrost

Measured depth to permafrost was significantly correlated (at p ＜0.01; R2regression correlation = 0.7) with Landsat RdNBR from post-fire images (Fig. 3b). At Landsat RdNBR values at or above 1000, representing HBS with complete consumption of all pre-fire forest biomass during the 2015 fires, permafrost was commonly measured at between 80- and 100-cm depths from the burned soil surface. At unburned forest sites, where the RdNBR values ranged between 0 and 300, permafrost was always measured at a depth of between 30 and 50 cm from the top of the organic surface layer.

RdNBR versus soil temperature and moisture content

Three measured moisture and temperature attributes, namely surface organic layer moisture (% VWC) and soil temperatures at 10-cm and 30-cm depth, were each significantly correlated (at p ＜0.05; R2regression correlation ＞0.3)with the Landsat RdNBR values (Fig. 5). At Landsat RdNBR values greater than 1000 representing complete forest biomass consumption, surface organic layer moisture was commonly measured at greater than 30% VWC and soil temperatures were between 3 and 10 °C; whereas for unburned forest sites, moisture levels of organic surface layers were generally lower than 10% VWC and soil temperatures were commonly lower than 2 °C. However, soil temperatures at 10-cm depth were more variable than at 30-cm depth for the relatively unburned forest sites.

Changes in soil temperature and moisture content

For both unburned (CBI = 0) and severely burned(CBI ＞2) forest sites, 19 soil measurement data sets werecompared for differences in depth to permafrost, temperature, and moisture content (Table 2). According to K-S test results, unburned sites had significantly shallower depth to permafrost, lower soil temperatures, and lower surface water and soil water (30-cm depth) than severely burned forest sites, all with p ＜0.001. The deepest depth to permafrost in an unburned site was 65 cm (with a mean = 42 cm), compared to the severely burned maximum depth to permafrost of 95 cm (with a mean = 88 cm).

Table 1 Live plant species observed in forest ground cover(sorted alphabetically)

Fig. 5 Correlations of the Landsat normalized burn ratio (RdNBR) with surface layer water content (W) and soil temperatures (T) for forest sites surveyed near Tanana in July 2017

Change in surface organic layer thickness

Ocular evaluation of paired (unburned to burned) true color photos of organic soil layer thickness revealed that severely burned forest sites (CBI ＞2) had lost from 5 to 10 cm of the thick live moss and lichen cover observed at every unburned forest site surveyed (Fig. 4 photo comparison).

Table 2 Comparisons of depth to permafrost (DPF), temperature (T; in °C), and moisture content (W; VWC %) between unburned forest sites(CBI = 0) and burned forest sites (CBI ＞2) from the Tanana fires of 2015

TIR image profiles

We measured a significant separation (p ＜0.05) in averaged soil temperature profiles between unburned and severely burned forest sites (CBI ＞2), beginning around 14-cm depth (Fig. 6). The soil TIR temperatures commonly stabilized at between 8 and 12 °C in HBS site profiles below 15 cm from the top of the remaining organic surface layer. In contrast, at unburned forest sites, measured TIR temperatures continued to decline gradually to below 0 °C at a typical depth of 25 cm from the top of the thick (10-cm) intact organic surface layer of moss and lichen cover. Examples of pit profile TIR temperatures showed the higher temperatures of 5-8 °C at 30-cm depth in the HBS soil profiles, compared to freezing temperatures in the bottom of the 30-cm unburned site profiles (Fig. 7).

Fig. 6 Average thermal infra-red temperature profiles for 19 burned(Composite Burn Index ＞2; dashed line) and 19 unburned (Composite Burn Index = 0; solid line) soil pits excavated to 30 cm depth.Error bars show 2 standard errors of the mean

Discussion

The exceptionally warm and dry conditions leading up to the summer of 2015 were followed by the largest wildfires seen in decades surrounding the town of Tanana in interior Alaska. Our results from field measurements in both unburned and nearby burned forest sites around Tanana in 2017 were consistent with the concept that the loss of surface organic layers is a major factor determining postfire soil water and temperature changes and the depth of permafrost thawing (Jiang et al. 2015). In severely burned forest sites, the complete consumption of the living moss organic layer was strongly associated with both warming at the soil surface layer and increases in soil water pooling,relative to unburned forest sites. In the most severely burned sites, measurements showed that soil temperature at both 10-cm and 30-cm depths were higher by 8-10 °C compared to unburned forest sites. Below 15 cm, the temperature of unburned sites dropped gradually to subzero (°C) levels by 30-cm depth, while soil temperatures at burned sites remained above 5 °C to the 30-cm depth. The VWC measured at 3-cm depth was typically 2-3 times higher at severely burned sites, commonly in excess of 30%, compared to unburned forest sites. All surface soils measured at unburned forest sites (CBI = 0) had VWC under 10% at 3-cm soil probe depth through the live organic layer in July 2017.

Our ocular estimates of the depth to which 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. (2006) that for every centimeter of organic mat thickness in boreal forests,soil temperatures under the organic layer remained about 0.5 °C cooler during summer months. The difference (increase) we measured in average temperatures at 10-cmdepth between severely burned and unburned sites was 5 °C which, according to Harden et al. (2006), would imply a loss of 10 cm in the organic moss layer thickness in severely burned (CBI ＞2) forest areas.

Fig. 7 Images of thermal infrared temperature profiles for a 3 burned (Composite Burn Index ＞2.9) and b 3 unburned(Composite Burn Index = 0)soil pits excavated to 30-cm depth. Color bars are in units of degrees C

The strong linear correlation (R2= 0.7) revealed between Landsat RdNBR and depth to permafrost measured across all forest sites near Tanana in July 2017 showed that HBS forest areas had a thawed soil layer greater than 60-cm deep, and in many cases, the ice-free profile was measured at nearly 50 cm deeper than in unburned forest soils. At an average moisture content of 37% VWC measured in severely burned forest soils, this equates to approximately 18 cm3of water per m2surface area that had been thawed and converted from solid ice to liquid due to the 2015 Tanana fires. Our results were comparable to those reported by Nossov et al. (2013) 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 temperatures at the surface(to + 2.1 °C) and at 1-m depth (to + 0.4 °C).

Extrapolating the volume of water thawed from permafrost melting across the total HBS area of the 2015 Spicer and Bering Creek Fires, based on these field site measurements, we calculated that 8123 m3(6.6 acre feet)of melt water remained in these burned forest soils in July of 2017. If the total MBS area of the 2015 Spicer and Bering Creek Fires was added to this total, assuming that roughly half the volume of ice layer was melted in MBS areas (compared to HBS areas), then an additional 2620 m3(2.1 acre feet) of melt water remained in these burned forest soils in July of 2017. A total of more than 10,740 m3melt water added to the surface soil storage across these two 2015 fires alone was thereby equivalent to about 104 2.54 cm (1-inch) rain storms, or nearly 10 years of total annual precipitation for the Tanana area.

With more than 200,000 ha of forest burned during the 2015 Tanana area wildfires, the outcomes of recovery and regrowth pathways for the next few decades will be of significant consequence to the local community members who have depended on spruce forests for subsistence hunting and trapping. The deep surface organic material of low bulk density in black spruce stands generally precludes deciduous boreal species from establishing seedlings(Johnstone et al. 2010). Black spruce seeds, which are larger than seeds of most deciduous tree species, have greater capacity to withstand thick, dry surface organic layers during the establishment phase of regrowth.

However, relatively thin post-fire organic layers, such as those measured during our field surveys of the Spicer and Bering Creek Fires of 2015, may cause profound changes in the successional outcomes of severely burned black spruce forests, including a shift from the conifer-dominated thick organic layer to an increase in the dominance of deciduous or shrub species (Johnstone et al. 2010; Barrett et al. 2011) and changes to permafrost conditions (Jorgenson et al. 2010). Barrett et al. (2011) reported that areas with less than 3 cm of surface organic layer after boreal forest fires will be susceptible to deciduous-dominated regeneration and permafrost loss, whereas areas with 3-10 cm of organic layer will be susceptible to co-dominant regeneration by both coniferous and deciduous trees,during which continued degradation of permafrost would be expected.

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 the summer of 2015 consumed between 5 and 10 cm of the former live surface layer and left behind only a residual dead, charred moss and lichen cover 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 and was not nearly as loosely packed and porous as the live organic layers. Not only were the mineral soil layers in HBS areas significantly more water saturated (at ＞30% VWC) than the soils measured under unburned forest strata, presumably from the melting of ice permafrost layers in 2015, but we can speculate that surface runoff from subsequent precipitation events is now more rapid and complete due to the charred moss layers with lowered infiltration rates in severely burned areas(Koch et al. 2014).

Over the next few years, the local community of several hundred residents in Tanana may see unprecedented impacts of the 2015 wildfires on conditions of their access(dirt) roads, traditional hunting grounds, trapping trails, and stream banks. Region-wide, the majority of indigenous communities in Alaska interviewed by Brinkman et al.(2016) for their perceptions of climate change impacts emphasized the potential negative consequences of more limited access to essential resources due to physically obstructed travel, such as fallen trees after a wildfire. This study found that subsistence harvesters (hunters, trappers,fishermen) in Alaska needed to maintain control over decisions relating to their access routes, compared to control over large-scale changes in population dynamics and seasonal distribution of subsistence resources. Unfamiliar or unpredictable changes in forested landscapes following large wildfires may preclude safe travel to traditional hunting grounds. The Fish Lake area is one such traditional hunting grounds about 30 km to the southeast of Tanana. This area is a significant moose habitat, where calving, rutting, and wintering occur (ADNR 2014). Duck,geese, and brown bear are also hunted in the native-owned lands around Fish Lake. Access to this area is mainly by small boat, and high severity wildfires in 2015 have made the route into Fish Lake highly uncertain and hazardous with falling, burned trees.

Conclusions

The loss of surface organic layers was found to be closely associated with post-fire soil water and temperature changes and the depth of permafrost thawing after 2015 wildfires around Tanana Alaska. In the most severely burned forest sites, complete combustion and loss of the living moss organic layer was strongly associated with warming and wetting at the soil surface layer. Soil temperatures of unburned sites dropped gradually to frozen levels by the 30-cm depth, while soil temperatures at burned sites remained above 5 °C to the 30-cm depth. The strong correlation between burn index values and depth to permafrost measured across all forest sites showed that the new icefree profile in severely burned forest areas was commonly 50 cm deeper than in unburned forest soils.

AcknowledgementsThis work was supported by NASA Ames Research Center and the NASA ABoVE Logistics Office in Fairbanks, Alaska. Special thanks to 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.