Journal of Geophysical Research: Biogeosciences

Identification of unrecognized tundra fire events on the north slope of Alaska

Authors


Corresponding author: B. M. Jones, Alaska Science Center, U.S. Geological Survey, 4210 University Drive, Anchorage, AK 99508, USA. (bjones@usgs.gov)

Abstract

[1] Characteristics of the natural fire regime are poorly resolved in the Arctic, even though fire may play an important role cycling carbon stored in tundra vegetation and soils to the atmosphere. In the course of studying vegetation and permafrost-terrain characteristics along a chronosequence of tundra burn sites from AD 1977, 1993, and 2007 on the North Slope of Alaska, we discovered two large, previously unrecognized tundra fires. The Meade River fire burned an estimated 500 km2 and the Ketik River fire burned an estimated 1200 km2. Based on radiocarbon dating of charred twigs, analysis of historic aerial photography, and regional climate proxy data, these fires likely occurred between AD 1880 and 1920. Together, these events double the estimated burn area on the North Slope of Alaska over the last ~100 to 130 years. Assessment of vegetation succession along the century-scale chronosequence of tundra fire disturbances demonstrates for the first time on the North Slope of Alaska that tundra fires can facilitate the invasion of tundra by shrubs. Degradation of ice-rich permafrost was also evident at the fire sites and likely aided in the presumed changes of the tundra vegetation postfire. Other previously unrecognized tundra fire events likely exist in Alaska and other Arctic regions and identification of these sites is important for better understanding disturbance regimes and carbon cycling in Arctic tundra.

1 Introduction

[2] Fire influences vegetation distribution and structure, carbon cycling, land-atmosphere energy exchange, and climate, and it represents an important and widespread disturbance mechanism in several major biomes [Bowman et al., 2009]. However, the role of fire in shaping ecosystem patterns in Arctic tundra remains poorly resolved due to the rarity of reported fires, the geographic remoteness of the region, and the short observational record in the region [Barney and Comiskey, 1973; Wein, 1976; Giglio et al., 2006; Rocha et al., 2012]. As a result, an accurate representation of the Arctic in models depicting the geographic distribution of wildfires and potential shifts in global wildfire activity and pyrogenic gas emissions is lacking [Bond et al., 2005; Krawchuk et al., 2009; van der Werf et al., 2010]. Given ongoing climate change in the Arctic [ACIA, 2005; AMAP, 2011], the frequency, severity, and areal extent of tundra fires are predicted to increase [Higuera et al., 2008; Hu et al., 2010]. Thus, a better understanding of the role of fires in the tundra biome is needed since tundra vegetation, peat, and frozen soils in the Arctic store large, globally significant amounts of labile carbon [McGuire et al., 2009; Tarnocai et al., 2009], and pulse disturbances, such as fire, can play an important role in releasing this carbon [Grosse et al., 2011; Mack et al., 2011].

[3] Since the middle of the 20th Century, Arctic tundra fires have been primarily documented in Alaska [Wein, 1976; Barney and Comiskey, 1973; Racine et al., 1985; AICC, 2012]. Historically, a spatial gradient in annual area burned in tundra vegetation extended from the Seward Peninsula (frequent and extensive fires) toward the northeast (fewer and smaller fires) [Racine et al., 1985]. A more contemporary analysis of the fire history database [AICC, 2012] reveals a northward shift in annual area burned in the Alaskan tundra regions from 1950 to 2012, with an increase in the size of fires north of the Arctic Circle over the past two decades [Rocha et al. 2012]. Interestingly, analysis of a portion of the fire history database indicates a weak but positive correlation between declining summer sea ice extent in the Arctic Ocean and an increasing trend in the area of tundra burned since 1979 [Hu et al., 2010].

[4] The 2007 Anaktuvuk River fire burned more than 1000 km2 on the North Slope of Alaska [Jones et al., 2009] and factors prominently in these emerging trends. This large event has prompted the notion that the northern Alaska tundra fire regime may be shifting [Jones et al., 2009; Hu et al., 2010; Mack et al., 2011; Rocha et al. 2012]. While the Anaktuvuk River tundra fire has created a natural laboratory to study the impact of fire on Arctic ecosystems, it has also raised several questions, including (1) is it possible that other large tundra fires occurred prior to the period of record keeping, (2) how complete is the record of tundra fires in northern Alaska, and (3) what is the long-term impact of tundra fires on vegetation and permafrost-terrain characteristics? Here we report results from ground-based observations and measurements of vegetation and surface permafrost terrain characteristics in the aftermath of tundra fires that occurred in 1977, 1993, and 2007. Through these efforts, we were able to gain a better understanding of the role of tundra fire on landscape change on the North Slope of Alaska over decadal time scales. Analysis of postfire vegetation and landscape succession in concert with remotely sensed imagery led to the discovery of two large and eight small unrecognized tundra fire events in the region. Here we primarily describe and characterize previously unrecognized tundra fires in the Meade and Ketik River watersheds and discuss their potential significance in ecological and climate change contexts.

2 Study Area and Methods

[5] The study area consists of the northern portion of the Arctic Foothills on the North Slope of Alaska (Figure 1). Aerial observations and ground surveys were conducted between 69°N to 70°N latitude and 150°W to 163°W longitude, covering much of the loess belt, or a region which typically contains Pleistocene-aged, ice-rich permafrost deposits (Yedoma) [Kanevskiy et al., 2011]. Combined, these efforts led to the development of criteria for identifying old tundra fires based on postfire characteristics of geomorphology and vegetation. This information was then combined with remotely sensed imagery to map the previously unrecognized tundra fires. Additional field surveys were then conducted to validate the interpretation of the remotely sensed imagery.

Figure 1.

Regional map showing showing previously identified tundra fires, unrecognized tundra fire sites presented in this study, and field survey locations on the North Slope of Alaska. The blue polygon represents the likely distribution of Pleistocene-aged silt with high ice and carbon content (Yedoma) [Jorgenson and Grunblatt, 2013]. The question mark depicts the site shown in SOM Figure 2.

2.1 Landscape-Scale Aerial Observations

[6] In order to develop landscape-scale criteria for the impact of tundra fires on the North Slope, we conducted aerial observations of active fires as well as known, past burn sites. We did this to describe the basic patterns and processes of wildfires in tundra and to identify key characteristics that would aid in the recognition of prehistoric fires. Aerial observations of the tundra burn sites on the North Slope of Alaska began in September 2007 during the period of rapid expansion of the Anaktuvuk River Fire. Aerial reconnaissance conducted during this period allowed for the observation of fire behavior as it migrated across the landscape (Figures 2a and 2b). Aerial reconnaissance was also conducted at the 1993 and 1977 burns in July 2011 and June 2012 to assess postfire patterns of landscape change (Figures 2c and 2d). While flying over the region, we observed other areas that resembled the landscape characteristics of these previously burned sites but were not identified as having burned during the historic period (Figures 2e and 2f).

Figure 2.

Oblique aerial photography from tundra fire sites. (a) The 2007 Anaktuvuk River fire showing the heterogeneous pattern associated with the burn, (b) the 2012 Kucher Creek fire showing a patchy burn pattern as well as a small stream prohibiting fire expansion, (c) the 1993 DCKN190 Fire showing an increase in landscape texture in the burned (right side of creek) versus unburned tundra (left side of creek), (d) the 1977 Kokolik River fire showing shrubbier tundra in the foreground (in burn) and typical graminoid tussock tundra in the background (outside burn), (e) the recently identified Meade River fire site, and (f) the recently identified Ketik River fire site.

2.2 Ground Observations and Surveys

[7] Ground-based surveys at the 2007 Anaktuvuk River Fire [Jones et al., 2009], the 1993 DCKN190 Fire [Barrett et al., 2012], the 1977 Kokolik River Fire [Hall et al., 1978], two unburned control sites, and two putative fire sites were conducted in July 2011 and late-June 2012 (Figure 1). The surveys focused on upland tundra settings and provided information on vegetative and geomorphological differences between burned and unburned sites. We selected unburned sites that were moist tussock tundra dominated by tussock cottongrass (Eriophorum vaginatum), which is the most commonly burned vegetation community in Arctic Alaska [Rocha et al., 2012]. The burned sites were then selected in areas of similar elevation and topography that were inferred to be moist tussock tundra prefire based on the presence and abundance of live or dead E. vaginatum tussocks. The tussocks, especially in the younger fire scars, often showed evidence of charring at their base. Our aim in this sampling design was to establish a chronosequence of sites that vary in time since last fire to better understand postfire vegetation successional trajectories. While we cannot be certain plant communities followed the same postfire successional pathway in this space-for-time substitution, we use the chronosequence approach as an exploratory method given the absence of direct repeat postfire observations on the North Slope. A recent analysis found that chronosequences are most appropriate for studying plant communities that are following convergent successional trajectories and have low biodiversity, and low frequency and severity of disturbance [Walker et al., 2010] suggesting this approach has high potential to be applicable in the Arctic.

[8] Full floristic surveys were conducted using the point intercept sampling method recommended by the Alaska Interagency Fire Effects Task Group [2007]. Plant species and height were recorded every 1 m along three (burned sites) and two (unburned sites) 30 m transects. All taxa that touched the sampling pin were recorded as “hits.” Percent cover was calculated by dividing the number of hits for each taxa by the total number of points along each transect. The transects from the two unburned sites were pooled for a total of four transects in undisturbed moist tussock tundra. Nomenclature follows the Panarctic Species List (v. 1.0) developed for the Arctic Vegetation Archive [Walker and Raynolds 2011; Walker et al., 2013]. Plant species were grouped into broad functional types (SOM Table 1; shrub, forb, grass, sedge, rush, bryophyte, lichen, or other), averaged across transects within a site, and a chi-square contingency analysis was used to determine if functional types were randomly distributed across sites or more predominant at some sites compared to others. To test if canopy height differed among sites, we used an analysis of variance (ANOVA) followed by a posteriori (Student's T) contrasts to compare mean canopy height across sites. The putative fire sites were not included in this analysis since we could not perform an ANOVA with six sites and a sample size of only three to four transects per site.

Table 1. Age Control for the Meade River and Ketik River Fire Sitesf
Laboratory NumberField SiteLatitude (N)Longitude (W)Dated Material14C Agea1-σ Error (±)bδ13C (‰)c13C-Adjusted Radiocarbon Aged1-σ Error (±)b2-σ Calibrated Age Range (cal yr AD)eRelative Area Under Distribution
  1. a

    Radiocarbon years before present where present is AD 1950.

  2. b

    Quoted errors represent one relative standard deviation.

  3. c

    Measured 13C/12C ratios (δ13C) were calculated relative to the PDB-1 standard.

  4. d

    Represents the measured radiocarbon age corrected for isotopic fractionation and reported in years before present where present is AD 1950.

  5. e

    Using Calib6 [Reimer et al., 2009] and the 13C-adjusted radiocarbon age.

  6. f

    Results from radiocarbon dating of charred twig material found preserved in the soils at the previously unrecognized fire sites provided as a measured radiocarbon age, conventional radiocarbon age, 2-σ calibrated age, and the relative area under the distribution curve.

Beta-327427Meade69° 55.8'157° 44.3'Charred Twig7030−27.730301695–17260.20
          1813–18380.14
          1842–18530.02
          1868–18730.01
          1876–19180.60
          1952–19540.03
Beta-336038Ketik69° 39.1'159° 57.7'Charred Twig10030−27.560301694–17270.24
          1812–19190.74
          1952–19540.02

[9] Microtopographic surveys along linear transects across a series of ice-wedge polygons and troughs were conducted at each site using a survey-grade Leica Viva® differential global positioning system (DGPS). A base station was established at each site that recorded fixed positions, and DGPS data collection was acquired with a rover antenna using real time kinematic corrections. This resulted in high-resolution, horizontal (<1 cm), and vertical (2–4 cm), topographic data. These surveys allowed us to identify variation in relief due to degradation of ice wedges (thermokarst) in burned and unburned upland terrain settings. Given the time of the year associated with site visits, we were not able to conduct robust measures of variability in active layer thickness.

2.3 Analysis of Remotely Sensed Imagery

[10] Delineation of the putative fire sites relied primarily on the surface expression or apparent increase in texture in high-resolution (<2 m) remotely sensed imagery (Figure 3), aerial and ground-based surveys, and natural barriers such as river corridors that showed sharp contrasts between what was interpreted as burned and unburned tundra. The entire perimeter of the Meade River site was covered by high-resolution satellite imagery acquired between 2002 and 2012; whereas, the Ketik River site had approximately 50% coverage by high-resolution satellite imagery from 2002 to 2012. Where high-resolution imagery was lacking, we assessed spectral bands 4, 2, and 1 from a Landsat TM image (30 m) acquired on 9 July 2010 by applying a 2% linear histogram stretch within an upland tundra setting that exhibited the characteristic increase in image texture visible in the high-resolution imagery. This technique emphasized the vegetative characteristics associated with the putative fire sites and was used to further delineate the perimeters. By combining terrain surface roughness characteristics with remotely sensed vegetative characteristics, we manually delineated the estimated burn perimeters at a scale of 1:40,000. Vegetation metrics such as NDVI and EVI were determined for the Landsat scene; however, the utility in accurately delineating these features using these indices proved unfruitful.

Figure 3.

Increase in image texture following fire. High-resolution satellite images of upland tundra settings from (a) a site within the Anaktuvuk River fire prior to the burn, (b) the same site as in Figure 3a four years after the fire, (c) the 1993 DCKN190 fire site, (d) the 1977 Kokolik River fire site, (e) the Meade River fire site, and (f) the Ketik River fire site.

2.4 Field Verification of Unrecognized Fire Events

[11] The two putative tundra fire sites were visited in June 2012 to validate our remote sensing analysis. We did this by conducting aerial surveys across the mapped area as well as complementary vegetative and microtopographic surveys to determine whether the upland tundra settings in these sites were more similar to known burn sites or to the unburned sites. We also dug several soil pits at randomly selected locations to look for charcoal. In the field, we tested for the presence of charcoal by smearing samples on white paper. In the laboratory, we examined these samples under 10–50× magnification. Approximately 5 mg of charred woody material was extracted from each site and sent to Beta Analytic Inc. for radiocarbon dating using accelerator mass spectrometry. Calibration of the dated material was conducted using the δ13C-adjusted age and CALIB 6.1 [Reimer et al., 2009].

3 Results

3.1 Delineation of Unrecognized Tundra Fires

[12] We delineated the perimeters of two large and previously unrecognized tundra fires by combining field observations with remotely sensed imagery (Figure 4). The Meade River fire burned an estimated area of 500 km2. The centroid of the burn perimeter lies at 69° 56.4′ N, 157° 24.6′ W. The fire was bounded by the Meade River on its eastern margin and burned a north to south distance of 40 km. The Ketik River fire burned an estimated area of 1200 km2; however, the delineation of the perimeter at this site was hampered by the lack of contemporary high-resolution satellite imagery. Thus, portions of this fire extent were based on the histogram-stretched Landsat image and natural landscape barriers. The centroid of the burn perimeter lies at 69° 45.3′ N, 159° 36.0′ W. Based on this delineation, the Ketik River fire burned a distance of 60 km north and south and was largely confined between the Ketik River on the east and the Koalak River on the west.

Figure 4.

Enhanced Landsat images of the (a) Meade River and (b) Ketik River sites. The locations of survey sites mentioned in the text are marked with a white dot along with their corresponding figure number. The yellow dots represent the location of oblique geotagged photos taken during the aerial surveys in 2012.

3.2 Charred Material and Age Control

[13] In order to find indisputable evidence that these sites had burned, we extracted soil plugs to look for charcoal layers. We visited three sites within the Meade River fire and found charcoal preserved at all three sites. The charred layer was approximately 1 cm thick and was buried beneath 7–9 cm of organic matter that had accumulated postfire (Figure 5a). A total of eight sites were visited in the Ketik River fire and charred material was encountered at two. The best preserved site contained a 0.5 cm thick charred layer that was overlain by 12 cm of peat and vegetation (Figure 5b). At the sites where we did not find charcoal, there was approximately 10 to 12 cm of organic material accumulation directly atop mineral soil.

Figure 5.

Charred material preserved in soil column. Example of soil plugs taken from the (a) Meade River site and the (b) Ketik River site. Arrows depict charred horizon and smear tests shown in lower left.

[14] Several pieces of charred woody material from one location at each site were used to determine the approximate, maximum time since fire. The measured radiocarbon age of the charred material from the Meade River fire was 70 ± 30 14C yr BP and the measured radiocarbon age of the Ketik River fire was 100 ± 30 14C yr BP, where “BP” indicates radiocarbon years before AD 1950 (Table 1). Calibration of these sample ages to calendar years using the δ13C-adjusted ages resulted in six possible age ranges for the Meade River site that span from AD 1695 to 1954 and three possible age ranges for the Ketik River site that span approximately the same time interval. Of the six possible calibrated age ranges for the Meade River sample, the periods AD 1695–1726 and AD 1876–1918 account for 20% and 60% of the total probability under the calibrated age distribution, respectively (Table 1). Of the three possible age ranges for the Ketik River sample, the highest probability under the calibrated age distribution (74%) is the period AD 1812–1919 (Table 1).

3.3 Vegetative and Geomorphic Characteristics

[15] The distribution of plant functional types differs markedly among sample sites (Figures 6 and 7; SOM Table 1; χ2 = 236.728, d.f. = 35, P < 0.0001). At the three sites that burned since the mid-1970s, grass cover (primarily Arctagrostis latifolia and Calamagrostis lapponica) was higher relative to the unburned moist tussock tundra sites, which were dominated by sedge species (primarily Eriophorum vaginatum and Carex bigelowii). At the two newly identified fires, the greatest deciduous shrub cover was observed (primarily Salix pulchra, Betula nana and Rhododendron tomentosum ssp. decumbens), while grass cover was lower than that observed for the sites that burned more recently (Figure 6b). Bryophyte and lichen cover was greatest in the unburned site and oldest fire sites, while lichens were absent from the two more recent Anaktuvuk River and DCKN fires. The bryophytes at these two more recent fires were primarily the early colonizing pyrophytes including Marchantia polymorpha ssp. ruderalis, Ceratodon purpureus, Leptobryum pyriforme, and Pohlia nutans. In contrast, bryophyte taxa such as Sphagnum were only observed at the unburned sites. We also observed a significant difference among sites in the height of the vegetation, with canopy heights from one half to six times greater in burned versus unburned areas (Figure 6a; ANOVA, F3,9 =386.56, P < 0.0001). In particular, taller shrubs tended to be found in the burned upland tundra sites relative to the unburned upland tundra sites.

Figure 6.

Difference in vegetation at unburned and burned sites. (a) Mean canopy height among unburned and burned tundra sites (ANOVA, F3,9 =386.56, P < 0.0001) and (b) the distribution of vascular plant functional types (χ2 = 236.728, d.f. = 35, P < 0.0001). Bars in Figure 6a show mean values and brackets denote associated standard errors and site means that differ significantly (P < 0.05) are indicated by a subscript. The x axis is arranged from youngest to oldest fire event.

Figure 7.

Ground-based surveys along the chronosequence of tundra fire sites. Field photos and DGPS survey data showing the vegetative and geomorphic expression of (a) an unburned site, (b) the 2007 Anaktuvuk River fire, (c) the 1993 DCKN190 fire, (d) the 1977 Kokolik River fire, (e) the Meade River fire, and (f) the Ketik River fire. The pink flamingo lawn ornament measures 50 cm from head to tail. All x axes represent 50 m and all y axes represent 2 m.

[16] There were also differences in micro topography among sites with varying time since fire (Figure 7). The typical microrelief associated with ice-wedge troughs in the unburned, upland control sites was 0.2 to 0.5 m. In contrast, microrelief associated with ice-wedge troughs in the DCKN190 fire site ranged from 0.7 to 1.3 m, while at the Kokolik River site it ranged from 0.5 to 1.0 m. Microrelief across ice-wedge troughs at the Meade River fire survey site ranged from 0.7 to 1.1 m and from 0.5 to 1.0 m at the Ketik River fire survey site.

4 Discussion

4.1 Spatial and Temporal Context of Fire on the North Slope of Alaska

[17] Between 1950 and 2012, 42 tundra fires are known to have occurred on the North Slope of Alaska, with the earliest reported burn in 1959 [AICC, 2012]. These 42 fires burned a combined area of 1635 km2. The majority (~83%) of these fires have been small, burning areas <10 km2. Four of the five largest fires observed on the North Slope over this period occurred in the past two decades, which define the pattern described by Rocha et al. [2012]. There has also been an increase in the total number of fires detected on the North Slope, particularly in the last decade, but many of these are very small (< 0.5 km2), and their detection likely a result of better observational coverage [Miller, 2010]. Spatially, tundra fires have primarily occurred in the Arctic Foothills between the Brooks Range and the Arctic Coastal Plain (Figure 1). Only three fires have been documented on the Arctic Coastal Plain and all three of these occurred between 2011 and 2012.

[18] Together, the Meade River (500 km2) and Ketik River (1200 km2) fires roughly equal the area burned on the North Slope of Alaska over the period of record keeping [AICC, 2012]. Addition of these sites to the fire history database would make the Ketik River fire potentially the largest documented event and the Meade River fire the third largest documented event in the region. However, these delineated areas should be viewed as approximations as it is even difficult to accurately delineate fire perimeters that occur during the remote sensing period, which began around the 1950s [Kasischke and Turetsky, 2006; Barrett et al., 2012]. These delineated areas should also be viewed as maximum burn extents since tundra fires exhibit a heterogeneous burn pattern with numerous unburned inclusions. A patchy burn pattern at the Ketik River site could explain the lack of charcoal encountered within the estimated burn perimeter. This could also result from the presence of a more uniformly graminoid-rich tundra at the time of the burn because this vegetation type often leaves no partially burned charred material [Cofer et al., 1990]. Alternatively, it could indicate that the fire severity varied across the burn extent or that we have over-estimated the size of this fire.

[19] Both the Meade River and Ketik River fires likely occurred as single events sometime during the last 300 years (Table 1). Due to limitations associated with radiocarbon dating during this period, we can only provide the possibility of the fires in several different age ranges. In addition, the charcoal material we dated was fixing atmospheric carbon before these fires occurred. We consider the period between C fixation and fire (in-built age) for the material we dated (burned twigs) to be a few years and at most a few decades [Gavin, 2006]. Nevertheless, we can further constrain the timing of the two fires by assessing the probability distribution of calendar year calibration ages, historic aerial photographs, and regional climate proxy records.

[20] For both the Meade and Ketik sites, the most recent calibrated age range occurred between AD 1952 and 1954. However, this age window accounts for a low probability of the sample (2–3%) making it very unlikely to be the calendar age of the charcoal. Furthermore, analysis of aerial photography acquired in the late-1940s revealed that the increased landscape texture and inferred fire perimeters at the Meade and Ketik sites were already present at this time (SOM Figure 1). The oldest possible age range for both fires occurred between AD 1695 and 1725. The probability that the Meade and Ketik fires occurred during this time period is 20% and 24%, respectively. This period is coincident with the Little Ice Age (LIA), and in particular the coldest and driest phase of the LIA on the North Slope [Bird et al., 2009]. While, presumably this was a period with little thunderstorm activity, large tundra fires could have occurred owing to the dry conditions. The final possible calibrated calendar year age ranges for these events occur between AD 1810 and 1920. The probability distribution for both the Meade and Ketik sites is highest during this time, 77% and 74%, respectively. Within this time period, results from the Meade River sample indicate four possible age ranges, with AD 1876 to 1918 yielding a 60% probability. Results from the Ketik sample only indicate the one age range (AD 1812 to 1919) during this period. Jorgenson et al. [2006] noted the possibility of widespread thermokarst pit formation between AD 1850 and 1940 for a study site approximately 200–300 km to the northeast of these fires sites. Furthermore, Bird et al. [2009] suggest that the period after AD 1880 marked a transition to regionally warm and dry conditions. Thus, given the probability distribution of ages from radiocarbon dating, the presence of fire altered tundra in the 1940s photography, and regional climate proxy data, both fires likely occurred between AD 1880 and 1920 or shortly thereafter when considering the in-built age factor [Gavin, 2006].

[21] The discovery of the Meade and Ketik Fires indicates that large tundra fires on the North Slope of Alaska have likely occurred as recently as the last ~100 to 130 years. It is likely that other fire events of variable size have also occurred in the region that remain to be discovered (SOM Figure 2) and that the role of fire as a disturbance agent in Arctic tundra in general has likely been underestimated. Efforts that seek to determine the historical context of large fires on the North Slope are still in their nascent stage. In addition, our cursory analysis of a subset of the remote sensing archive for the region identified eight small tundra fires not documented in the fire history database (Figure 1). While the focus of this paper is on the detection of the large prehistoric events, a recent study highlighted the important role of small fire events on fire regimes and that globally there was a 35% underestimate in the area burned as a result of omission of small fires [Randerson et al., 2012].

[22] These data also add to the growing observations concerning the fire regime of the North Slope region. Based on recent (AD 1950 to present) [Rocha, et al., 2012] and paleo (last ~5000 years) [Hu et al., 2010; Higuera et al., 2011] estimates of fire rotation periods (the amount of time it would take for a specified region to burn) and fire return intervals (the number of years between successive fires at a given location) for the North Slope of Alaska converge on a time frame of 4000–5000 years, respectively. Addition of our sites to this estimate for the region using the highest probability calibrated age ranges (AD 1810 to 1920) and the definition of the North Slope as provided in Rocha et al. [2012] indicate a fire rotation period that ranges from approximately 3000 to 7000 years. Up to this point, the North Slope has a scant and evolving data set of past fire frequency compared to other systems where fire rotation periods have been estimated based on decades of research [e.g., Sousa, 1984]. Estimates of a fire rotation period for the study area have proven to be highly variable when using different calibrated age ranges of the fires discovered here. For example, using the period AD 1810–2012 results in a fire rotation period of ~7000 years and using the period AD 1920–2012 results in a fire rotation period of ~3000 years. Given the uncertainties in a minimal number of radiocarbon samples, the in-built age factor, and the fact that future work is needed to better reconstruct the fire history in the region, we have included these calculations in this paper a bit reluctantly. In addition, a recent modeling effort focused on fire return intervals in Alaska found values that ranged from 100 to 200 years, 500 to 700 years, and greater than 2000 years for various locations on the North Slope of Alaska [Pfeiffer et al. 2013]. These wide ranging estimates in the repeated burning of Arctic tundra in this region highlight the need for more concentrated work with respect to potential past tundra burning.

4.2 Wildland Fires and Shrub Invasion of Tundra

[23] Direct measures of the ecological impacts of tundra fires on vegetation on the North Slope of Alaska over decadal and centennial time scales are sparse to nonexistent. Observations at the Anaktuvuk River Fire indicate that graminoid cover is substantially greater than shrub cover in the first four years postfire [Jandt et al., 2012; Bret-Harte et al., 2013]. Observations at the 1977 Kokolik River site spanning the first five years postfire also support this pattern [Racine et al., 1987]. At the other large historic tundra fire site, the 1993 DCKN190, graminoid cover remained greater than shrub cover 17 years postfire, but the proportion of grasses relative to sedges had increased [Barrett et al., 2012]. Remotely sensed observations of vegetation change in tundra sites that burned in the last several decades also follow this general pattern with an increase in both albedo and greenness that is interpreted as an increase in grass cover [Rocha et al., 2012]. Our observations from one site located in the northern portion of the Anaktuvuk River Fire also follow this short-term trajectory, but we note a substantial difference in grass cover (primarily Arctagrostis latifolia and Calamagrostis lapponica) relative to our unburned upland moist tussock tundra site in the first five years postfire (Figures 6b and 7b). Our results also indicate a greater abundance of grass cover at the 1993 fire site (Figures 6b and 7c) similar to that found by Barrett et al. [2012], whereas vegetation present at the 1977 burned site resembled the younger fire sites but with the addition of lichens that were absent from the 2007 Anaktuvuk River and 1993 DCKN190 fires (Figure 6b and 7d).

[24] Addition of the Meade River and Ketik River sites to the chronosequence of burned upland tundra used to infer fire-related changes in vegetation indicates that the relative proportion of grasses is less than more recently burned sites, yet still far greater than unburned tundra, likely reflecting the legacy of past burning. Moreover, the proportion of shrubs is greatest at these two sites (Figures 6b, 7e, and 7f). We interpret the greater abundance of shrubs to reflect secondary succession from grass to shrub dominance postfire. On the more southerly, warmer Seward Peninsula and Mackenzie Delta, postfire secondary succession in tundra is known to be accompanied by a transition from grasses to shrubs over a time scale of several decades [Racine et al., 2006; Lantz et al. 2013]. However, on the North Slope of Alaska, this transition has not been documented previously, and our results provide the first compelling evidence of a shift toward shrubbier conditions postfire in the region. These findings have implications when interpreting the modern distribution of shrubs on the North Slope of Alaska [Beck et al., 2011] and indicate that fire could play a key role in the ongoing “shrubification” of the region [Sturm et al. 2001; Tape et al., 2006].

4.3 Fire History and Permafrost Degradation

[25] Tundra fires can trigger thermokarst by removing insulating layers of vegetation and peat and decreasing albedo at the ground surface. In the southern portion of the Anaktuvuk River Fire, Mann et al. [2010] noted the widespread occurrence of active layer detachment slides and several thaw slumps forming over the first three years following the burn. Rocha et al. [2012] synthesized thaw depth measurements for all known tundra burn sites in Alaska and found that there was a sustained increase in the thaw depth within burned sites even three decades postfire. While we lack robust measures of active layer thickness variation and did not see widespread evidence of stabilized detachment slides and thaw slumps, we did notice widespread formation of thermokarst pits at our sites and conducted microtopographic surveys to capture the relief associated with ice-wedge troughs, as the formation of thermokarst pits is also an indicator of permafrost degradation [Jorgenson et al., 2006].

[26] The microtography of ice-wedge polygons (raised centers to troughs) in undisturbed upland tussock tundra settings was 0.2 to 0.5 m (Figure 7a). When viewing these sites in high-resolution remotely sensed imagery, it is difficult to distinguish individual polygons (Figure 3a). However, in images from the Anaktuvuk River fire four years after the burn, the increase in texture is evident (Figure 3b), and the ice-wedge trough microtopography show initial signs of thermokarst (Figure 7b). For the sites that had burned in 1977 and 1993, the relief associated with ice-wedge troughs ranges from 0.5 to 1.3 m indicating that the fires disrupted the soil thermal regime leading to permafrost degradation (Figures 7c and 7d). This shows up in the high-resolution imagery as an increase in texture (Figures 3c and 3d). DGPS surveys conducted at the Meade River and Ketik River sites were more similar to the burn sites than the unburned sites, with microrelief associated with the ice-wedge troughs on the order of 0.5 to 1.1 m (Figures 7e and 7f). However, one notable difference between these sites and the sites that burned in 1977 and 1993 (Figures 3c and 3d) was the lack of ponded water in the ice-wedge troughs (Figures 3e and 3f). We interpreted that this could be the result of a more integrated drainage network in these older burn sites or paludification of the troughs and infilling with peat and vegetation.

[27] The presence of near-surface, ice-rich permafrost terrain with massive ice in the form of wedges was essential for the identification of the previously unrecognized tundra fire events. The degradation of ice-rich permafrost terrain likely plays an important role in postfire vegetation succession trajectories as noted by Bret-Harte et al. [2013]. Thus, the increase in shrubbiness at the century-old fire sites is likely due in part to ice-rich permafrost degradation and it was the combination of vegetative and geomorphic change that aided in our ability to identify these sites. While it appears that large fires on the North Slope tend to occur in the eolian silt band of the northern Foothills where thick, Pleistocene-aged, ice-rich permafrost (Yedoma) is found, several small historic fires have occurred in areas not indicative of ice-rich permafrost terrain (Figure 1). Detection of fires that predate the period of record keeping in these ground-ice poor areas would be difficult to detect by using the techniques applied in this study. The distribution of Yedoma permafrost may also have implications for potential future successional trajectories associated with the Anaktuvuk River fire. Figure 1 shows that Yedoma is likely present below the northern two thirds of the area impacted by the Anaktuvuk River fire. This may result in more widespread ground subsidence as a result of postfire permafrost degradation relative to the southern one third of the burned area, where surface permafrost deposits tend to be less ice rich. Thus, spatially variable responses in vegetation succession should be expected in response to this fire event.

4.4 Broader Implications

[28] Identification of the Meade River and Ketik River fires may have Arctic-wide implications. Mack et al. [2011] found that the 2007 Anaktuvuk River fire released 2.1 Tg of C to the atmosphere, which is equivalent to the annual net C sink for the entire Arctic biome. If the Meade River and Ketik River fires released similar amounts of C during combustion, these events may have had a similar immediate impact on atmospheric carbon. There may also be an important long-term impact of these disturbances because the fires here may promote the degradation of the Yedoma permafrost deposits (areas with Pleistocene-aged permafrost deposits that can be high in ice and carbon content) that occur in this region (Figure 1) [Kanevskiy et al., 2011]. The apparent shift from sedge-dominated tundra to shrub-dominated tundra that we documented in the older burn sites could also have a positive feedback on climate warming due to the combination of a decrease in albedo and an increase in evapotranspiration associated with this transition [Euskirchen et al., 2009; Bonfils et al., 2012]. Thus, during a period of already rapid shrub expansion in the Arctic [Myers-Smith et al., 2011] in which tundra fires are becoming larger and more frequent [Rocha et al., 2012], this process may be exacerbated [Lantz et al., 2013]. Further, an increase in shrub abundance may also lead to an increase in the frequency, severity, and areal extent of tundra fires [Higuera et al., 2008], which may lead to an increase in C release [Mack et al., 2011]. However, very little is known about the balance of C storage following tundra fires. The complexities associated with the long-term C budget at burned tundra sites underscore the importance of better understanding the spatial and temporal context of tundra fires in the Arctic.

5 Conclusions

[29] We discovered two large and previously unrecognized tundra fire events on the North Slope of Alaska using the increase in landscape texture that resulted from vegetation changes and ice-wedge degradation postfire. The Meade River fire burned an estimated 500 km2 and the Ketik River fire burned an estimated 1200 km2. Based on radiocarbon dating and analysis of historic aerial photography, and climate proxy data, these fires likely occurred between AD 1880 and 1920, making them the oldest delineated tundra fire perimeters in Arctic Alaska. Recognition of these fires potentially doubles the estimated area of burned tundra on the North Slope of Alaska during the last ~100 to 130 years. Identification of these sites also provides a centennial-scale chronosequence of postfire vegetation and geomorphic change providing a valuable temporal context for better understanding the impacts of tundra fires during a period in which they are expected to increase in their number, size, and frequency.

Acknowledgments

[30] The authors gratefully acknowledge support provided by the U.S. Geological Survey - Alaska Science Center and the Bureau of Land Management - Arctic Field Office. We thank Dr. Philip Higuera, Dr. Eric Kasischke, and Eric Miller for their constructive and helpful reviews. We are also grateful to Drs. Misha Zhurbenko and Olga Afonina of the Komarov Botanical Institute at the Russian Academy of Sciences for identification of lichens and bryophytes. We would like to thank Jim Webster of Webster's Flying Service as his expertise over the last several years has been invaluable. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.