The Arctic is changing at unprecedented rates. The changes in the seasonal timing and decreased duration of frozen conditions combined with increased air temperatures has already manifested itself in visible changes in the Arctic landscape including increased plant productivity [1
], thermokarst, and drying of lakes [2
]. Changes in wildfire frequency and severity are suspected but undocumented in the tundra. In boreal ecosystems wildfire has already been documented as increasing in frequency and severity over the last 50 years [3
]. However, the baseline fire regime in the tundra is not well quantified due primarily to the relatively low level of human habitation in Arctic regions, and thus limited fire management and suppression efforts. Historically, resources spent mapping fires have been directly correlated to human presence in the region. In turn, tundra fire records are not maintained to the level they have been in boreal regions [4
]. Also contributing to the lack of tundra fire data records is that the optical satellite data record over the Arctic has limitations due to persistent cloud cover, lack of algorithms suitable to detection of burns in tundra, and quick green up of tundra vegetation within one year of fire [5
]. Additionally, the physical and ecological effects of fire disturbance on the tundra are poorly understood due to the logistical challenges of obtaining field measurements, and especially repeat measurements, in remote locations.
An exception to our limited understanding of fire in the tundra is observations of the 2007 Anaktuvuk River Fire on the North Slope of Alaska. This is the largest fire on record (1039 km2
burned) for the tundra biome and it doubled the cumulative area burned north of 68°N in that region since 1950 [6
]. This fire has been well-studied [6
], but appears to be a novel expression in the tundra fire record as a fire that started mid-summer (July) and persisted late into the growing season (October) and exhibited greater burn severity than typical tundra fire events [8
]. Alternatively, it has been suggested that with climate change the Anaktuvuk River Fire may represent the new normal [6
]. The Anaktuvuk fire scar is less than a decade old, thus many questions on recovery of a large, high-burn-severity tundra fire and long-term landscape change remain unanswered.
It is generally known that the factors affecting fire occurrence and the effects of fire on the landscape differ between the tundra and the more extensively-studied boreal regions. In the tundra, as the boreal region, most fires start as a result of lightning strikes. However, while fire events in the boreal zone are of relatively high frequency (159 year fire return interval for 1860–1919 and 105 years for 1920–2009 [10
]) and can be of very large size (average 203 km2
for high fire years and 78 km2
for low fire years for the period 1950–1999 [11
]), in the tundra fire events are generally rare and small in size [12
]). Historically, tundra fire events have occurred in June and July [13
], with average size of 30–55 km2
] with an estimate of cumulative decadal burning of 744 km2
on the North Slope of Alaska [15
Several ecoregion [16
] and vegetation maps [20
] exist for Alaska and the pan-Arctic that can be used to delineate the geographic extent of tundra. While differences exist in these map products, approximately six ecoregions are covered within tundra vegetation extents in Alaska. These geographic areas include, from north to south: North Slope Coastal Plain, Brooks Range Foothills, Brooks Range, Kotzebue Lowlands, Seward Peninsula, and Southwest Alaska. Differences in the fire regime between these geographic areas exist [6
], but despite differences in fire frequency and size, the persistent patterns of vegetation across the different regions may have similar post fire recovery.
There is evidence that climate change has led to an increase in fire occurrence in tundra regions. Hu et al.
] show through paleoecological evidence that recent tundra burning is unprecedented in the central Alaska Arctic within the last 5000 years. Fuel loads (plant biomass) are expected to increase over time in high northern latitudes as shrub dominated land cover increases [23
]. These enhanced fuel reserves are likely to result in increased burned area and fire severity which would be detrimental to ecosystem services such as wildlife habitat.
The Alaskan tundra falls within the zone of continuous permafrost. Fire events are known to locally disturb permafrost by increasing the active layer—the depth of seasonal soil thaw. Permafrost recovery is largely a function of vegetation recovery and thus pre-fire vegetation, slope characteristics, and fire characteristics [25
]. Thermokarst occurs through large-scale permafrost degradation, and can significantly alter the local hydrology though the draining and creation of thermokarst lakes [26
]. The extent to which fire increases active layer, thermokarst, and alters surface hydrology in the tundra has not been widely investigated.
Electro-optical and thermal satellite sensors may be used to detect initial changes in temperature and surface composition resulting from a fire event [28
]. Synthetic Aperture Radar (SAR) sensors are sensitive to changes in surface roughness and soil moisture, making SAR useful for characterizing longer-term patterns and trends that occur post-fire [29
]. In the persistently cloudy and hazy Arctic environments, SAR systems have the added benefit of more useable image observations as compared to electro-optical systems due to all-weather imaging capabilities. SAR data has also proven useful for monitoring other ecological parameters in high northern latitudes [32
Recent research [5
] in the North American tundra using Landsat imagery shows that the electro-optical spectral signature of burned areas deteriorate rapidly, resulting in fire-disturbed sites being poorly distinguishable from unburned tundra by the end of the first post-fire season. In contrast, the remote sensing signature in SAR imagery is likely to be much more long-lived due to the geophysical changes detected with SAR backscatter imagery. Research in the boreal region using SAR data has shown that fire scars are detectable for five to seven years post-fire [31
]. In boreal regions fire scars are typically three to six dB brighter than adjacent unburned forests in the spring (May) after snowmelt due to changes in surface roughness, moisture, and removal of tree canopies [34
]. The decreased evapotranspiration, thawing of frozen ground and deepening of active layers makes the burned regions wetter than adjacent unburned forests and this is detected by the SAR sensors as enhanced backscatter.
The study presented addresses the hypothesis that the SAR signature from fire-disturbed sites in tundra will persist for several years post-fire, as has been documented in boreal sites. This paper presents a previously undocumented temporal assessment of radiometric response (i.e., SAR backscatter) captured by the SAR instruments on board the Earth Resources Satellite/European Remote-Sensing Satellite (ERS) at three fire scars in the Alaskan tundra. SAR data are shown to be useful for detection, monitoring, and quantifying temporal changes in fire disturbed landscapes. The goal of this analysis is to assess the landscape response to fire and to quantify the longevity of these effects as observed through radar images.
Recent tundra fire scars appear brighter (higher backscatter values) in the SAR images than the surrounding landscape (Figure 2
). Typically, fire scars are brightest one year post fire with the brightness gradually decreasing each subsequent year post-fire. The fires evaluated are approximately 3.0 to 3.3 dB brighter than adjacent unburned areas during the end of the growing season one year post fire. For the Anaktuvuk and Uvgoon datasets the difference between burned and unburned is smaller (0.1 to 0.6 dB) in early May, with the difference increasing over the growing season, and reaching a maximum in early to mid-August (Figure 3
). This trend cannot be evaluated in the DCKN178 plot (Figure 2
), because May and August data were not available one year after the fire event. Generally, the May data for all three fires showed variable response year-to-year but May images consistently showed less differentiation between burned and unburned signatures.
Plots of the radiometric response over the entire ERS data record (Figure 4
) clearly show the fire event (dashed line in Figure 4
plots) and the lasting impact on the record. Visual inspection of the plots shows the divergence in the burned versus
unburned series that persists for approximately four to five years post fire for all three scenarios. The Uvgoon fire generally has higher and more stable backscatter values than the Anaktuvuk and DCKN178 fires. The Uvgoon data record shows dB values in the range on −6 to −11 with the approximately 3 dB separation in the burned versus
unburned polygons. The North Slope fires, Anaktuvuk and DCKN178, have backscatter values in the range of −6 to −17 dB with a decreasing trend over time. The fire event is again clearly visible in these temporal plots.
The ANOVA results show approximately four to five years are needed for landscape recovery, as defined by a return to the pre-fire signature, of burned areas in the SAR imagery. Plots of the burn years versus
the effect of burn for each fire (Figure 5
) show a return to zero effect, within the 95% confidence envelope, at 2004 (five years post fire) for Uvgoon Creek and at 1998 (four years post fire) for DCKN178. The Anaktuvuk River Fire does not return to zero effect of burn given the available data, but 2008, 2009, and 2010 (three years post fire) are all above the zero effect line.
The Tukey HSD tests were implemented to investigate if differences existed in the means of the different regions within each burn. The results were variable and did show some regional effects for the Anaktuvuk and Uvgoon Creek Fires (Table 3
). No significant difference between Regions 1 and 2 (p
= 0.097) were found for the DCKN178 Fire. For the Anaktuvuk River Fire no difference was found between Regions 1 and 2 (p
= 0.428) but statistically significant differences were found between Region 3 and Regions 1 and 2 (p
< 0.001 for both). For the Uvgoon Creek Fire no significant difference was detected between Region 2 and 3 (p
= 0.956) but a significant difference between Region 1 and Regions 2 and 3 (p
= 0.001 and p
= 0.004, respectively) was observed.
Previous research in boreal forests found a three to six dB difference in ERS SAR backscatter in burned versus
unburned sites. In this analysis of tundra fire sites a three dB difference has been detected for the tundra sites studied. A 1994 boreal wildfire near Gerstle River, Alaska had average ERS-1 backscatter of −4.5 dB compared to −8 to −10 dB (3.5 to 5.5 dB difference) for adjacent unburned forests in the spring of 1995 (one year post-burn). This compares to the 1993 DCKN178 fire which had peak backscatter in late-July 1994 of −6 dB and the adjacent unburned tundra had on average −9 dB backscatter (3 dB difference). The effect of standing dead trees in burned boreal forests was at first thought to be causing a double bounce (enhanced backscatter) effect. However, the effects of the trees was determined as negligible due to low moisture content of the boles; instead surface roughness was determined to be the greatest factor, coupled with high moisture content, in causing the bright backscatter return from burned boreal forests [38
]. Similarly, in the tundra the C-band energy is scattering from the rough ground surface causing an enhanced signature when the ground is wet. Distinctions between tundra and boreal that could affect the backscatter differences between burned and unburned areas of these two ecoregions include greater variation in vegetation structure and composition pre-fire in boreal, differences in seasonal timing of ground thaw, greater severity of burning within duff and organic layers in boreal, slow post-burn vegetation recovery in boreal versus
tundra, shallower active layer and organic soil depths in the tundra. Further investigation into the dominant scattering mechanisms, surface roughness conditions, and seasonal trends in soil moisture are needed in the tundra and boreal to fully understand the differences observed by the C-band sensors.
Intra-annual observations in the years post fire reveal a greater separation in backscatter values as the growing season progresses. Differences in springtime (May) were found to be very low, most likely due to frozen ground conditions. For this reason May data were omitted from the statistical analysis. Peak differentiation between burned and unburned polygons was found late-July to mid-August, depending on image availability. Wetness increases during the growing season due to permafrost thaw and reaches a peak in mid-August when active layer measurements are typically taken. This shows that the best time to detect fire scars in the Arctic is as late in the growing season as possible before frost and frozen ground conditions develop. For the North Slope and Noatak National Preserve study regions this corresponds to mid-August.
The Anaktuvuk River Fire was a much larger fire than DCKN178 or Uvgoon, which accounts for the larger range of variability in polygon backscatter signatures for Anaktuvuk. North to south, the Anaktuvuk fire scar spans different topography, types and proportions of land cover classes, as well as differences in hydrology which all affect the observed backscatter. Similarly Uvgoon is larger than DCKN178 and thus displays a larger range of backscatter values.
The results from the ANOVA effects model show landscape recovery, as detected by statistically significant changes in backscatter, 4–5 years post-fire. This means there is a 4–5 year window to detect burned areas in the tundra, and a comprehensive mapping effort using SAR data could be conducted every 4–5 years and theoretically map all fires that occurred during this the previous 4–5 year time period. These results show that the landscape is affected over a longer period of time than is observed through electro-optical satellite data. Previous electro-optical studies [5
] show recovery as early as late season for early season burns (less than one year) and one year post fire. Electro-optical and microwave sensors are detecting different landscape parameters. Measures of electro-optical recovery are mainly detecting changes in vegetation and vegetation greenness; whereas SAR sensors are detecting moisture and surface roughness changes post-fire. Further research is needed to better develop the relationships between soil moisture and surface roughness on the detected backscatter over time.
Visual observations of the individual SAR images show the persistence of the fire scar longer than the four to five years as detected though statistical means. Based on our experience with these three fires, the human eye can detect the diminishing fire scar up to ten years post fire, although admittedly, the remnant signature is patchy and only small portions remain discernable. This is comparable to research results as reported in boreal systems, accounting for differences in methodology and number of SAR images analyzed. It is also interesting to note that visual observation of the individual ERS image scenes for the three fires do not show any progression of the fire scar boundary over time. It was initially hypothesized that the fire could affect the radar response directly adjacent and outside the burn perimeter due to melting of permafrost and other hydrologic changes from the fire, but this does not appear to occur at the three fires investigated.
The result of the investigation into regional effects within each fire is not surprising given the size of the three different test fires. No regional effects were observed in the smallest fire, DCKN178. Regional effects were found at the medium and large fires, Uvgoon and Anaktuvuk, but in both scenarios, only one of the three regions was statistically different from the other two. For the Anaktuvuk Fire the far northern polygons (Figure 2
) correspond to the statistically different region. They may be the result of different land cover or vegetation types in this region or the topographical position on the landscape. The far northern polygons also have a higher percentage of surface water features and may have higher soil moisture values which could be introducing increased intra- and inter-annual variability in the backscatter values for these regions. For the Uvgoon Creek fire, the polygon pairs in the far western portion (Figure 2
) of the overall fire scar correspond to the statistically different region. The available ancillary data layers for this region do not reveal the same noteworthy differences as were found for Anaktuvuk, but the p
-values were not as small as were found at Anaktuvuk either. The results for the three fires show that spatial autocorrelation does affect the radiometric response, and provides evidence for developing different regional models for large fire sizes. This is most likely not necessary for small fires, given the observations from this study.
The Noatak and the North Slope fires display a difference in radiometric response over time. Regardless of the timing of the fire event within the radar data record, the Noatak trend is relatively stable over time, but the North Slope sites show a general downward trend over time. This finding is significant and may indicate regional drying that is occurring on the North Slope but not in the Noatak [39
]. Further test areas distributed across the North Slope and both north and south of the Brooks Range are needed in combination with in situ
measurements to further investigate this finding.
SAR is a powerful tool for fire detection and fire effects monitoring in the Arctic. Persistent cloud-cover and haze severely limits electro-optical satellite applications in this region. SAR data can be used to provide image looks when electro-optical data are not available, and integration of SAR data within current fire detection algorithms could increase the number of detections. The fire perimeter for the DCKN178 fire was not accurate in the AICC Alaska Large Fire Database potentially due to the lack of cloud-free Landsat data one year post-fire. SAR image data could be used to generate perimeters, and thus area estimates, of burned areas in the tundra as was demonstrated as feasible in boreal regions [30
]. Increased fire detections and more accurate estimates of area burned would provide the data for better characterization of the baseline fire regime in this region. Without accurate baseline data it is difficult, if not impossible, to determine if fire regime is changing in the Arctic and the extent to which ecosystem services are affected. Improved baseline data is also needed to refine fire emissions estimates and carbon accounting.
Electro-optical and radar sensors are measuring different landscape parameters, and SAR data can be used to detect additional and complementary information to what can be extracted from optical systems. Within the wildfire community, algorithms currently exist to map burn severity and the inclusion of SAR data within these algorithms may provide better correlation between satellite and in situ data and result in more accurate map outputs. Other research fields that could benefit from inclusion of SAR wildfire monitoring data are studies in permafrost and surface hydrology. Algorithm development estimating active layer depth as a function of backscatter would be beneficial to many stakeholders beyond the wildfire community, but fire events provide opportunities for detection and monitoring of rapid changes and recovery that typically are not available within baseline studies.
Two decades of ERS SAR data are available in the data archive, but no new images are being acquired. This analysis could be extended with the inclusion of both archive and new data from of other satellite SAR systems. C-band archive data from Radarsat-1 and 2 and Envisat exist, and new data requests can be submitted for Radarsat-2. Additionally, the Sentinal-1 mission is planned for launch in 2014 and is designed to provide C-band data continuity building on the legacy of ERS, Envisat, and Radarsat. A constellation of Radarsat-2 satellites is also planned and could provide additional data options.