Effects of wildfire on peat plateau soil thermal regimes

The soil thermal regime at each of the 16 peat plateau sites (Fig. 1a, Supplementary Fig. 3, and Supplementary Table 1,) was characterized by monitoring air and soil temperatures at 40 cm depth for a full year, and via repeat measurements of depth to frost table at 100 point locations in 3 m spaced grids. Depths to frost table were measured up to seven times per site between May and late September. It was apparent from sudden large increases in depth to the frost table that many point locations had continuously thawed soil layers between the permafrost and the seasonally frozen layer, i.e. taliks (Fig. 2, and Supplementary Fig. 4a). We derived two measures of soil thermal regime from the within-site distribution of depth to the frost table in September; the proportion of point locations with taliks, and the active layer depth as defined by the typical depth to frost table at point locations without taliks (see Methods; Supplementary Fig. 4b, and Supplementary Tables 3, 4). Differences in soil thermal regimes were considered to be primarily due to differences in fire histories among sites, since sites all had similar peat depths, and current or pre-fire tree densities, and since the variability in mean annual air temperatures between −0.8 and −3.1 °C among unburned sites did not explain any of the variability in their active layer depth or talik coverage (p > 0.5, linear regressions) (Supplementary Table 1).

We observed a >60% increase of the active layer in recently burned sites, from ~50 cm in unburned sites to ~85 cm in sites that burned within the last five years prior to the study (Fig. 3a). Recently burned sites had a deeper depth to the frost table than unburned sites already in June, and this difference increased throughout the summer and into fall (Supplementary Fig. 5). Accordingly, thaw at 40 cm depth occurred more than 2 weeks earlier at the recently burned than unburned sites, as indicated by soil temperature records (Fig. 3b). This suggested that fire was associated with increased downward ground heat flux throughout both summer and fall, possibly accompanied by an earlier initiation of the seasonal thaw development. We consider it likely that increased ground heat flux resulted from increased net radiation at the surface following the loss of shading from black spruce trees, and from reduced ground albedo30 as light-colored lichens had been replaced with black char (Fig. 3c, Supplementary Figs. 1 and 3). The observed effects of wildfire on active layer depth and rate of thaw throughout the summer were most apparent at recently burned sites, lessened at sites that burned 10–20 years ago, and were not detectable at older burn sites (Fig. 3a, b).

Fig. 3 Trajectories of soil thermal regime and vegetation reestablishment on peat plateaus following wildfire. a Active layer depth (cm), and talik coverage (%). Taliks are continuously thawed soil layers between the permafrost and the seasonally frozen soil. No active layer estimates were possible for sites burned 7 and 16 years ago due to complete talik coverage. b Timing of thaw at 40 cm depth (Julian Date, JD), as indicated by timing of the first exceedance of 0.5 °C soil temperature, and the maximum annual soil temperature (°C) at 40 cm. Soil temperature loggers malfunctions at sites burned 7 and 49 years ago, and is missing. c Trajectories of vegetation reestablishment, including ground cover of lichens (%), and median height of black spruce trees (m), where error bars indicate interquartile range. In each panel, the average of 6 unburned sites is shown on the left with gray background, where error bars indicate ±1 standard deviation Full size image

Our results further indicate that wildfire caused substantial development of taliks and increased deep soil temperatures. However, these effects were delayed and most pronounced at sites that burned 10–20 years ago, and only started to recover once active layer depths had returned to pre-fire conditions (Fig. 3a, b). Talik coverage increased from ~20% in unburned sites to between 70 and 100% in sites burned 10–20 years ago (Fig. 3a), while maximum annual soil temperature at 40 cm increased from between 2 and 5 °C to between 9 and 11 °C (Fig. 3b). The delayed response suggests that these effects represent a cumulative effect of wildfire which carries over from year to year. We speculate that wildfire increases heat penetration during the summer but also reduces soil heat loss during winter, which together prevents complete re-freeze of the active layer. This in turn allows for a continued deepening of the frost table and further talik expansion in the following year. Winter heat losses after fire may be reduced due to the loss of an intercepting tree canopy which causes increased snow depths and thus increased insulation31. Our results, indicating strong effects of wildfire on soil thermal regimes in peat plateaus are contrasting to the minor effects of wildfire found in more southern non-permafrost bogs32. This could be due to greater ground-cover dominance in non-permafrost bogs of Sphagnum fuscum hummocks that remain light-colored following fire32, or due to differing effect of fire on near-surface soil moisture and consequently on soil thermal conductivity.

Fire severity did not appear to have any influence on the post-fire soil thermal regime. Fire severity is generally higher during droughts and for fires that occur later in the season33. However, neither the Canadian drought code, which is a rating of the average moisture content of deep organic layers, nor the Julian date of the fires explained any of the variability in soil thermal regime among the burned peat plateau sites (p > 0.5, multiple linear regressions with active layer depth or talik coverage as dependent variables, and years since fire, Julian date, and drought code as independent variables) (Supplementary Table 1). The lack of an influence of fire severity may be due to the observed near-complete tree mortality and loss of lichens at all recently burned sites in this study (Fig. 3c and Supplementary Fig. 3). Hence, tree mortality and lichen loss appear to occur regardless of fire severity, which suggests that fire severity does not moderate the immediate effects of fire on shading or albedo. Fire severity furthermore did not appear to influence vegetation recovery. Variability in fire severity is known to affect timing and trajectory of vegetation recovery in some Alaskan ecosystems with relatively shallow soils, since high severity fires have the potential to cause almost total loss of soil organic matter34,35. While there likely was substantial variability among sites with regards to depth of burn among our sites, the thick organic soils of the peat plateaus prevented complete peat combustion36, which likely explain the consistent trajectory of vegetation recovery across burned sites (Fig. 3c).

The recovery of the soil thermal regime to pre-fire conditions coincided with vegetation reestablishment (Fig. 3a–c). Slow, decadal, vegetation recovery was likely linked to the dry, cold, and nutrient poor conditions on burned peat plateaus. Median height of regenerating black spruce trees remained <0.5 m even 10 years after fire, and lichens were absent in all sites that burned <20 years ago. Lichen coverage had only returned to pre-fire coverage in the two sites that burned >35 years ago, and tree height was not fully recovered even 50 years after fire. Influence of vegetation on shading, albedo, and snow pack dynamics during reestablishment may thus all have contributed to the recovery of the soil thermal regime30,37,38. The last aspect of the soil thermal regime to recover was the contraction of taliks, which occurred ~30 years after fire. Since taliks are likely to prevent effective heat loss from the permafrost core during winter, we consequently did not expect increased rates of thermokarst bog development due to complete permafrost thaw along peat plateau edges to last beyond 30 years after fire (Fig. 2).

Effects of wildfire on thermokarst bog development

Young thermokarst bogs are generally found along thawing peat plateau edges, and they have a distinct vegetation composition that is clearly discernable in high resolution satellite imagery (Figs. 1b and 4). The transition from young to mature thermokarst bog is defined by a shift in dominance from Sphagnum riparium and Scheuchzeria palustris to Sphagnum fuscum and ericaceous shrubs17. Radiocarbon dating of cores from the study region show that this transition from young to mature thermokarst bog occurs between 60 and 140 years after the initial permafrost thaw that led to the development of the young thermokarst bog (Supplementary Table 5). In order to determine the effect of wildfire on thermokarst bog development, we chose to study four large peatlands that were partially affected by wildfire 20–30 years ago. The four peatlands were chosen for this analysis since the 20–30 years since fire coincided with the duration over which wildfire was found to influence peat plateau soil thermal regime, and thus likely also the period over which it would influence the rate of thermokarst bog development. Hence, we expected the majority of the cumulative effect of wildfire on thermokarst bog development to be accounted for by choosing sites that burned 20–30 year ago. However, analysis using the chosen sites cannot rule out effects of wildfire on thermokarst bog expansion extending beyond this time frame, and as such our analysis is potentially conservative. By assessing the spatial extents of peat plateaus, young thermokarst bogs and mature thermokarst bogs within each peatland we were able to estimate rates of thermokarst bog development over the last 60–140 years, a period which includes but extends beyond the influence of the more recent wildfires (Fig. 1a, Supplementary Table 2).

Fig. 4 Classification of peat plateau, young thermokarst bog, and mature thermokarst bog using high resolution satellite imagery in peatlands partially affected by historical wildfires. a and b Examples of satellite imagery (WorldView-2, 0.6 m resolution) for 250 × 250 m sections within unburned and burned peatland parts, respectively. Satellite images were acquired in 2011, the burn occurred in 1987. c and d Examples of supervised classification of the unburned and burned sections, respectively Full size image

Supervised classification was done for between 30 and 65 sections (250 × 250 m) within burned and unburned parts of each of the four peatland sites (Supplementary Fig. 6, Supplementary Table 2). Burned and unburned peatland parts at each site were located <10 km apart. Precision of the supervised classification was assessed by comparison with field-determined dGPS locations of transitions between peat plateau, young thermokarst bog, and mature thermokarst bogs at the Zama site (Supplementary Fig. 7). The distance between field-determined ecological transitions and transitions in the supervised classification was within 1 m 80% of the time, and without bias, in both burned and unburned sections (Supplementary Fig. 8). The field validation thus showed that the supervised classification of young thermokarst bogs would be able to provide a both precise and unbiased measure of differences in thermokarst bog expansion between burned and unburned peatland parts.

Average coverage of young thermokarst bogs within burned and unburned peatland parts was 8.6 and 5.3%, respectively, and the average difference between burned and unburned parts of paired sites was found to be 3.4 ± 0.5% (±1 SD) (Fig. 5a). A pairwise t-test indicated a significant influence of fire on young thermokarst bog coverage (t = −10.889, p < 0.01) when comparing average young thermokarst bog coverage in burned and unburned parts of the four paired sites. While the effect of wildfire was largely consistent between sites, we did observed a large variability in average young thermokarst bog coverage between unburned sites, ranging from 2.7 to 10.5% (Fig. 5a). We note that the lowest young thermokarst bog coverage was found at the higher elevation sites, Zama and Trout Lake (Supplementary Table 2). These sites are located at elevations ~300 m higher than the two other sites, and are indicated to have low subarctic climate in contrast to the lower elevation sites that are located in high boreal or mid-boreal ecoregions18. Field data confirmed that the Zama site had a colder climate than the Fort Simpson site during the year of our study, at −0.8 and −3.1 °C. While no direct climate data for the 60th Parallel site was available, this site was located both at a low elevation and in the southernmost part of the study region, and would thus be expected to have the warmest climate, thus possibly explaining the greatest young thermokarst bog coverage within the unburned areas among our four sites. This implied effect of climate on thermokarst bog development in unburned peatland parts23 contrasted with the lack of an observed difference in soil thermal regimes among unburned sites (Fig. 3). The greater young thermokarst bog coverage in burned than unburned parts at the Zama and Trout sites (+100–150% greater coverage in burned than unburned parts) than at the 60th Parallel and Fort Simpson sites (+30–70% greater coverage in burned than unburned parts) thus suggests that wildfire has had a relatively more pronounced influence on thermokarst bog expansion at colder sites (Fig. 5a).

Fig. 5 Effect of wildfire on permafrost thaw through development of young thermokarst bogs in western Canadian peatlands. a Current-day coverage of young thermokarst bogs in burned and unburned sections within four peatlands partially affected by wildfire 20–30 years ago. Box plots indicate median, interquartile range, and minimum and maximum young thermokarst bog coverage among classified 250 × 250 m sections, with number of classified sections indicated (n). A pairwise t-test showed a significant (p < 0.01) effect of fire on young thermokarst bog coverage. Sites are ordered left to right by likely decreasing mean annual air temperature, see text for justification. b Estimated historical development currently present young thermokarst bogs within peatlands in the study region. Young thermokarst bogs persist for 60–140 years before succession into mature thermokarst bogs17, suggesting that a significant proportion of young thermokarst bogs currently present developed prior to the more recent fires. Thermokarst bog development rates are stated with 95% CI (Methods), which are also indicated by shading Full size image

The effect of wildfire on thermokarst bog development must however be greater than indicated by differences in current coverage of young thermokarst bogs, since much of the currently present young thermokarst bogs developed prior to the fires that occurred 20–30 years ago (Fig. 5b). In order to estimate rates of young thermokarst bog development at each of the four sites after the fire, expressed as percent of total peatland area developed into young thermokarst bogs each year, we made three assumptions. The first assumption was that burned and unburned peatland parts had similar rates of young thermokarst bog development prior to the fire. The second assumption was that young thermokarst bogs persist in the landscape 100 ± 50 years (95% confidence interval) before developing into mature thermokarst bogs (Supplementary Table 5), signifying that no young thermokarst bog currently present developed >150 years ago. The third assumption was that the rate of thermokarst bog expansion is likely to have increased over the last 30 years also in unburned peatland parts, due to ongoing climate change20,21,22 (see Methods for further details). Using these three assumptions, we estimated the rate of young thermokarst bog development prior to the fire, following fire in burned parts, and following fire in unburned parts for each of our four sites (Supplementary Fig. 9). Across our four sites, this analysis indicated that the rate of young thermokarst bog development within peatlands nearly tripled after fire, to 0.28 ± 0.11% yr−1 from 0.10 ± 0.07% yr−1 (95% CI) (Fig. 5b).

Rate of young thermokarst bog development can also be expressed as rates of peat plateau loss, i.e. as a change in young thermokarst bog area over time divided by the sum of peat plateau and young thermokarst bog land coverage. Across the four classified peatland sites, coverage of peat plateaus varied between 50 and 75% (Supplementary Table 2). As such, our estimated rates of peat plateau loss following fire were 0.39 ± 0.18% yr−1 and 0.16 ± 0.12% yr−1 (95% CI) in burned and unburned peatlands, respectively. Our resulting estimated rate of peat plateau loss within unburned peatlands was similar, but in the low range, to what other studies have estimated in the study region using historical image change detection (0.26–0.34% yr−1 peat plateau loss)20,21.

In order to estimate the total area of peat plateau loss due to thermokarst bog development within the study region over the last 30 years, we combined the distribution of peat plateaus, the distribution and timing of fires (Fig. 1), and our estimated rates and uncertainties of peat plateau loss within and outside burned areas (Methods). We assumed that the soils maps and fire maps have negligible errors, and that the effect of fire on the rate of thermokarst bog development lasts 30 years. Our remote sensing analysis cannot rule out any effects beyond 30 years after fire, and as such this is potentially a conservative measure of the effect of wildfire on thermokarst bog development. We estimated the total peat plateau loss to 9800 ± 4100 km2 (95% CI), of which 2200 ± 1500 km2, or ~23%, was directly attributed to the increased rate of thermokarst bog development that occurs following wildfire. This major role of wildfire occurred with wildfire having affected 25% of peat plateaus in the study region during the last 30 years. It is not clear from this study whether the role of wildfire as a driver of thermokarst bog expansion has become relatively more important, as it likely that thermokarst bog development due to both climate warming and increased fire occurrence have increased over the last 30 years compared to earlier periods. Overall, this study shows that wildfire has been a major driver of peat plateau loss due to accelerated thermokarst bog development in boreal western Canada, and thus likely a major cause of permafrost thaw in this region where a majority of the permafrost is found in peatlands.