Shallow peat is most vulnerable to high peat burn severity during wildfire

The study area is in the UNESCO Georgian Bay Biosphere Reserve (GBBR) in Ontario, Canada. Over 11 000 ha of the GBBR burned between July 18 and October 31, 2018 in the Parry Sound #33 wildfire (figure 1). The area is characterized by granite bedrock ridges and valleys, with abundant bedrock depressions of various depths along the ridges. The low bedrock permeability in these depressions supports peat accumulation, where a thin layer of organic/mineral soil is overlaid with peat and a living moss layer (typically Sphagnum spp.; Moore et al 2018). The rock barrens hydrogeological setting allows for frequent hydrological isolation of peat deposits of varying depth; where hydrological connectivity is dependent on fill-and-spill events, when water inputs exceed the storage capacity of the depression (Spence and Woo 2003). The surface cover of the surrounding bedrock ridge tends to consist of exposed bedrock, moss (Polytrichum spp.) cushions, and lichen (Cladonia spp.) mats, while the intervening valleys more commonly consist of deeper mineral soil, open water ponds, or deep peatlands. Common tree species in the area include tamarack (Larix laricina), black spruce (Picea mariana), white pine (Pinus strobus), and jack pine (Pinus banksiana).

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The region may have previously experienced low-severity cultural burning, however, there are no large wildfires documented in recent records for this area (records since 1950 for the Ontario Shield ecozone; Stocks et al 2002, Natural Resources Canada 2018). The climate is humid continental with a relatively drier summer (June–August) compared to wetter shoulder seasons (Environment Canada 2019). In 2018, the study area received approximately 130%, 30%, and 46% of the average precipitation for April, May, and June, respectively, compared to 1981–2010 climate normals (Environment Canada 2019). At the time of wildfire ignition (July 18, 2018) the area had received only 0.25% of the typical precipitation for July (Environment Canada 2019).

Our research was carried out at nine different sites within the Parry Sound #33 fire footprint (figure 1(A)). Peat burn severity was analysed using a stratified-random approach to ensure the distribution of both peat depths and DOB on the landscape was captured, based on a priori knowledge of the unburned landscape. Each site was visually divided into two landscape units defined as: (i) the peatland; subdivided into the peatland middle (P_mid) and peatland margin (P_marg, along the perimeter of the peatland), and (ii) the upland; forested areas on the adjacent upland rock barrens, with peaty soils and a thin, underlying mineral soil layer. Hence each 'site' contains a peatland middle, peatland margin and upland landscape unit that are referred to by their respective abbreviation, given above, and site ID (table S1 (available online at stacks.iop.org/ERL/15/104032/mmedia)). A range of peatland areal extents were assessed, including three sites containing relatively large peatlands (0.62–1.11 ha), three with medium peatlands (0.14–0.20 ha), and three with small peatlands (0.03–0.06 ha), where maximum peat depth in peatland middles was ⩾0.4 m and peat depth in peatland margins was typically, but not always, <0.4 m. Uplands had variable depths, usually <0.4 m. Dominant tree species were tamarack, black spruce and jack pine for P_mid, P_marg, and upland landscape units, respectively.

At each site we randomly selected 10 plots in each of the P_mid, P_marg, and upland landscape units for a total of 270 plots across the nine sites. DOB was estimated 7 months post-fire by assuming that the pre-fire surface between two reference points (tree roots and/or surfaces unaltered by the fire) was flat prior to fire. Post-fire vegetative recovery was minimal during this time, which was predominantly winter months characterised by the presence of frozen ground and a snowpack. Further, one to two year time lags in ground cover re-establishment are typical in northern peatlands (Benscoter and Vitt 2008; Lukenbach et al 2015a). The difference between the burned surface and the estimated pre-fire surface was taken to be the DOB, similar to several studies (e.g. Kasischke and Johnstone 2005, Mack et al 2011). Tree root reference points were calibrated by determining the average depth of peat and/or the live moss layer above roots in two unburned sites located approximately 1.5 km from the study sites, similar to Lukenbach et al (2015b). A total of five DOB measurements were made in each plot, n = 150 per site (1350 total). We also made three remnant peat depth measurements per plot using a peat probe, n = 90 per site (810 total). We estimated pre-fire peat depth in each plot by summing the average post-fire peat depth and average DOB, n = 30 per site (270 total) and this was used to calculate the proportion of the peat profile that burned.

To demonstrate how combustion impacts peatland carbon balances, the median value of peatland DOB measurements (P_mid and P_marg) was applied across the areal extent of each peatland giving an estimate of the total mass of carbon combusted from each peatland. Carbon loss was estimated using peat bulk density and organic matter content (0.05–0.1 m depth increments) from peat cores at nearby unburned peatlands (table S2). Organic matter was converted to organic carbon using the 42.3% estimate for Sphagnum peat from Loisel et al (2014). The areal extent of each of the nine peatlands was digitized in ArcGIS 10.6.1 using 0.2 m resolution imagery from the 2016 Central Ontario Orthophotography Project (COOP; OMNRF, 2016).

WTD was continuously monitored at 14 unburned sites (peatlands to shallow peat deposits) located ∼60 km south of the burned sites. The unburned sites are also located within the rock barrens hydrogeological setting and experience similar hydroclimatological conditions to the burned sites (figure 1(B)), where WTD is responsive to changes in short-term climate due to isolation from large-scale groundwater systems (Spence and Woo 2003). Water table measurements were taken for the period between the end of snow melt (April 6, 2018) and the beginning of the wildfire (July 18, 2018). Solinst Levelogger pressure transducers recorded WTD in 0.05 m diameter PVC wells installed in the deepest part of the unburned sites, typically reaching bedrock at the base. The water table drawdown rate (mm day⁻¹) for all 14 unburned sites was calculated for the longest rain-free period preceding the wildfire from May 26 to June 12, 2018.

All statistical analyses were conducted in R 3.6.1 (R Core Team 2018). Linear regressions were conducted to examine relationships between pre-fire peat depth and DOB, percent burned, and percent carbon loss. A breakpoint analysis using a piecewise linear regression (segmented—R; Muggeo 2008) was conducted to determine the pre-fire peat depth at which the slope of the linear relation between the percent burned and pre-fire peat depth changed. Data tended to be non-normal, hence, non-parametric Kruskal–Wallis (KW) tests were conducted to determine whether the median DOB and percent burned differed significantly between the P_mid, P_marg and upland, or between plots deeper and shallower than the identified breakpoint. Similarly, KW tests were employed to test the difference between water table drawdown rates in unburned sites. Median values ± interquartile range are displayed unless stated.

DOB was significantly different between P_mid, P_marg, and upland landscape units (χ² = 147.42, df= 2, p < 0.001). DOB values were 0.02 ± 0.06, 0.18 ± 0.09 and 0.12 ± 0.07 m for P_mid, P_marg and upland, respectively. P_marg had the largest range of DOB values (0.002–0.40 m), while P_mid was the only landscape unit which contained any plots with DOB values of 0 m. Relationships between DOB and pre-fire peat depth varied between the landscape units (figure 2). In the upland, DOB (m) and pre-fire peat depth (m) had a strong positive correlation (DOB = 0.59(peat depth) + 0.02, adjusted R² = 0.65, p < 0.001). In contrast, in P_mid DOB decreased as pre-fire peat depth increased (DOB = −0.05(peat depth) + 0.08, adjusted R² = 0.13, p < 0.001) and P_marg showed no significant relationship between DOB and pre-fire peat depth (p = 0.42). However, when analysing all landscape units together, DOB (m) was found to decrease significantly with increasing pre-fire peat depth (m) (DOB = −0.11(peat depth) + 0.17, adjusted R² = 0.19, p < 0.001) (figure 2).

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The proportion of the profile that burned (percent burned) varied significantly between the landscape units (χ² = 164.1, df = 2, p < 0.001), where uplands had the highest percent burned (73.1 ± 20.2), followed by P_marg (61.3 ± 35.7), and P_mid had the lowest (2.4 ± 8.3). Within the peatland (P_mid and P_marg), the percent burned decreased significantly with increasing pre-fire peat depth (for n = 8/9 sites) (table S3), differing in slope based on their pre-fire peat depth distributions (figure S1). A piecewise linear regression (adjusted R² = 0.75, p < 0.001) showed that the percent burned (%) decreased sharply with increasing pre-fire peat depth (m) (percent burned = −129.43(peat depth) + 95.56) until a breakpoint of 0.66 ± 0.04 m pre-fire peat depth, where, whilst still significant, this relationship changed (percent burned = −14.01(peat depth) + 19.88; figure 3). A piecewise linear regression model was selected over a general linear regression based on Akaike information criterion values that were 2271.7 and 2355.6 for piecewise and generic linear regressions, respectively. Percent burned was significantly greater at plots where pre-fire peat depth was below the breakpoint (i.e. <0.66 m; χ² = 117.8, df = 1, p < 0.001). Median percent burned was 2.2 and 65.1% for peat deeper than and shallower than the breakpoint, respectively.

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Mean (± std) carbon loss from the peatlands (P_mid and P_marg) was 1.61 ± 0.97 kg C m⁻², ranging from 0.24–3.12 kg C m⁻². The percent carbon loss (%) decreased significantly with increasing median peat depth (m) (carbon loss = −48.39(peat depth) + 40.76, adjusted R² = 0.48, p < 0.05). Site 111, with a median peat depth of 0.26 m (maximum 0.4 m), had the greatest proportional carbon loss, losing an estimated 45% of its total stored organic carbon (figure S2). The site with the deepest peat (site 102), with a median peat depth of 0.98 m, had the third lowest percent carbon loss, with an estimated 2.0% of its total stored carbon being combusted (figure S2).

Unburned sites with a maximum peat depth shallower than the breakpoint (0.66 m) had water tables near the base of the peat profile or not present at the time of the wildfire (figure 4). Conversely, unburned sites with a maximum peat depth greater than the breakpoint had water tables present in the peat profile (figure 4). For the May 26–June 12 rain-free period the rate of water table drawdown (mm day⁻¹) decreased with increasing maximum peat depth (m) per site (figure 5). A log-linear curve describes the relationship reasonably well (drawdown rate = −8.788ln(max peat depth) + 13.560; R² = 0.702) and water table drawdown rate was significantly greater for peatlands that lost their water table prior to the start of the wildfire (maximum peat depth below breakpoint) compared to those which did not (χ² = 8.0, df = 1, p < 0.01; maximum peat depth above breakpoint), over the same time period.

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Our results support our hypothesis that shallower peat is more vulnerable to high peat burn severity, where DOB decreases with increasing pre-fire peat depth, albeit with variability (figure 2). We identified a threshold pre-fire peat depth of 0.66 ± 0.04 m, where profiles deeper than this were generally resistant to deep combustion (figure 3). In peat deeper than the threshold, median percent burned was 2.2% and 27/67 plots had an average DOB of <0.01 m. Conversely, in peat shallower than the threshold we recorded up to 98% of the profile burned in some plots, where the median was 65%. Although vertical peat combustion is typically moisture-limited rather than fuel-limited (Usup et al 2004, Prat-Guitart et al 2016), our results show that shallow peat deposits can burn through almost their entire profile, suggesting that the combustion of the majority of the organic matter present was the cause of the cessation of the smouldering reaction in the shallowest peat (see 1:1 line; figure 2), similar to shallow peat profiles studied by Miyanishi and Johnson (2002). Shallow peat deposits have also been found to burn deeply in the transitional margin ecotones of peatlands in the Boreal Plains, Alberta (Hokanson et al 2016), and in Alaska with up to 86% of the pre-fire organic soil profile lost to combustion (Walker et al 2019). However, a threshold of pre-fire peat depth describing the vulnerability of peat to severe burning has not previously been established.

The resistance of deeper peat to combustion may be due to the increased water storage capacity of deeper peat deposits (Price and Schlotzhauer 1999), and the ability of deeper peatlands to maintain a water table within the peat profile during the summer dry period. In fact, on the day the 2018 Parry Sound #33 wildfire started, we found that only unburned sites with a maximum peat depth deeper than the threshold had a water table within the peat profile, whereas shallower sites had water tables below their peat profiles (figure 4). The strong capillary forces in peat enable water to be transported from the water table towards the surface (Mccarter and Price 2014) whilst the water table is less than ∼0.8 m deep (Lukenbach et al 2015a), maintaining hydrostatic equilibrium in the peat profile. When the WTD increases past this depth, or is lost from the peat profile, water content in the upper layers can become decoupled from the water table (e.g. Lukenbach et al 2015a) causing surface peat to be rapidly driven towards residual water content (Moore and Waddington 2015, Dixon et al 2017).

Maintaining moisture in the peat profile, especially the near-surface, is crucial to limit the ignition of peat and propagation of the smouldering reaction (Van Wagner 1972, Frandsen 1987, Benscoter et al 2011, Prat-Guitart et al 2016). Because of the high latent heat of vaporisation of water, peat moisture acts as the main energy sink in the combustion reaction (Benscoter et al 2011) where water must be evaporated before the peat fuel is heated to combustion temperature, enabling the propagation of smouldering (Van Wagner 1972). The critical peat moisture content limiting peat smouldering has been found to vary between 125 and 395 g g⁻¹ where samples with high peat density experienced smouldering at a higher moisture content (Rein et al 2008, Benscoter et al 2011). Under continued atmospheric demand the loss of the water table in shallow peat (figure 4) will cause decreasing moisture content in near-surface peat and increase peat smouldering potential (Van Wagner 1972, Rein et al 2008, Benscoter et al 2011).

In nearby unburned peatlands, we found that water table drawdown rate (mm day⁻¹) decreased with increasing maximum peat depth during a rain-free period preceding the Parry Sound #33 wildfire (figure 5). By extension, the regulation of water table drawdown in deeper peatlands in the burned landscape likely helped maintain a water table within the peat profile, leading to lower peat burn severity. Higher water table drawdown rates in shallow peatlands and peat deposits could be a result of lower Sy due to greater decomposition. Frequent oxidation of the peat profile likely increases peat decomposition (Freeman et al 1993, Strack et al 2006) leading to increased peat bulk density that is associated with decreasing specific yield (Sy) and greater water table fluctuations, promoting further decreases in Sy (Waddington et al 2015). Similar mechanisms have been suggested to increase the smouldering potential of peat in the margins of treed peatlands in the Boreal Plains (Wilkinson et al 2019). Because higher density peat can burn at a higher moisture content (Frandsen 1987, Rein et al 2008, Benscoter et al 2011) this could help further explain the high peat burn severity recorded across this landscape, although specific pre-fire peat characteristics are unknown for these burned sites.

Despite the prevalence of shallow peat in the study sites, average carbon loss from the peatlands (1.61 ± 0.97 kg C m⁻²) was within the typical range of carbon loss from northern peatlands (e.g. Benscoter and Wieder 2003, Hokanson et al 2016). However, the impact on the carbon balance is dependent on net carbon accumulation rate, which for northern peatlands is ∼0.029 kg C m⁻² yr⁻¹, but is strongly controlled by the rate of decomposition (Gorham 1991). Moreover, due to the shallow depths of peat deposits and the high percent of the profiles experiencing combustion (figure 3), the proportion of pre-fire carbon combusted from individual peatlands (max = 45%; figure S2) was much higher than other undisturbed northern peatlands (e.g. 2.8%; Wilkinson et al 2018), and more similar to heavily-drained peatlands (e.g. 51%; Wilkinson et al 2018). Furthermore, the exposure of mineral soil promotes the recruitment of deciduous species and non-peat-accumulating vegetation (Johnstone and Chapin 2006) that are detrimental for the re-accumulation of long-term carbon stores and can result in an altered wildfire regime (Kettridge et al 2015). Overall, we find that deeper peat is more resistant to high peat burn severity and excessive carbon loss, owing the ability to regulate water table position within the peat profile to a number of complex and interacting ecohydrological feedbacks. This is an important finding given the extensive distribution and cover of shallow peat across regions experiencing both natural and managed fire regimes (Jaenicke et al 2008, JNCC 2011, Vompersky et al 2011).

We suggest that peat depth can act as a key indicator for vulnerability of peat deposits to high peat burn severity. In-situ measurements of peat depth do not require continuous monitoring, such as precipitation or WTD, making it an efficient and simple indicator to be used in fire vulnerability mapping as part of broader fire management strategies. Additionally, peat depth can be used conceptually to identify the types of peatlands and peat deposits which may be at the greatest risk of not maintaining their carbon sink function over successive fire disturbances (cf Ingram et al 2019;, Walker et al 2019), such as: younger and/or slow-accumulating peatlands (Vardy et al 2000), recently restored peatlands (Granath et al 2016), and moss and lichen mats on the upland rock barrens (Moore et al 2018). Regional and pan-regional risk assessments based on peat depth will require advances in peat depth mapping techniques by utilizing remote sensing approaches, leading to improved estimates of northern peat depths, which are currently limited (Yu 2012).

The identification of the pre-fire peat depth threshold allows for a classification of fire risk based on peat depth where, in this region, deposits >∼0.7 m deep are at lower risk of high peat burn severity, and peat deposits shallower than this have a higher risk of peat smouldering leading to high peat burn severity. Given that our pre-fire peat depth threshold for peat resistance to deep combustion is controlled by a number of interacting factors, we suggest that while the specific resistance threshold will vary between ecozones and landscapes due to the interactive effects of hydroclimate, hydrogeological setting, and land-use and land-cover change (e.g. Hokanson et al 2018, Wilkinson et al 2019), this 'survival of the deepest' concept is widely applicable. We encourage further field research and coupled hydrological and combustion modelling (e.g. Thompson et al 2019) studies to examine peat burn severity thresholds in other landscapes and climates.

In addition to improving the targeting of fire management strategies, these results also highlight the value of proactive restoration. Restoration efforts should be designed to accumulate peat quickly, particularly through the establishment of Sphagnum moss (Granath et al 2016). Efficient methodologies have been developed to successfully restore peatlands after harvesting and drainage, i.e. the moss layer transfer technique (Rochefort et al 2003), with more recent work focusing on shallow peat deposits (Grand-Clement et al 2015). Furthermore, we suggest that similar practices could be applied to burned peatlands to promote a quicker return to a carbon-accumulating system and to limit future smouldering potential (Granath et al 2016), thereby supporting a return to a net carbon sink status and protecting globally significant carbon stores.