Field experiments were carried out in the Great Slave Lake region of the Canadian Northwest Territories in summer 2015. This boreal region has a dry, sub-humid continental climate with an annual precipitation of ~300 mm, a wildfire season usually between May and September, and a fire recurrence interval in the region of 150 to 200 years. The forest is dominated by black spruce (Picea mariana) and jack pine (Pinus banksiana) with often thick organic soil layers. The topography is largely flat with an elevation between 100 and 200 m.a.s.l. (Alexander et al., 2004; Stocks et al., 2004).

Two types of PyC and two types of uncharred wood materials were used in the fire exposure experiments (Figures 1A,B):

A–C pieces were 2–3 cm in diameter and 2–4 cm in length. D blocks were 3 × 3 × 3 cm. All were free of bark. Sets were made with one piece of each type (A+B+C+D), and the four pieces were linked together by thin wire and with a metal label added to facilitate relocation after fire (Figure 1B). A total of 30 sets were placed onto the forest floor <24 h prior to exposure to two types of boreal wildfire conditions: a high-intensity crown fire and a low-intensity surface fire (i.e., 15 sets for each fire). Individual pieces were situated at least 5 cm apart (Figure 1A).

K-type thermocouples (≤ 1 mm diameter) were attached directly to the surface of the samples and connected to data loggers (Lascar, Easylog) buried in the adjacent soil to record the temperature at the sample surface-flame interface during the fire (at 10 s intervals). Given the limited number of loggers available, we chose to attach thermocouples to only one type of charcoal (A: wildfire charcoal) on the basis that any differences in temperature/duration data between the two types of charcoal might be expected to be less than between the charcoal and the contrasting types of wood. All B, C, and D samples had thermocouples attached.

Immediately after the fires samples were identified (Figure 1B), thermocouples and loggers removed and samples placed in sealed containers for subsequent laboratory analysis.

(1) The high-intensity crown fire was an experimental fire conducted on 15/6/2015 as part of the Canadian Boreal Community FireSmart Project at Ft. Providence, (61°34'55” N; 117°10'13” W) and aimed at simulating wildfire conditions. The experimental plot (~2,000 m²) was a mature stand of black spruce and jack pine, originating from a stand-replacing wildfire in 1931, with a tree density (live and dead, >1.5 m high) of approximately 5,500 stems ha⁻¹ and an average canopy tree height of 11 m (a typical mature stand height in this boreal region). The understorey was very sparse, the forest floor composed of litter, mosses and lichens, with some down wood present. Fifteen sets of A-D samples (Figures 1A,B) were placed randomly at least 1 m apart from each other within an area of 15 × 15 m. The fire was fast moving with a total burn time of <5 mins and flame lengths >5 m above canopy height (Figure 1C). The overall fireline intensity was estimated as ~8,000 kW m⁻¹, which is within the typical range of high-intensity boreal crown fires in Canada (de Groot et al., 2009). The fire resulted in the homogenous and complete consumption of the canopy, thin branches and the understory, with no unburnt or low severity patches remaining within the plot.

(2) The low-intensity fire was situated ~170 km east of the high-intensity fire site (60°49′38″ N; 114°24′28″ W). It was a slow-moving, surface wildfire (Fire 28, Hay River) affecting a mixed black spruce and jack pine stand of comparable stand characteristics to the high-intensity fire, but with a peaty soil layer of up 50 cm depth. This lightning-caused wildfire burnt across the site on 29/6/2017 during calm atmospheric conditions, advancing at < 5 m h⁻¹. This allowed sufficient time to place samples and thermocouples with loggers ahead of one of the flanks of the fire. Fifteen sets of A-D samples were placed 2 m apart along a transect parallel to the fire front. This surface fire led to the patchy smoldering combustion of the forest floor to a depth of up to 30 cm (Figure 1D), charring of trees to ~1 m height and the “torching” of a few individual trees, with some areas remaining unburned. The fireline intensity was estimated to not exceed 500 kW m⁻¹ based on flame-length/intensity relationships for jack pine forest (Cruz and Alexander, 2010). Due to the patchy nature of this burn, not all samples were exposed to fire and, but in some cases, deep burning of the organic soil layer also led to the destruction of loggers and loss of data.

Further details about the fires are given in Table 1.

The air-dry weight of all samples (N = 120) was determined before and after exposure to fire, and mass loss calculated as:

Of the total 120 samples, 90 had thermocouples attached, and of those, 19 loggers were lost or did not record properly during the fires. A subset of 40 samples (5 of each type A-D for each fire) spanning a representative range of mass losses and temperature/duration records, and pre-fire reference samples (n = 1–5 for each type A–D) were selected for further analyses to allow the potential relationships between mass loss, sample characteristics, and temperature exposure during the fires to be examined.

Statistical analyses were performed with the software IBM SPSS Statistics 20. The level of significance used for all tests was 5% (i.e., α = 0.05). Pearson product-moment correlation coefficients (r) were calculated to test linear correlations between pairs of variables. Variables examined were: mass loss (%), TG-DSC parameters (T50, Q1, Q3), C, N, H contents (%), initial C remaining (%Ci), and Ro (%), as well as maximum temperature recorded on samples (Tmax), time >300°C (s), integral of duration >300°C (°Cs). The threshold of 300°C was chosen because is widely considered the temperature above which charring begins in woody fuels (Drysdale, 2011), the main chemical transformations of biomass-derived materials during heating start around 200–300°C (Keiluweit et al., 2010), and it having been used in previous studies focusing on PyC (e.g., Santín et al., 2016b, 2017).

To analyze the differences in mass loss (%) between sample types, one-factor ANOVA was performed using sample type as the independent factor. Before the ANOVAs, the equality of variances for the studied groups was tested by the Levene's homoscedasticity test and the post-hoc Duncan's multiple range test (groups had equal variances) were performed to identify classes with significantly different means.

The two unburned wood samples used in this study (types C and D) showed the lowest % C, the highest contents of thermolabile biomass and the lowest thermal recalcitrance (Q1 >33%; T50 <420°C), with the cedar wood (D) being more thermolabile than the pine wood (Table 2, Figure 2). In comparison, both charcoal types (A and B) showed much higher C content and thermal recalcitrance, and lower thermolability (Q1 <21%; T50 >447°C). As expected, the slash-pine charcoal (B) presented, by far, the highest C% and thermal recalcitrance of all the samples before fire (Table 2).

Considering data from both fires together (N = 101), wildfire charcoal (type A), exhibited a significantly higher mean mass loss (66.5 ± 25.2%) compared to slash-pile charcoal (Type B, 41.7 ± 27.2%). Pine wood (type C) showed higher mean mass losses (78.2 ± 14.9%) compared to both charcoal types A&B, and cedar wood (type D) the greatest losses (83.8 ± 18.9%). However, given the high variability in the data, the differences between mean mass losses between the groups of wildfire charcoal, pine wood and cedar wood were not significant.

For the combination of all the samples with temperature data associated (N = 71) and of the three fire parameters analyzed, T_max showed the strongest positive correlation (r = 0.64) with mass loss (Table 3, Figure 3). Correlation between mass loss and the integral >300°C was also significant, but weaker (r = 0.48). No significant correlation was found between mass loss and time >300°C (r = 0.33) (Supplementary Table 1).

When comparing the two fires (Figure 4), the mean maximum temperature (T_max) recorded on all samples in the high-intensity fire was 855°C (range 661–1005°C; N = 39), whereas in the low-intensity fire it was only 434°C (range 40–792°C; N = 32). Indeed nearly all T_max values recorded on samples in the high-intensity fire exceed those of low-intensity fire (Figure 4). The high-intensity fire also exhibited longer mean residence times (>300°C) than the low-intensity fire, although these differences between fires were less pronounced (Figure 4). Mean sample exposure durations >300°C were 245 s (range 70–570 s) and 142 s (0–620 s) for the high- and low-intensity fire respectively, and the mean integrals >300°C were 53,656°C s (range 20,689–122,798°C s) and 18,589°C s (range 0–99,327°C s) for these respective fires. All samples exposed to the high-intensity fire, and most samples exposed to the low-intensity fire experienced mass losses, with these mean losses being significantly higher in the wood pieces (80.7 ± 16.8%; n = 49) than in charcoal pieces (54.7 ± 28.5%; n = 52) and in the high-intensity than in the low-intensity fire (80.4 ± 18.6%; n = 47 vs. 53.7 ± 27.4%; n = 54). These significant differences in losses between fire types agree with their contrasting temperature records (Table 4, Figure 4). Regarding the differences in mass losses between the two fires for the different sample types, mass losses were consistently smaller in the low-intensity than in the high-intensity fire and in both cases following the order D>C>A>B (Figure 5). The variability of these losses was higher in the low-intensity fire in all cases except type D, the slash-pile charcoal (Figure 5).

Results of the elemental-, DSC- and reflectance analyses for each fire are given in Table 2, and DSC curves for each sample type in Figure 2. As highlighted before, the DSC analyses show that, prior to the fires, the slash-pile charcoal was the most thermally recalcitrant (i.e., least thermolabile) material used in this experiment (mean T50 = 510°C) followed by wildfire charcoal (447°C), pine wood (417°C), and cedar wood (395°C). This order is reflected also in the increasing values for Q1 and decreasing values for Q3 (Table 2).

After sample exposure to fire, the same order is retained, but with substantial increases in thermal recalcitrance for the wood samples following charring, and only modest increases for the charcoal samples (Table 2, Figure 2). These increases in recalcitrance were greater in the high- than the low-intensity fire, which matches the greater thermal exposure of the samples in the former (Table 4).

The C concentration also increased in all sample types following fire exposure, but this change was much more pronounced for the wood samples than charcoal samples (Table 2). Given that exposure to fire does not only lead to mass- and therefore C losses through combustion, but also to relative enrichment in C in the remaining PyC through pyrolysis, we also calculated the % of PyC remaining in relation to the total C content of the samples prior to exposure (Table 4). Mean values of PyC% remaining were, respectively for high- and low-intensity fires: 26.2 vs. 48.3 (wildfire charcoal), 44.8 vs. 80.8 (slash charcoal), 15.7 vs. 54.2 (pine wood) and 8.2 vs. 30.8 (cedar wood). As highlighted in the methodology, for the fully charred wood samples C&D, this parameter represents the fuel C to PyC conversion rate used in Santín et al. (2015). It is interesting to note that the low-intensity fire resulted in a higher mean PyC production ratio (54.2 and 30.8%) than the high-intensity fire (15.7 and 8.2%) for the pine and cedar wood samples respectively.

Regarding N and H concentrations, N increased and H decreased after fire for most sample types (Table 2). Charcoal reflectance (Ro) following the fires was highest for slash-pile charcoal with values for the other sample types being around half this value (Table 2). Values following the high-intensity fire overall exceed those following the low-intensity fire with respectively mean Ro values of 2.80 vs. 2.37 (slash charcoal), 1.73 vs. 0.92 (wildfire charcoal), 1.47 vs. 0.97 (pine wood), and 1.46 vs. 1.33 (cedar wood).

When considering all samples exposed to the fires together, significant correlations were found between some sample chemical and reflectance characteristics, and with fire properties (Table 3). For example, T50 showed positive correlations with C% (r = 0.91), Ro (r = 0.72) and a negative correlation with mass loss (−0.38) and with H% (r = −0.86). C% correlated with Ro (r = 0.72) and mass loss (−0.42). Maximum temperature at the fuel-flame interface (T_max) showed not only a positive correlation with mass loss (r = 0.64) as presented in more detail above, but also with Ro (r = 0.46) and H% (r = −0.49). The duration integral of exposure >300°C was similarly well correlated to Ro (r = 0.55), and weaker (but still significantly) correlated to mass loss than T_max (r = 0.48). All other relationships between fire characteristics, including exposure duration >300°C, showed weaker and non-significant relationships with sample chemical and reflectance characteristics (Table S1).

The only two previous studies examining PyC recombustion under field conditions reported mean mass losses of <8% for charcoal pieces of ~0.5–1.5 cm (Saiz et al., 2014) and of 23% (median value <15%) for charcoal pieces of 2–4 cm (Santín et al., 2013). In the current study, mass losses were substantially higher except for the slash-pile charcoal exposed to the low intensity fire, which had a mean loss of 17.0% (median 11.8%) (Table 4). In the high-intensity fire, slash-pile charcoal showed mean losses of 63.6% (median 58.6). Wildfire charcoal exhibited losses of 84.0% (median 81.9%) and 49.7% (median 58.6%) in the high- and low-intensity fires, respectively (Table 4). Thus, all losses recorded here across the two types of PyC and fires (i) exceed those previously reported from in-situ field studies and, except for the slash-pile charcoal in the low intensity fire, (ii) exceed also the mean losses of 36% for PyC fragments >6 mm reported in the laboratory study by Tinkham et al. (2016). The data presented in Table 1 allows comparison of the current study with fire and PyC characteristics in these previous studies.

The comparatively low losses in the study by Saiz et al. (2014) (<8%; Table 1) could be a reflection of a relatively high thermal resistance of their PyC combined with a low capacity of the fire to lead to PyC combustion. The source material, Acacia aneura, has roughly twice the density (1 g cm⁻³) of pine wood and was produced in a slash burn, which typically have much longer flame residence times than wildfire and therefore lead to a greater thermal degradation resistance of the charcoal produced than in a wildfire (Massman et al., 2008). The fire, although considered intense in the context of savanna fires, had the lowest fuel load and highest wind speed of all experimental fires listed in Table 1 and hence is likely to have led to a burn of comparatively low intensity and flame residence time.

The experimental crown fire examined by Santín et al. (2013) was carried out in the same fuel complex and study region as the current study and used the same source of slash-pile charcoal. Fuel load, atmospheric conditions and fireline intensity (~8,000 kW m⁻²) were also broadly comparable to the high intensity fire of current study (Table 1). The main difference between these two studies is that, in the current study, PyC pieces were placed on the forest floor surface, whereas in Santín et al. (2013) they were placed at ~2 cm depth in the forest floor layer. In the 2013 study, this could have resulted in the initial protection from thermal exposure of the charcoal pieces until the litter layer itself was consumed to this depth. This may have contributed to the substantially lower combustion of the slash-charcoal of 23% reported by Santín et al. (2013) compared to the 64% in the high-intensity fire of the current study (Table 1).

The mass losses of both slash-pile charcoal in the high-intensity fire and of wildfire charcoal in both fires of the current study are substantially greater than the 36% found in the laboratory burns of Tinkham et al. (2016) (Table 1). Their experiment, however, is not directly comparable to the current study not only due to it being carried out under laboratory conditions using masticated fuels. They also considered only PyC pieces >6 mm and within those, only the fraction that is resistant to chemical degradation based on the CTO375 method (Hatten and Zabowski, 2009). In addition, the sample placement on the litter surface, the substantially higher fuel loads and warmer and drier atmospheric conditions in the current study are all more conducive to a greater combustion completeness than those in the laboratory burn by Tinkham et al. (2016).

Our research design allowed us to link PyC losses directly to temperature recorded on the fuel-flame interface of the respective PyC pieces. That mass losses show a reasonably strong correlation with T_max (r = 0.65; Table 3) is perhaps not surprising. The thermocouple tip (≤ 1 mm diameter) reflects the temperature at the fuel-flame interface, which is influenced by the energy received from the surrounding combustion combined with that released from the burning PyC piece itself. T_max could therefore be expected to be at least somewhat related to the mass loss of a PyC piece, with higher temperatures reflecting greater losses. What may be surprising is that mass losses are only weakly correlated with exposure duration >300°C (r = 0.33, not significant; Table S1). Three Hundred degree is widely considered the threshold above which charring begins in woody fuels (Drysdale, 2011) and hence the duration above this threshold could be expected to correlate more closely with mass loss than T_max. Given that the correlation between mass loss and the integral >300°C was stronger (r = 0.48) than for time >300°C (and significant), suggests that the faster rate of combustion associated with the process that generates higher temperature readings in thermocouples is more important in the overall mass loss process observed here than the total time during which mass loss occurs. This notion is supported by a study examining organic matter in a soil block affected by experimental laboratory burns, where the time >300°C also showed no correlation with C loss (Merino et al., 2018).

The fact that the samples exposed to the high-intensity fire recorded substantially higher mean values for T_max, the time integral (s) >300°C and exposure duration >300°C than the lower intensity fire (Table 4) provide not only an additional set of measures confirming the contrasting intensities of these fires. It also suggests that once combustion of individual PyC pieces begins, the greater rate of thermal energy release during the more intense fire facilitates a greater rate of loss of individual PyC pieces. This, together with the fact that exposure duration >300°C showed only a weak (and not significant) correlation with mass loss (Table S1), suggest that the energy released from the combustion of the surrounding fuel may be a more important factor in PyC mass loss than that released from the combustion of a PyC piece itself.

Our data also allows PyC losses in a fire to be evaluated in relation to the specific PyC characteristics. The importance of considering PyC type when evaluating potential losses in fires is clearly highlighted by our finding that slash-pile charcoal, which exhibits higher thermal degradation resistance according to the DSC analysis (Table 2), indeed exhibited a significantly lower mass loss under actual wildfire conditions (41.7 ± 27.2%; mean of both fires combined) compared to wildfire charcoal (66.5 ± 25.2%). This supports the speculation made by Santín et al. (2013), that the slash-pile charcoal placed in their fire could be expected to be more thermally resistant and may therefore have given lower rates of losses than if wildfire charcoal had been used. Prior to the new fire exposure, the slash-pile charcoal was more thermally recalcitrant (mean T50 = 510°C) than wildfire charcoal (447°C; Table 2) due to its origin in a pile burn, which generates prolonged burning conditions with restricted oxygen supply compared to typical wildfires (Santín and Doerr, 2016). This difference in thermal resistance is also reflected in the higher C content and lower values of Q1, and higher values of Q3 for the slash-pile compared to the wildfire charcoal (Table 2). Following fire exposure, both charcoal types exhibit an additional, albeit small, increase in mean values of T50, C%, and Q3 (Table 2, Figure 2). This suggests that the exposure of this charcoal to a new fire has led to its further increased thermal recalcitrance, particularly in the high-intensity fire, which can be expected to lower the susceptibility to combustion of this remaining PyC in a subsequent fire. Unsurprisingly, the wood pieces that were exposed to the high-intensity fire showed greater mass loss than in the low-intensity fire.

It is noteworthy that the low-intensity fire led to a higher PyC production rate than the high-intensity fire (as reflected by percentage PyC remaining for the wood samples; Table 4). This could suggest that more intense fires produce less PyC in relation to fuel consumed and hence emit a greater proportion of C to the atmosphere than is converted to PyC and remains in situ for a given amount of fuel affected by burning (Santín et al., 2015).

The significant correlations between chemical characteristics of samples, and their relationship with fire properties (Table 3), are also noteworthy. For example, they provide valuable evidence from real wildfires that some chemical parameters, which are often utilized to indicate thermal and chemical recalcitrance (C%, T50, Q3) are well correlated (Table 3) and indeed provide useful indicators of the resistance of PyC materials to combustion in field-scale fire conditions.

Furthermore, our data support the notion that charcoal reflectance (Ro) can be related to post-fire chemical recalcitrance parameters such as resistance to dichromate etch and the size of polyaromatic units remaining used in other studies (Ascough et al., 2010, 2011). The significant correlations relationships of Ro% with T50 (r = 0.72) and C% (0.77; Table 3), as well as with other DSC parameters (Table S1), suggest that Ro may be a useful indicator of charcoal resistance to thermal degradation. This is important because it implies that PyC exhibiting higher Ro% will not only be more chemically inert in the post-fire environment (e.g., Ascough et al., 2010; Belcher et al., 2018), but also that such PyC will be more resistant to thermal degradation in subsequent fires. The potential of this additional metric for exploring charcoal recalcitrance, its resistance to thermal degradation in future wildfires, and more generally fire effects on C storage in wood and charcoal, is explored in more detail in Belcher et al. (2018). The results also highlight that the DSC analysis methods used here provide a relatively rapid and economic approach for the assessment of the resistance of PyC to removal from subsequent fires.

The data presented here add to the still small, but growing body of evidence for the relevance of fire as a removal mechanism of any PyC exposed at the ground surface. Whilst experimental conditions and PyC analysis methods are not fully comparable across the four studies examining this process experimentally to date (Table 1), the current study stands out with the highest overall PyC losses from recombustion reported to date. The mean losses of 82 and 56% for wildfire charcoal in the high- and low-intensity boreal wildfires found here respectively, suggest that fire can indeed be a significant removal mechanism for any PyC that was produced in previous fires and remains exposed at the ground surface. These values, however, should be considered as a being in the extreme upper range of what might be expected under natural conditions for the following reasons. First, the PyC pieces were placed on the surface and not inside the litter layer. Under most natural situations, PyC would be subject to bioturbation and burial in the soil or within the accumulating litter, resulting in reduced exposure to thermal energy and hence combustion. This may be particularly relevant in the North American boreal region where the typical intervals between fires is relatively long, ranging from 150 to 200 years (Stocks et al., 2003; Balshi et al., 2009). Second, the pieces used here were within a size range of 1–3 cm, which represents only a fraction of the full size-range of PyC in the environment (Santín et al., 2015). For example, PyC present on the surface of larger pieces of down wood or standing timber could be expected to show lower losses due to their relatively smaller fuel surface area/volume ratio. To further improve our understanding of the role of wildfire as a removal mechanism of PyC, we need not only more experimental data on PyC recombustion from different types of fires and environments, but critically also a better understanding of the fate and movement of PyC between fires [see review by (Abney and Behre, 2018)] in order to assess how much of the PyC produced in a given fire is likely to be exposed to combustion in a subsequent fire.

What this study has demonstrated for the first time under field conditions is (i) the contrasting resistance of different types of PyC to combustion even when produced from the same material (in this case pine wood), and (ii) the contrasting net PyC losses different types of fire can cause. The significantly greater losses found for wildfire PyC compared to slash-pile PyC found here also match what would be expected based on the chemical and reflectance characteristics of the PyC types used here (Table 2) and supports the usefulness of these characteristics in reflecting thermal degradation resistance under field conditions.

The results of this study also demonstrate that wildfire charcoals do present different chemical characteristics and resistance to combustion than other types of man-made PyC such as the slash-pile charcoal used here. The latter may be more comparable to the more degradation resistant charcoal produced in kilns or, to some extent, also to biochar than to wildfire charcoal, confirming the need for considering their direct comparability with caution as highlighted by Santín et al. (2017) in their study comparing wildfire charcoal and biochar. This is especially relevant in the context of the C sequestration abilities of different PyC materials (Santín et al., 2017).

Based on the results presented here, it could be argued that more intense flaming fires that lead to higher temperatures at the fuel-flame interface will produce new PyC of higher recalcitrance compared to lower-intensity (and “cooler”) flaming fires. However, as also seen here, more intense flaming fires are likely to also produce less PyC in relation to fuel consumed, but (ii) potentially also consume overall more of any old PyC that may be available for combustion. If these notions prove to be widely applicable, they have implications for the accounting of PyC production and fate in local- to global-scale carbon budget modeling of fires.

In this context, it is important to consider that deep-burning smoldering fires, that would also be considered as low-intensity, would not fit this pattern as they lead to relatively complete combustion of the affected fuel (Rein, 2013), which would include any PyC present. To what degree fire intensity (or indeed fire behavior in general) and other factors such as fuel characteristics, PyC redistribution and its breakdown in the landscape between fires, influence (i) the total amount of PyC present in the landscape and (ii) its resistance to biotic and abiotic degradation, remains poorly understood today. Recent work by Hudspith et al. (2017), for example, has highlighted important differences between specific fuel types in how much charcoal they produce under comparable laboratory burn conditions. Further work representative of a range of environmental and fire behavior characteristics is clearly needed to provide a more complete understanding of the production and fate of PyC, and its role in the global carbon cycle. Developing this understanding is particularly relevant given that human-induced climate- and land cover changes are leading to large-scale changes in fuel characteristic, fire occurrence, and fire severity (Jolly et al., 2015; Abatzoglou and Williams, 2016). These shifts will affect the overall role of fire in the global carbon balance, which in turn will be affected by the balance of PyC production and its fate (Bird et al., 2015; Santín et al., 2016a).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer, RL, declared a shared affiliation, with no collaboration, with one of the authors, CB, to the handling editor at time of review.