Thomas Duchnik Hessilt

Major advances have been made in understanding carbon emissions from boreal forest fires, yet substantial knowledge gaps remain for tundra fires. Tundra ecosystems are increasingly susceptible to fire activity due to climate change, posing a threat to their large organic soil carbon stocks and the permafrost they insulate. Additionally, studies assessing remote sensing fire severity metrics in tundra landscapes are limited. Here, we report carbon (C) combustion estimates from two large tussock tundra fires (1048 km²) that occurred in Southwest Alaska, USA, in 2022. We quantified above-and belowground carbon stocks and combustion (kg C m⁻²) at 45 field plots (36 burned, 9 unburned) one year post-fire. Soil burn depth was determined using paired moss patches, and aboveground carbon stocks were calculated using allometric equations for shrubs and tussocks. Fire severity was estimated using the field-based geometrically structured Composite Burn Index (GeoCBI) at these 45 plots and at 41 additional plots where only GeoCBI was measured. We upscaled our field-based findings using the differenced normalized burn ratio (dNBR) derived from Sentinel-2 imagery. Tussock tundra landscapes emitted an average of (± standard deviation) 1.59 ± 0.55 kg C m⁻², consuming 11% of the total pre-fire carbon stock. The majority of the loss (75%) came from belowground stocks, with an average burn depth of 6.9 ± 2.1 cm. By scaling carbon combustion with the dNBR ( R² = 0.42, p < 0.001), we estimated total carbon emissions from tussock tundra across both fires at 0.81 ± 0.22 Tg. GeoCBI estimates were moderately correlated with the dNBR ( R² = 0.63, p < 0.001) across vegetation types. This work provides important information on the impacts of fires on the carbon balance of tundra ecosystems and demonstrates that dNBR, a well-established remote sensing proxy for forest fires, is also effective for mapping fire severity in tundra landscapes.

Fire is globally one of the main disturbance agents that regulate and shape ecosystems and this has been so for millennia. Globally, there has been a discrepancy between the global emergence of extreme fire weather from a warming climate which promotes fires and a recent decrease in burned area of around 25% between 1998 and 2015. However, in parts of the world, known as the extratropics, a positive relationship between more extreme fire weather and burned area has been observed. Especially the vast areas of boreal forest are less influenced by human activity, and thus burned area is driven by lightning ignitions. Globally, 2.5% of the annual burned area stems from boreal forest fires while a recent study found that 14% of the annual global carbon emission comes from boreal forest fires. The high fuel consumption and thus high emissions of carbon per unit burned area in the boreal forest has also caused the global fire carbon emissions to stabilize even with declines in global burned area. The important role of boreal fires on a local, regional, and global scale and its intricate interactions with carbon and vegetation dynamics as well as climate change, illustrates the need for a better understanding of the prerequisite drivers of boreal fire ignitions and the newly discovered overwintering fires. This thesis responds to the need by quantifying the importance of snow disappearance and the dynamic interplay between lightning, fuel, and fire weather on ignition and overwintering fires in boreal North America. Recent changes in snow disappearance timing and the number of fire ignitions across boreal North America showed a clear west-east divergence. Regardless of the spatial and temporal trends, the snow disappearance timing correlated significantly positively with the timing of the ignitions across all ecoregions in boreal North America. This relationship, however, was observed for all ignition sources i.e. human, lightning, and unknown. The annual earliest 20% ignitions resulted in fires growing around 77% larger on average compared to the late-season ignitions. Warm and dry weather as well as fuel loads also promoted snowmelt and ignition timing. Similarly, the lightning ignition efficiency over Alaska, USA, and the Northwest Territories, Canada was largely driven by top-down fire weather conditions. The short-term (within a day) drying of organic soils and the lack of precipitation accompanying the thunderstorm influenced the lightning ignition efficiency strongly. Especially the occurrence of dry lightning (less than 2.5 mm precipitation) exerted an important control over lightning ignitions in boreal forests, but it can vary at landscape scale. Recent climate changes have also led to changes in the fire season length with overwintering fires being more prevalent. By spatiotemporally extrapolating our algorithm, we estimate that around 1% and 1.2% of the total burned area stemmed from reburning of overwintering fires between 1986 and 2020 in Interior Boreal Alaska and the Taiga Plains. Denser evergreen forest in microtopographical uplands were more prone to overwintering reburns. This indicates that productive forests may provide mechanisms for overwintering reburns by combustion of tree boles and roots.

The snow cover extent across the Northern Hemisphere has diminished, while the number of lightning ignitions and amount of burned area have increased over the last 5 decades with accelerated warming. However, the effects of earlier snow disappearance on fire are largely unknown. Here, we assessed the influence of snow disappearance timing on fire ignitions across 16 ecoregions of boreal North America. We found spatially divergent trends in earlier (later) snow disappearance, which led to an increasing (decreasing) number of ignitions for the northwestern (southeastern) ecoregions between 1980 and 2019. Similar northwest–southeast divergent trends were observed in the changing length of the snow-free season and correspondingly the fire season length. We observed increases (decreases) over northwestern (southeastern) boreal North America which coincided with a continental dipole in air temperature changes between 2001 and 2019. Earlier snow disappearance induced earlier ignitions of between 0.22 and 1.43 d earlier per day of earlier snow disappearance in all ecoregions between 2001 and 2019. Early-season ignitions (defined by the 20 % earliest fire ignitions per year) developed into significantly larger fires in 8 out of 16 ecoregions, being on average 77 % larger across the whole domain. Using a piecewise structural equation model, we found that earlier snow disappearance is a good direct proxy for earlier ignitions but may also result in a cascade of effects from earlier desiccation of fuels and favorable weather conditions that lead to earlier ignitions. This indicates that snow disappearance timing is an important trigger of land–atmosphere dynamics. Future warming and consequent changes in snow disappearance timing may contribute to further increases in western boreal fires, while it remains unclear how the number and timing of fire ignitions in eastern boreal North America may change with climate change.

The snow cover extent across the Northern Hemisphere has diminished while fire extent and severity has increased over the last five decades with accelerated warming. However, the effects of earlier snowmelt on fire is largely unknown. Here, we assessed the influence of snowmelt timing on fire ignitions across 16 ecoregions of boreal North America. We found spatially divergent trends in earlier (later) snowmelt led to an increasing (decreasing) number of ignitions for the northwestern (southeastern) ecoregions between 1980 and 2019. Similar northwest-southeast divergent trends were observed in the changing length of the snow-free season and correspondingly the fire season length. We observed increases (decreases) over Northwest (Southeast) boreal North America which coincided with a continental dipole in air temperature changes between 2001 and 2019. Earlier snowmelt induced earlier ignitions of between 0.22 and 1.43 days earlier per day of earlier snowmelt in all ecoregions between 2001 to 2019. Early-season ignitions (defined by the 20 % earliest fires per year) developed into significantly larger fires in 8 out of 16 ecoregions and 77 % larger across the whole domain. Using a piecewise structural equation model, we found that earlier snowmelt is a proxy for earlier ignitions but may also result in a cascade of effects from earlier desiccation of fuels and favorable weather conditions that led to earlier ignitions. This indicates that snowmelt timing is an important trigger of land-atmosphere dynamics. Future warming and consequent changes in snowmelt timing may contribute to further increases in western boreal fires while the number of fires in eastern boreal North America may increase too with climate change.

The snow cover extent across the Northern Hemisphere has diminished while fire extent and severity has increased over the last five decades with accelerated warming. However, the effects of earlier snowmelt on fire is largely unknown. Here, we assessed the influence of snowmelt timing on fire ignitions across 16 ecoregions of boreal North America. We found spatially divergent trends in earlier (later) snowmelt led to an increasing (decreasing) number of ignitions for the northwestern (southeastern) ecoregions between 1980 and 2019. Similar northwest-southeast divergent trends were observed in the changing length of the snow-free season and correspondingly the fire season length. We observed increases (decreases) over Northwest (Southeast) boreal North America which coincided with a continental dipole in air temperature changes between 2001 and 2019. Earlier snowmelt induced earlier ignitions of between 0.22 and 1.43 days earlier per day of earlier snowmelt in all ecoregions between 2001 to 2019. Early-season ignitions (defined by the 20 % earliest fires per year) developed into significantly larger fires in 8 out of 16 ecoregions and 77 % larger across the whole domain. Using a piecewise structural equation model, we found that earlier snowmelt is a proxy for earlier ignitions but may also result in a cascade of effects from earlier desiccation of fuels and favorable weather conditions that led to earlier ignitions. This indicates that snowmelt timing is an important trigger of land-atmosphere dynamics. Future warming and consequent changes in snowmelt timing may contribute to further increases in western boreal fires while the number of fires in eastern boreal North America may increase too with climate change.

The snow cover extent across the Northern Hemisphere has diminished while fire extent and severity has increased over the last five decades with accelerated warming. However, the effects of earlier snowmelt on fire is largely unknown. Here, we assessed the influence of snowmelt timing on fire ignitions across 16 ecoregions of boreal North America. We found spatially divergent trends in earlier (later) snowmelt led to an increasing (decreasing) number of ignitions for the northwestern (southeastern) ecoregions between 1980 and 2019. Similar northwest-southeast divergent trends were observed in the changing length of the snow-free season and correspondingly the fire season length. We observed increases (decreases) over Northwest (Southeast) boreal North America which coincided with a continental dipole in air temperature changes between 2001 and 2019. Earlier snowmelt induced earlier ignitions of between 0.22 and 1.43 days earlier per day of earlier snowmelt in all ecoregions between 2001 to 2019. Early-season ignitions (defined by the 20 % earliest fires per year) developed into significantly larger fires in 8 out of 16 ecoregions and 77 % larger across the whole domain. Using a piecewise structural equation model, we found that earlier snowmelt is a proxy for earlier ignitions but may also result in a cascade of effects from earlier desiccation of fuels and favorable weather conditions that led to earlier ignitions. This indicates that snowmelt timing is an important trigger of land-atmosphere dynamics. Future warming and consequent changes in snowmelt timing may contribute to further increases in western boreal fires while the number of fires in eastern boreal North America may increase too with climate change.

The snow cover extent across the Northern Hemisphere has diminished while fire extent and severity has increased over the last five decades with accelerated warming. However, the effects of earlier snowmelt on fire is largely unknown. Here, we assessed the influence of snowmelt timing on fire ignitions across 16 ecoregions of boreal North America. We found spatially divergent trends in earlier (later) snowmelt led to an increasing (decreasing) number of ignitions for the northwestern (southeastern) ecoregions between 1980 and 2019. Similar northwest-southeast divergent trends were observed in the changing length of the snow-free season and correspondingly the fire season length. We observed increases (decreases) over Northwest (Southeast) boreal North America which coincided with a continental dipole in air temperature changes between 2001 and 2019. Earlier snowmelt induced earlier ignitions of between 0.22 and 1.43 days earlier per day of earlier snowmelt in all ecoregions between 2001 to 2019. Early-season ignitions (defined by the 20 % earliest fires per year) developed into significantly larger fires in 8 out of 16 ecoregions and 77 % larger across the whole domain. Using a piecewise structural equation model, we found that earlier snowmelt is a proxy for earlier ignitions but may also result in a cascade of effects from earlier desiccation of fuels and favorable weather conditions that led to earlier ignitions. This indicates that snowmelt timing is an important trigger of land-atmosphere dynamics. Future warming and consequent changes in snowmelt timing may contribute to further increases in western boreal fires while the number of fires in eastern boreal North America may increase too with climate change.

Fire is a major disturbance in the boreal forests of the high northern latitude. Fire extent and severity have been increasing in recent decades, and the occurrence of overwintering ‘zombie’ fires has been linked to recent fire extremes. Overwintering fires are fires which were seemingly extinguished at the end of the boreal fire season yet smolder during winter to re-emerge as a flaming fire in the subsequent spring. So far, overwintering fires have only been investigated using satellite imagery. Here, for the first time, we show preliminary results from a field campaign that measured in situ impacts of fires that overwintered from 2014 to 2015 in the Canadian Northwest Territories. We measured among other the burn depth in organic soils, and characterized micro-topography. We also qualitatively assessed how fires may have overwintered. We compared nine overwintering fire sites, which burned during both 2014 and 2015, with six sites that only burned in 2014 and five nearby unburned sites. The average burn depth (±SD) of the overwintering fires was 6.8 ± 1.6 cm and significantly deeper compared to 6.1 ± 1.2 cm in the single fire sites (P < 0.01). Somewhat surprisingly, the majority of overwintering fires occurred in mesic sites with large productive trees. Only two overwintering sites were sampled in mesic-subhygric to subhygric sites dominated by black spruce (Picea mariana). The unburned control sites often featured a micro-topography of hummocks and hollows. This micro-topography was leveled in overwintering fires sites because of severe burning in organic soils. In overwintering sites, most of the organic layer was consumed. This may have led to prolonged smoldering in the root systems of trees. Our results are the first to quantify the burn depth of overwintering fires, and also show that overwintering does not only happen through deep smoldering in organic soils, yet can also occur from smoldering in tree boles and root systems of burned and fallen trees.

Holdover fires are usually associated with lightning-ignited wildfires (LIWs), which can experience a smoldering phase or go undetected for several hours, days or even weeks before being reported. Since the existence and duration of the smoldering combustion in LIWs is usually unknown, holdover time is conventionally defined as the time between the lightning event that ignited the fire and the time the fire is detected. Therefore, all LIWs have an associated holdover time, which may range from a few minutes to several days. However, we lack a comprehensive understanding of holdover times. Here, we introduce a global database on holdover times of LIWs. We have collected holdover time data from 29 different studies across the world through a literature review and datasets assembled by authors of the original studies. The database is composed of three data files (censored data, non-censored data, ancillary data) and three metadata files (description of database variables, list of references, reproducible examples). Censored data are the core of the database and consist of different frequency distributions reporting the number or relative frequency of LIWs per interval of holdover time. In addition, ancillary data provide further information to understand the methods and contexts in which the data were generated in the original studies. The first version of the database contains 42 frequency distributions of holdover time built with data on more than 152 375 LIWs from 13 countries in five continents covering a time span from 1921 to 2020. This database is the first freely available, harmonized and ready-to-use global source of holdover time data, which may be used in different ways to investigate LIWs and model the holdover phenomenon. The complete database can be downloaded at https://doi.org/10.5281/zenodo.7352172 (Moris et al., 2022).