The Ignition Enigma
Authors:   , Hessilt Thomas Duchnik 
Published: October 2024

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.