The 2019/2020 mega-fires exposed Australian ecosystems to an unprecedented extent of high-severity fire

Abstract

Extreme fire seasons characterised by very large ‘mega-fires’ have demonstrably increased area burnt across forested regions globally. However, the effect of extreme fire seasons on fire severity, a measure of fire impacts on ecosystems, remains unclear. Very large wildfires burnt an unprecedented area of temperate forest, woodland and shrubland across south-eastern Australia in 2019/2020, providing an opportunity to examine the impact of extreme fires on fire severity patterns. We developed an atlas of wildfire severity across south-eastern Australia between 1988 and 2020 to test (a) whether the 2019/2020 fire season was more severe than previous fire seasons, and (b) if the proportion of high-severity fire within the burn extent (HSp) increases with wildfire size and annual area burnt. We demonstrate that the 2019/2020 wildfires in south-eastern Australia were generally greater in extent but not proportionally more severe than previous fires, owing to constant scaling between HSp and annual fire extent across the dominant dry-forest communities. However, HSp did increase with increasing annual fire extent across wet-forests and the less-common rainforest and woodland communities. The absolute area of high-severity fire in 2019/2020 (∼1.8 M ha) was larger than previously seen, accounting for ∼44% of the area burnt by high-severity fire over the past 33 years. Our results demonstrate that extreme fire seasons are a rare but defining feature of fire regimes across forested regions, owing to the disproportionate influence of mega-fires on area burnt.

The Effect of Antecedent Fire Severity on Reburn Severity and Fuel Structure in a Resprouting Eucalypt Forest in Victoria, Australia

“…These findings are consistent with a growing body of evidence showing that high-severity canopy-disturbing fires cause transitions towards fuel states with greater ignitability and propensity to burn at a high severity [14,30,68]. There is evidence that the increasing frequency of severe fire weather has already driven the contraction of interfire intervals and increased the area affected by high severity fire across large areas of south-eastern Australia [83][84][85]. These changes to fire weather and fire regimes, coupled with increased canopy fuel connectivity resulting from exposure to high-severity fire, have likely increased the propensity for high-severity fires across areas of southern Australia.…”

Extreme Winds Flip Influence of Fuels and Topography on Megafire Burn Severity in Mesic Conifer Forests Under Record Fuel Aridity

“…Overall, the burn severity patterns found in our study area (ca. 62%) were higher than those founds among other mega re events in wet-temperate climate regions elsewhere (9,24).…”

“…Global increases in mega res are of paramount concern to ecosystems and human well-being yet the speci c mechanisms have driven this growth are not well understood. More work in the PNW, across the western US, and elsewhere (24,30) is needed to understand large-scale and synoptic conditions that favor extreme re behavior under warming (e.g., (76, 107-109)particularly with respect to climaterestricted re regimes common to mesic temperate forests. Rapid climate change has and will continue to disequilibrate historical relationships between productivity gradients and burn activity (110,111), leading to increases re activity within fuel-rich areas where re has historically been climate-limited.…”

“…Ecosystems with climate-restricted re regimes may be particularly sensitive to changes in aridity (13)(14)(15)(16)). These re regimes, where long periods (> 100-300 years) of quiescent re activity area are punctuated by episodic, regional-scale re events (17,18), are common within mesic, temperate forests, including those found in Eurasia, Southern Chile, SE Oceana, and in North America in the subalpine Rocky Mountains and the Paci c Northwest (PNW) (19)(20)(21)(22)(23)(24)(25), all of which have experienced mega res during the 2016-2020 period (26). While mesic, temperate forests compose only a fraction of the global temperate forest biome, they represent some of the most carbon-dense locations on the planet (27) and highly valued for their productivity.…”

Catastrophic Bushfires, Indigenous Fire Knowledge and Reframing Science in Southeast Australia

“…Some countries have banned its planting in an attempt to mitigate wildfire risk [24]. Despite most Eucalyptus depending on fire for their reproduction in various ways [25][26][27][28], these Eucalyptus-dominated forests across Southeast Australia have not experienced such intense and widespread fires as the Black Summer bushfires through the historic period [25,29]. Presently, these forests often have a dense understorey of flammable woody shrubs that provide fuel for fire and which communicate fire rapidly from the ground to the canopy (Figure 2a) [30].…”