The IUCN Red List criteria are a globally accepted method of assessing species extinction risk, and countries around the world are adapting these criteria for domestic use. First, we compared trends in IUCN Red List criteria used in threatened plant species listings in Australia and globally. Second, using the state of New South Wales (NSW), Australia, as a study region, we conducted two complementary analyses: (1) An assessment of ~ 5000 currently unlisted NSW plant species against the thresholds for the geographic range criterion (Criterion B) to identify species which may require full assessment; and (2) A rapid assessment of currently listed threatened plant species, applying the IUCN Red List Critically Endangered thresholds for all criteria, to identify species likely to be at the highest risk of extinction from further decline. Impacts on these species could be considered to be "serious and irreversible impacts" (SAII). Geographic range size was the most common criterion used in Australia and globally for plant listings. Our assessment of unlisted NSW plant species revealed 92 species (75 endemic to NSW) met the geographic range thresholds for Critically Endangered. Our rapid assessments of currently listed NSW threatened plant species identified 53.5% as having an extremely high risk of extinction should further decline occur. Of these, most were flagged under Criterion B (88.8%). Geographic range and the other IUCN Red List criteria thresholds for Critically Endangered provide a useful framework to identify species at an extremely high risk of extinction from ongoing decline.

NOTE: This is a pre-print, still under consideration for publication, and this version has not undergone peer review.

California experienced five of the six largest fires in the state's recorded history (Higuera and Abatzoglou 2021). On the east coast of Australia, a series of megafires -including the largest ever recorded in the country -consumed 21% of the Australian temperate forest biome during the 2019-2020 fire season (Boer et al. 2020; Nolan et al. 2020), potentially pushing many ecosystems toward thresholds for "threatened" classification under International Union for Conservation of Nature (IUCN) Red List of Ecosystems criteria (Bland et al. 2017). Driven jointly by climate change, fire suppression/exclusion, and landuse changes (Stephens et al. 2014), this increase in Australian megafire occurrence is consistent with climate-change projections (Abram et al. 2021; van Oldenborgh et al. 2021) and is likely to continue.Fire is a natural ecosystem process, particularly in fireadapted communities (Stephens et al. 2014;Fidelis 2020), but changes to fire regimes can have negative consequences that are exacerbated by more frequent occurrence of large, intense megafires (Stephens et al. 2014). Such fires may increase the likelihood of burning of fire-sensitive vegetation (eg rainforests) and lead to high plant mortality, as reproductive strategies that drive regeneration either act independently of or are impaired by fire (Barlow et al. 2020). Fire-sensitive vegetation recovers more slowly from fire and in some cases may not recover at all, increasing the risk of a state transition (Tepley et al. 2018). Although fire-driven transitions between alternative stable states are a natural part of fire-vegetation dynamics in many fireprone systems (Pausas 2015), interactions with climate change and other drivers (eg altered fire regimes) may lead to irreversible shifts or ecosystem collapse (Batllori et al. 2019;Kelly et al. 2020). Despite improved understanding of fire ecology in these communities, the impacts of extremely large, intense fires on persistence and the interactions with temporal elements of the fire regime require further research (Batllori et al. 2019).