Rainfall-Linked Megafires as Innate Fire Regime Elements in Arid Australian Spinifex (Triodia spp.) Grasslands

Abstract

Large, high-severity wildfires, or “megafires,” occur periodically in arid Australian spinifex (Triodia spp.) grasslands after high rainfall periods that trigger fuel accumulation. Proponents of the patch-burn mosaic (PBM) hypothesis suggest that these fires are unprecedented in the modern era and were formerly constrained by Aboriginal patch burning that kept landscape fuel levels low. This assumption deserves scrutiny, as evidence from fire-prone systems globally indicates that weather factors are the primary determinant behind megafire incidence, and that fuel management does not mitigate such fires during periods of climatic extreme. We reviewed explorer’s diaries, anthropologist’s reports, and remotely sensed data from the Australian Western Desert for evidence of large rainfall-linked fires during the pre-contact period when traditional Aboriginal patch burning was still being practiced. We used only observations that contained empiric estimates of fire sizes. Concurrently, we employed remote rainfall data and the Oceanic Niño Index to relate fire size to likely seasonal conditions at the time the observations were made. Numerous records were found of small fires during periods of average and below-average rainfall conditions, but no evidence of large-scale fires during these times. By contrast, there was strong evidence of large-scale wildfires during a high-rainfall period in the early 1870s, some of which are estimated to have burnt areas up to 700,000 ha. Our literature review also identified several Western Desert Aboriginal mythologies that refer to large-scale conflagrations. As oral traditions sometimes corroborate historic events, these myths may add further evidence that large fires are an inherent feature of spinifex grassland fire regimes. Overall, the results suggest that, contrary to predictions of the PBM hypothesis, traditional Aboriginal burning did not modulate spinifex fire size during periods of extreme-high arid zone rainfall. The mechanism behind this is that plant assemblages in seral spinifex vegetation comprise highly flammable non-spinifex tussock grasses that rapidly accumulate high fuel loads under favorable precipitation conditions. Our finding that fuel management does not prevent megafires under extreme conditions in arid Australia has parallels with the primacy of climatic factors as drivers of megafires in the forests of temperate Australia.

Evidence that predator satiation drives reproductive synchrony in the desert masting grass, soft spinifex (Triodia pungens)

“…In arid Triodia grasslands, masting is driven by periods of exceptionally high rainfall (e.g. >500 mm over 12 months) (Jacobs 1973;, and such high rainfall in the Australian arid zone also stimulates biomass accumulation and subsequent large-scale lightning-ignited wildfires (Allan et al 2003;Allan and Southgate 2002;Gill and Allan 2008;Wright et al 2021). Given that establishment in unburnt spinifex grasslands is constrained by competition (Nano & Clarke 2010), seeding after high rainfall periods that trigger wildfires may be adaptive because competition is reduced in post-fire environment and T. pungens seedling survival rates are enhanced.…”

Breaking the wildfire cycle: progressive fire management can shift fire regimes and improve ecosystem condition. A case study from a large conservation reserve in northern Australia

“…This has created a landscape more resilient to occasional LDS wildfires, but no longer dominated by them. Although a large extensive wildfire in spinifex dominated systems is considered a natural component of arid Australia (Wright et al 2021), in multiuse semi-arid savannas where rainfall can promote shorter fire-return periods, there is clear recognition that prescribed burning to promote more mosaic landscapes is critical for threatened species (Radford et al 2020).…”

“…Apart from a conservation benefit, this has a significant effect on the annual budget for prescribed burning, as well as wildfire management. For example, fire management at Boodjamulla no longer includes wildfire response, and now costs approximately AU$75000 over 5 years, and Strong associations between high-rainfall events and large LDS wildfires are well documented across Australia (Wright et al 2021). For Boodjamulla, it is difficult to separate the contribution of high rainfall from that of limited EDS prescribed burning in driving large LDS wildfire events in its recent history.…”

“…At gazettal of the reserve in 1984, a 5-year cycle of large-scale and high-intensity LDS wildfires was entrenched, resulting in young, even-aged, contiguous stands of the very fire-prone spinifex grass (Trioda spp. ), the dominant vegetation type of the reserve (Wright et al 2021). These extensive LDS wildfires caused significant environmental and financial cost.…”

A dynamic and evidence-based approach to mapping burn potential

“…After European colonisation, it is likely that desert depopulation resulted in a shift to fire patterns characterised by cycles of hot fires that burn large areas in spring and summer (Burrows and Christensen 1990;Burrows et al 2006;Burrows and Chapman 2018). However, it is also possible that climatic factors contributed to large fire events also being typical of the period of traditional fire management (Wright et al 2021).…”