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Prescribed burning is a widely accepted wildfire hazard reduction technique; however, knowledge of its effectiveness remains limited. To address this, we employ simulations of a widely used fire behaviour model across the ecologically diverse Australian island state of Tasmania. We simulate three broad scenarios: (1) no fuel treatment, (2) a maximal treatment, with the most possible prescribed burning within ecological constraints, and (3) 12 hypothetically more implementable state-wide prescribed-burning plans. In all simulations, we standardised fire-weather inputs to represent regionally typical dangerous fire-weather conditions. Statistical modelling showed that an unrealistically large maximal treatment scenario could reduce fire intensity in three flammable vegetation types, and reduce fire probability in almost every vegetation type. However, leverage analysis of the 12 more-realistic implementable plans indicated that such prescribed burning would have only a minimal effect, if any, on fire extent and that none of these prescribed-burning plans substantially reduced fire intensity. The study highlights that prescribed burning can theoretically mitigate wildfire, but that an unrealistically large area would need to be treated to affect fire behaviour across the island. Rather, optimisation of prescribed burning requires careful landscape design at the local scale. Such designs should be based on improved fire behaviour modelling, empirical measurement of fuels and analysis of actual wildfires.

Savanna fire management is a topic of global debate, with early dry season burning promoted as a large-scale emissions reduction opportunity. To date, discussions have centred on carbon abatement efficacy, biodiversity and cultural benefits and/or risks. Here we use a case study of Darwin, Australia to highlight smoke pollution as another critical consideration. Smoke pollution from savanna fires is a major public health issue, yet absent so far from discussions of program design. Here, we assess the likely impacts of increased early dry season burning on smoke pollution in Darwin between 2004 and 2019, spanning the introduction and expansion of carbon abatement programs. We found increased smoke pollution in the early dry season but little change in the late dry season, contributing to a net annual increase in air quality standard exceedances. Geospatial analysis suggests this relates to increased burning in the path of early dry season trade winds. This study highlights the complex health trade-offs involved with any large-scale prescribed burning, including for carbon abatement.

1. The relationships between productivity, fire frequency and fire severity shape the distribution of plant communities globally. Dry forests are expected to burn frequently and wet forests to burn infrequently. However, the effect of productivity on intensity and severity of wildfire is less consistent and poorly understood.One productive ecosystem where this is especially true is the Australian tall wet Eucalyptus-dominated forest (TWEF), which spans wet areas across the continent.This study aims to characterise how climate shapes the likelihood of low-and high-severity wildfire across Australian TWEF.2. We performed a continental-scale analysis of fuels in 48 permanent plots in earlymature stage TWEF across four climate regions in Australia. We estimated fuel loads and measured understorey microclimate. We then obtained historical fireweather observations from nearby meteorological stations and used fuel moisture and fire behaviour equations to predict the historical frequency with which TWEF could burn and what fire severities were expected. We investigated how this varies across the different TWEF climate regions. Lastly, we validated our approach by remeasuring eight plots that burned unexpectedly post-measurement.3. We found that surface fuels in cooler, moister regions were available to burn 1-16 days per year historically, with only low-severity, surface fire possible most of these days: high-severity fire was only possible under rare, extreme fire-weather conditions. However, in warmer, drier regions, fuels were available to burn 23-35 days annually, and high-severity fire was more likely than low-severity fire.Validation showed that we slightly overestimated flame heights, inflating highseverity risk estimates. If we used elevated fuel loads to predict flame heights, however, high-severity fire was more likely than low-severity fire everywhere.Lastly, the likelihood of high-severity fire increased with increasing temperature and worsening fire weather. Synthesis.Fire activity in early-mature TWEF is limited by climatic constraints on fire weather and availability to burn, with high-severity fire more likely in warmer, drier regions than in cooler, wetter ones. This indicates a particularly worrisome vulnerability to climate change, given TWEF's diminished ability to recover from disturbance in a warmer world. The occurrence of both low-and high-severity fire means the fire regimes of TWEF are best described as mixed severity.