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Mechanical mastication is growing in popularity as a wildfire mitigation technique. Yet few studies quantify its effects on fire behaviour. Such information is needed by fire managers to evaluate its efficacy. Our aim was to develop an understanding of how mastication alters flaming and smouldering durations and the longevity of any effects. Flaming and smouldering duration are important determinants of soil heating and smoke emissions. We used a paired sampling design, collecting litter bed (hereafter surface fuel) samples from 15 sites with masticated and untreated vegetation in shrub-encroached Eucalyptus woodlands. We measured a range of fuel bed properties and then burnt the samples in the laboratory. Average smouldering durations increased 88% from 25 to 47min in untreated v. masticated fuel; flaming durations increased 100% from 6 to 12min. These changes were attributable to higher fine and coarse fuel loads in the masticated fuel bed. However, fine and coarse fuel load declined significantly over 4 years, meaning the effects of mastication on combustion duration are likely to be short-lived. Despite being a laboratory study, the results assist with evaluating mastication as a fuel treatment by demonstrating the potential magnitude of changes to flaming and smouldering duration.

Shrub encroachment of grassland and woodland ecosystems can alter wildfire behaviour and threaten ecological values. Australian fire managers are using mechanical mastication to reduce the fire risk in encroached ecosystems but are yet to evaluate its effectiveness or ecological impact. We asked: (1) How does fuel load and structure change following mastication?; (2) Is mastication likely to affect wildfire rates of spread and flame heights?; and (3) What is the impact of mastication on flora species richness and diversity? At thirteen paired sites (masticated versus control; n = 26), located in Victoria, Australia, we measured fuel properties (structure, load and hazard) and floristic diversity (richness and Shannon’s H) in 400 mP2 plots. To quantify the effects of mastication, data were analysed using parametric and non-parametric paired sample techniques. Masticated sites were grouped into two categories, 0–2 and 3–4 years post treatment. Fire behaviour was predicted using the Dry Eucalypt Forest Fire Model. Mastication with follow-up herbicide reduced the density of taller shrubs, greater than 50 cm in height, for at least 4 years. The most recently masticated sites (0–2 years) had an almost 3-fold increase in dead fine fuel loads and an 11-fold increase in dead coarse fuel loads on the forest floor compared with the controls. Higher dead coarse fuel loads were still evident after 3–4 years. Changes to fuel properties produced a reduction in predicted flame heights from 22 m to 5–6 m under severe fire weather conditions, but no change in the predicted fire rate of spread. Reductions in flame height would be beneficial for wildfire suppression and could reduce the damage to property from wildfires. Mastication did not have a meaningful effect on native species diversity, but promoted the abundance of some exotic species.

Understanding the conditions when litter beds will ignite from firebrands is critical for predicting spot fire occurrence. Such research is either field- or laboratory-based, with limited analysis to compare the approaches. We examined the ability of a laboratory method to represent field-scale ignitability. The laboratory method involved collecting litter-bed samples concurrently with the field experiments and then reconstructing and burning the litter-bed samples in the laboratory. We measured the number of successful and sustained ignitions in the laboratory (n = 5) and field (n = 30 attempts). The laboratory and field results were more similar for successful (bias = 0.05) than sustained ignitions (bias = 0.08). Wind, fuel structure (in the field) and near-surface fuel moisture influenced the differences between the methods. Our study highlights the value in conducting simultaneous laboratory and field experiments to understand the scalability of laboratory studies. For our ignitability method, our results suggest that small-scale laboratory experiments could be an effective substitute for field experiments in forests where litter beds are the dominant fuel layer and where the cover of the near-surface fuel is low.