When soil becomes fuel: identifying a safe window for prescribed burning of Tasmanian vegetation growing on organic soils

Background: Flammable vegetation in Tasmania, Australia often grows on organic soils that can burn if sufficiently dry. Aims: To develop an approach to identify a safe window for prescribed burning of vegetation on Tasmanian organic soils, when vegetation is dry enough to be combustible, yet organic soils are unlikely to burn. Methods: We compiled a dataset of when organic soils did and did not burn when exposed to vegetation fires. Focussing on moorland, we used binomial modelling to estimate the probability of organic soil burning in relation to soil dryness index (SDI) computed from climate data. Vegetation combustibility was inferred from fuel moisture content estimated from climate data and records of area burnt. Key results: Risk of soil fire varied with vegetation. In moorland, modelling predicted a 17% risk when SDI was 10, a conservative estimate because our dataset was biased towards positive records of soil fire. Using an SDI threshold of 10, the average annual number of 'safe combustible' days varied across Tasmania from 26 to 53. Conclusions: This approach can be used to refine safe burning guidelines on organic soil. Implications: This approach, when applied to an improved dataset, will assist fire management on organic soils.