Fire risk and severity decline with stand development in Tasmanian giant Eucalyptus forest

Forest fire risk, and how it changes over time, has important influences on forest dynamics. Two common modelsdescribing how fire risk changes with stand development, known as flammability functions, are (a) the ‘moisturemodel’, where fire risk initially increases, then decreases, as a stand develops after a fire, and (b) the ‘Olsonmodel’, where fire risk increases asymptotically as a function of time since previous fire. These two flammabilityfunctions have both been used to describe the world’s tallest angiosperm forest, the Australian tall wet Eucalyptusforest (TWEF). It is unclear, however, which function is more appropriate for TWEF, as there are little empiricaldata describing fuels and microclimate, two important influences on fire risk and potential severity, across thelong lifespan of these forests. Accordingly, we use a chronosequence of TWEF stands in southeast Tasmania,Australia, to see how fuels, microclimate, and resulting fire risk and potential fire severity changed amongst fourstand-development stages ranging from regrowth to old forests. We measured fuel loads, understorey microclimate,and forest physiognomy. We then used these data with historical fire weather data and fire behaviourmodels to estimate how often low- and high-severity fire was possible historically. We also investigated if theseverity of the previous disturbance influenced the likelihood of a subsequent high-severity fire. We found that,while fuel loads remained unchanged across the chronosequence, later development stages had a significantlymoister understorey, an increased abundance of rainforest trees, and more vertically discontinuous fuels. Thesefactors resulted in a significantly reduced fire risk, with high-severity fire much more likely in the early stages.Further, we found that stands that had been initiated by stand-replacing fire were more susceptible to subsequenthigh-severity fire than those that experienced non stand-replacing fire, due to a lack of a remaining maturecanopy. We concluded that, unlike most other fire-dependent ecosystems where the Olson curve is an appropriatemodel, the flammability function in TWEF is best described by the moisture model. Our results indicatethat TWEF is vulnerable to a ‘landscape trap’ effect, where intensive disturbance creates large areas of regrowthstands with increased risk of high-severity fire, which increases the likelihood of landscape-wide, demographiccollapse. We suggest that fire and forest management incorporate techniques mimicking low-severity disturbancesto create more resilient landscapes