Short fire-return intervals modify fuel structure, changing flammability and fire behavior in dry forests of southeastern Australia

Shortening fire-return intervals are pushing fire-tolerant ecosystems in temperate Australia beyond their ecological limits, producing alternative vegetation states with markedly different composition and structure. While the structural effects of these regime shifts on fire-tolerant eucalypt forests are increasingly documented, their consequences for flammability remain largely untested. We compared fuel structure and load, surface fuel flammability, and modelled point-based fire behaviour between areas retaining a Eucalyptus overstorey under typical fire regimes (reference state) and alternative states in fire-tolerant native forests of southeastern Australia. Fuel hazard was visually assessed using an established field protocol, and surface fuels were sampled for laboratory flammability testing. Point-based fire behaviour was modelled using Vesta Mk2 across five weather scenarios spanning prescribed-burn to extreme wildfire conditions. Fuel hazard scores were significantly higher for surface and elevated strata in alternative states, but higher for near-surface and bark strata in reference states. However, laboratory flammability tests showed that surface fuels from alternative states were less ignitable, combustible, and consumable than those from reference states. Modelled fire behaviour showed consistently taller flames in alternative states across all weather scenarios, though rate of spread was greater only under the most extreme conditions. Spotting distances were consistently higher in reference states. These results show that alternative-state eucalypt forests produce a distinct fire behaviour signature: lower surface fuel flammability, greater flame height, but reduced spotting potential. The landscape-scale implications of this altered fire behaviour for subsequent fire regimes and the persistence of alternative states warrant further investigation.