Science

Eucalyptus Trees Are So Oily They Can Explode During Bushfires

Eucalyptus trees are packed with highly flammable volatile oils that evaporate in hot weather, forming a blue haze around the treetops and occasionally igniting into fireballs ahead of the main fire front. The oils make eucalyptus one of the most combustible trees on Earth, and the Australian Blue Mountains are named after this distinctive bluish vapour. In an ironic twist, eucalyptus has evolved to depend on fire for seed germination.

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Eucalyptus leaves and bark contain large concentrations of cineole and other volatile terpenoid compounds that serve as natural insect repellents and antifungal agents. In summer heat, these compounds evaporate and accumulate in a fine oily mist above the canopy, which is what gives the Blue Mountains region its characteristic azure haze. During bushfires, this vapour can ignite explosively, sending fireballs racing ahead of the fire front and causing fires to spread in unpredictable directions. Rather than being a flaw, this flammability appears to be an evolutionary strategy. Many eucalyptus species have thick insulating bark and dormant buds beneath the surface that survive fire and sprout rapidly afterwards. Some species have fire-activated seed pods called lignotubers, which open and release seeds only after heat exposure. This means the trees actively create conditions that eliminate competitor species while their own seeds thrive in the ash-enriched soil left behind.