Eucalyptus trees use flammable oil to encourage forest fires
Eucalyptus trees have evolved to survive, and even thrive, in forest fires. Their leaves contain highly flammable oils that encourage intense, fast-moving blazes. While the fire destroys competing plant species, the eucalyptus survives thanks to its fire-resistant bark and deep epicormic buds, which sprout new growth immediately after the fire passes.
The Chemistry of Flammable Foliage
The leaves of eucalyptus trees are densely packed with volatile, aromatic essential oils, primarily composed of terpenes such as cineole. While these chemical compounds serve as deterrents against herbivorous insects and grazing animals, they also have a dramatic physical consequence: they are extraordinarily combustible. In high temperatures or during hot, dry weather, these oils can vaporize and form a flammable haze above and around the canopy. When a spark or flame reaches this vapor, it can ignite rapidly, producing explosive crown fires that leap through the upper foliage.
Beyond the living canopy, eucalyptus trees contribute continuously to the fire risk on the forest floor. They shed significant volumes of bark, dry leaves, and dead twigs, all of which retain residual oils and decompose very slowly in arid conditions. Certain species shed long ribbons of fibrous bark that remain hanging from branches or litter the ground, creating loose, aerated fuel ladders. When ignited, these burning ribbons can be carried aloft by thermal drafts, travelling kilometres ahead of the main fire front to spark new spot fires.
Recovery from the Trunk: Epicormic Buds
While an intense fire will often incinerate competing vegetation and strip the eucalyptus of its leaves, the tree itself possesses specialized anatomical structures designed to survive the inferno. Beneath the thick, protective outer bark lie dormant epicormic buds. These buds are embedded deep within the living tissues of the trunk and primary branches, shielded from the brief, intense heat pulse of a passing blaze.
Under normal conditions, hormonal signals from the active canopy suppress the growth of these hidden buds. However, once fire consumes or scorches the foliage and breaks this hormonal suppression, the epicormic buds activate almost immediately. Within weeks of a severe burn, trunks and limbs that appear completely charred produce vibrant green shoots along their entire length. This rapid leaf production allows the tree to resume photosynthesis and re-establish its crown long before surrounding non-adapted plants can recover.
Subterranean Survival and Lignotubers
In environments where fires are so extreme that even thick branch bark fails to protect the above-ground trunk, many eucalyptus species rely on a subterranean survival structure called a lignotuber. This is a swollen woody swelling that forms at the base of the stem or root crown, buried safely beneath the insulating layer of the soil. Lignotubers contain substantial reserves of stored carbohydrates and dense clusters of dormant vegetative buds.
When top-kill occurs—meaning the entire visible trunk and canopy are destroyed—the underground lignotuber remains unharmed by the heat above. Drawing upon its stored starch reserves, the lignotuber rapidly pushes up multiple new vegetative stems. This multi-stemmed regrowth form, commonly seen in the mallee eucalypts of arid and semi-arid regions, allows individual plants to survive repeated cycles of complete above-ground incineration across decades or centuries.
Seeding the Ash Bed
Fire also plays a direct role in the reproductive cycle of many eucalyptus species through a process known as serotiny. Eucalyptus seeds are held within tough, woody capsules commonly referred to as gumnuts. In many species, these capsules remain closed on the tree for years, protecting the delicate seeds from seed-eating insects and birds, as well as from the heat of passing flames.
The intense heat of a bushfire desiccates the capsules, triggering them to open in the days and weeks following the blaze. As the capsules open, millions of tiny seeds fall onto the forest floor. They land on an environment perfectly prepared for their survival: the ground is covered in a nutrient-rich layer of fresh ash, the competing understory has been cleared, and the destroyed canopy allows abundant sunlight to reach the ground. This ash bed effect gives the new eucalyptus seedlings an optimal environment to germinate and establish themselves quickly.
Evolutionary Success and Landscape Dominance
The suite of fire-promoting traits and survival mechanisms found in eucalyptus did not evolve in isolation; they are deeply tied to the climatic history of the Australian continent. As Australia drifted northward and became progressively more arid over millions of years, ancient rainforests retreated, and fire became a regular environmental disturbance. Eucalyptus species diversified and expanded, outcompeting plant lineages that could not withstand frequent burning.
By producing fuel that encourages hot, fast-moving fires, eucalyptus actively modifies its ecosystem. The recurring fires suppress fire-sensitive species, such as rainforest flora, while favoring pyrophytic, or fire-adapted, vegetation. This creates an ecological feedback loop where the presence of eucalyptus promotes fire, and fire perpetuates conditions that ensure the dominance of eucalyptus across diverse woodland and forest habitats.
Global Introduction and Wildfire Hazards
Due to their rapid growth, hardiness, and valuable timber and pulp, eucalyptus species have been planted extensively outside Australia, including throughout the Mediterranean, California, South Africa, and South America. However, importing the genus into ecosystems that did not co-evolve with its specific fire dynamics has frequently led to severe ecological and public safety challenges.
In non-native settings, the combination of heavy, oil-rich leaf litter, combustible hanging bark, and volatile foliage can dramatically alter local fire regimes. Plantations and escaped stands of eucalyptus burn with an intensity that local flora cannot tolerate and that municipal firefighting infrastructure struggles to contain. What represents an evolutionary masterstroke of survival in the Australian bush becomes a potent wildfire hazard when introduced to populated or non-adapted landscapes around the world.
Key takeaways
•Eucalyptus leaves contain volatile terpene oils and shed dense, oil-rich debris that actively promote rapid, high-intensity fires.
•Trees survive canopy loss through protected epicormic buds under the bark and underground lignotubers that sprout rapidly after burning.
•Woody seed capsules release their seeds onto post-fire ash beds, where seedlings benefit from high sunlight, abundant nutrients, and eliminated competition.
•While these adaptations ensure ecological dominance in fire-prone Australian landscapes, eucalyptus species introduce severe wildfire hazards when planted globally.