A “gum tree” is any of the roughly 800 species of eucalyptus native to Australia, along with their close relatives in the genera Corymbia and Angophora. The name comes from the thick, sticky exudate called kino that oozes from wounds in the bark. Though the term is colloquial and sometimes loosely applied, gum trees share a set of striking traits that set them apart from nearly every other group of trees on Earth: a distinctive flower bud capped by a lid-like structure, aromatic oil-packed leaves, bark that ranges from papery and peeling to deeply furrowed, and remarkable adaptations to fire, drought, and nutrient-poor soils.
Why They Are Called Gum Trees
The “gum” in gum tree refers to kino, a dark red to reddish-brown exudate that seeps out whenever the bark or cambium is damaged. Kino is rich in polyphenols, and it flows through a network of canals called kino veins that the tree produces in response to injury. These canals encircle the trunk and branches, forming a connected defense system that seals wounds and discourages pathogens and insects.1Trees. Holocrine secretion and kino flow in angiosperms: their role and physiological advantages in plant defence mechanisms Early European settlers in Australia adopted the name “gum tree” after seeing kino pooling on trunks, and the term stuck. Aboriginal Australians had long recognized kino’s medicinal properties, using it alongside other eucalyptus-derived preparations for treating inflammation and wound care.2PubMed. Revisiting kinos–an Australian perspective
The Operculum and Other Flower Features
If there is one trait that botanists have traditionally used to define eucalypts, it is the operculum, a cap or lid that covers the flower bud and pops off when the flower opens. In most eucalypt species, the petals and sepals have fused together during development to form this single structure, which protects the reproductive parts of the bud until pollination time.3Plant Systematics and Evolution. Operculum development in the Eudesmieae B eucalyptus and Eucalyptus caesia (Myrtaceae) The result is a flower that, once the lid lifts, looks like a burst of stamens with no visible petals at all. Genetic studies in Eucalyptus globulus have found that a handful of genomic regions strongly influence operculum shape, with the most powerful one explaining about a quarter of the variation in how elongated the cap becomes.4PubMed Central. Genetic control of the operculum and capsule morphology of Eucalyptus globulus
The operculum is also at the heart of how eucalypt taxonomy has gotten complicated. For a long time, having an operculum was what made something a eucalypt. But Angophora, a genus closely related to eucalypts, has separate petals and sepals rather than a fused cap, yet molecular studies place it nested within the group that includes Corymbia, the bloodwood eucalypts. Researchers have concluded that the operculum has evolved, been lost, and possibly re-evolved more than once across the eucalypt lineage, which makes it an unreliable single marker for drawing strict genus boundaries.5Journal of Systematics and Evolution. Perianth evolution and implications for generic delimitation in the eucalypts (Myrtaceae), including the description of the new genus, Blakella
Leaves That Change Shape and Smell
One of the more unusual things about gum trees is heteroblasty: the juvenile leaves look dramatically different from the adult ones. In Eucalyptus globulus (the Tasmanian blue gum), young leaves are rounded, waxy blue, and arranged in pairs clasping the stem, while mature leaves are long, sickle-shaped, and hang vertically. These are not just cosmetic differences. Juvenile leaves invest more carbon into producing isoprene, a volatile organic compound, and use water less efficiently than the adult foliage.6PubMed. Characterization of juvenile and adult leaves of Eucalyptus globulus showing distinct heteroblastic development: photosynthesis and volatile isoprenoids The vertical hang of mature leaves is thought to minimize overheating by presenting their edges rather than their flat surfaces to the midday sun.
The leaves are packed with oil glands visible as tiny translucent dots when you hold a leaf to the light. These glands house terpenes, the aromatic compounds responsible for the signature eucalyptus smell. In at least some species, leaves contain two distinct types of glands that store different chemicals. In Eucalyptus brevistylis, translucent glands hold sesquiterpene alcohols while golden-brown glands concentrate an entirely different class of antimicrobial compounds called beta-triketones. As trees age, the ratio shifts: younger plants have more of the triketone-producing glands, while older trees develop more sesquiterpene glands.7Tree Physiology. Differential metabolic specialization of foliar oil glands in Eucalyptus brevistylis Brooker (Myrtaceae) This chemical complexity is part of what makes eucalyptus essential oil so varied depending on species, leaf age, and season.
Bark Diversity
Walk through an Australian forest and you will notice that gum trees wear their bark in strikingly different ways. Some are smooth and white, shedding long strips of papery bark each summer. Others are deeply furrowed with thick, rough bark that stays firmly attached. Researchers have catalogued at least twelve distinct bark types among eucalypts, and these are not just ornamental. Bark type is tightly linked to how a tree copes with fire and drought, and the distribution of different bark types across the landscape reflects the fire frequency and intensity of the local environment.8Journal of Biogeography. Bark Type as an Indicator of Pyro‐Ecological Strategy in the Eucalypts Smooth-barked species like the lemon-scented gum tend to grow in lower-fire-frequency areas or shed their bark to drop burning material away from the canopy, while species with thick, persistent bark, like ironbarks, rely on insulation to protect the living tissue beneath from intense heat.
Where Gum Trees Grow Naturally
Eucalypts are overwhelmingly Australian. More than 800 species are found naturally across the continent, dominating landscapes from coastal rainforest margins to arid inland woodlands. The handful of exceptions include Eucalyptus deglupta, the rainbow eucalyptus of Southeast Asian islands, and E. urophylla from Timor and neighboring islands.9Forest Ecology and Management. Native forests and climate change: Lessons from eucalypts Within Australia, eucalypt-dominated ecosystems have been drastically reduced since European colonization. More than 47 million hectares of eucalypt woodland have been cleared for agriculture since 1770, particularly in eastern Australia and southwestern Western Australia.9Forest Ecology and Management. Native forests and climate change: Lessons from eucalypts
Gum trees thrive on soils that would starve most other trees. Australian soils are among the oldest and most nutrient-depleted on Earth, particularly in phosphorus. Eucalypts have evolved to make do with very little. When phosphorus is scarce, some species ramp up how efficiently their photosynthetic machinery uses what is available, and at least one species reshuffles the fat composition of its cell membranes to free up phosphorus locked in membrane lipids.10PubMed Central. Low phosphorus induces differential metabolic responses in eucalyptus species improving nutrient use efficiency This ability to function on impoverished soils is one reason eucalypts dominate so much of the Australian landscape and also why they can become aggressive colonizers when planted in richer soils elsewhere.
How Gum Trees Handle Fire
Eucalypts and fire have a relationship that goes back tens of millions of years, and it shows. Many species are not just fire-tolerant but appear to be fire-dependent, relying on periodic burning to clear competitors, release seeds, and trigger regrowth.
The key adaptation is epicormic resprouting. In most broadleaf trees, dormant buds sit in the outer bark, where fire kills them. In eucalypts, the greatest concentration of bud-initiating tissue lies deep, at the level of the vascular cambium, protected by the full thickness of the bark.11New Phytologist. Epicormic strand structure in Angophora, Eucalyptus and Lophostemon (Myrtaceae) – implications for fire resistance and recovery After even an intense crown fire that scorches every leaf, a eucalypt can sprout a flush of new green shoots along its trunk and branches within weeks. This ability is shared across the eucalypt lineage and also features in their relatives Angophora and Corymbia.12PubMed. Resprouting as a key functional trait: how buds, protection and resources drive persistence after fire
Many eucalypts also develop lignotubers, swollen woody structures at the base of the trunk packed with dormant buds and starch reserves. If the aboveground portion of the tree is killed entirely, the lignotuber can send up new stems. Research on Eucalyptus obliqua seedlings found that epicormic and lignotuberous regrowth after stress was preceded by root tip growth, underscoring how dependent recovery is on a healthy root system.13Arboriculture & Urban Forestry. Root Tip Growth and the Presence of Leaves Affect Epicormic and Lignotuberous Shoot Development and Survival of Stressed Eucalyptus obliqua L’Herit. Seedlings
Fire season matters for recruitment too. In the jarrah forests of southwestern Australia, seedling emergence was highest in the first year after autumn burns, but by the final survey period, the highest seedling densities appeared after moderate-to-high-severity spring fires. Seedlings that sprouted after spring burns delayed their emergence, apparently holding off until winter rains arrived.14Forest Ecology and Management. Assessing the effects of fire season, fire severity and rainfall on seedling establishment and mortality in dry eucalypt forests Mortality was highest after autumn fires, suggesting that the timing and intensity of fire shapes not just which trees survive but which new ones get established.
Drought Strategies
Australia is the driest inhabited continent, and gum trees have accordingly developed a range of drought-coping mechanisms. Clonal studies using Eucalyptus globulus illustrate how this works at the individual level. When water becomes scarce, drought-tolerant trees maintain higher water status in their leaves, keep their stomata open longer to continue photosynthesizing, and shift more growth resources to their root systems. In a controlled drought experiment, a tolerant clone maintained active carbon uptake substantially longer than a sensitive one, because it balanced water loss from leaves with water uptake by a more extensive root network.15PubMed. Responses to water stress in two Eucalyptus globulus clones differing in drought tolerance The practical takeaway is that drought tolerance in eucalypts is not about shutting down; it is about investing in roots that keep the water supply running.
Koalas and the Chemistry of Being Edible
No discussion of gum trees would be complete without the animal most famously associated with them. Koalas eat almost nothing but eucalyptus leaves, which is a remarkable dietary choice given how toxic those leaves are to most mammals. The leaves are loaded with monoterpenes and other secondary metabolites that serve as the tree’s chemical defenses.
Koalas are selective about which trees they eat from. Research has shown that a class of compounds called formylated phloroglucinol compounds, or FPCs, strongly deters koala feeding. In captive feeding trials, koalas consistently ate less from trees with high FPC concentrations in a single night.16PubMed Central. Eucalyptus foliar chemistry explains selective feeding by koalas Monoterpenes pose a different problem. Because they dissolve readily in fat, they get absorbed through the gut and enter the bloodstream, meaning a koala faces chronic systemic exposure to compounds that in large amounts can disrupt immune function.17PubMed. Ingestion and Absorption of Eucalypt Monoterpenes in the Specialist Feeder, the Koala (Phascolarctos cinereus)
The koala’s solution is a powerful liver detoxification system. Studies of the compound 1,8-cineole (the main component of eucalyptus oil) found that koalas break it down through extensive oxidation, producing at least seven metabolites. The dominant products were hydroxycineolic acids, accounting for about 85% of all metabolites. The koala accomplishes this without relying on the conjugation pathways that most other mammals use, suggesting a specialized biochemical toolkit shaped by millions of years of eating little else.18PubMed. Metabolites of dietary 1,8-cineole in the male koala (Phascolarctos cinereus)
How Old Are Eucalypts
Gum trees have been around for a very long time, though exactly how long depends on whether you trust the molecular clock or the fossil record. Molecular estimates for the eucalypt tribe (Eucalypteae) suggest they appeared roughly 65 million years ago, around the time of the mass extinction that ended the age of dinosaurs.19Australian Journal of Botany. How old are the eucalypts? A review of the microfossil and phylogenetic evidence Fossils paint a slightly younger picture. The oldest definitive eucalyptus macrofossils date to about 52 million years ago from the early Eocene, and they were found not in Australia but in Patagonia, Argentina. These included multiple leaves, flower buds, and seed capsules that phylogenetic analysis placed firmly within the genus Eucalyptus.20PubMed Central. Oldest known Eucalyptus macrofossils are from South America Fossils of comparable age have also been found in southeastern Australia and eastern Antarctica, which at that time were still connected via Gondwana. The South American fossils are significant because they show the eucalypt group was not always confined to Australasia.
Pests, Pathogens, and Myrtle Rust
Eucalypts are tough trees, but they are not invulnerable. One of the more concerning threats in recent years is myrtle rust, caused by the fungus Puccinia psidii. This pathogen attacks a wide range of plants in the myrtle family (Myrtaceae), which includes eucalypts. In Brazilian plantations, myrtle rust has caused reduced growth, deformed stems, and tree death in severe cases. The strain that arrived in Australia has so far not caused serious disease in eucalypt plantations there, but it remains a concern for native ecosystems where susceptible species may lack resistance.21PubMed. First Report of Puccinia psidii (Myrtle Rust) in Eucalyptus Plantations in Australia The worry is less about plantation trees, which can be screened for resistance, and more about wild populations of rare Myrtaceae species with small ranges and no breeding programs to fall back on.
Climate Change and What It Means for Gum Trees
Climate projections for eucalypts make for sobering reading. Modeling suggests that species in Australia’s central desert and open woodland regions will be hardest hit, potentially losing around 20% of their suitable climate space under a mid-range warming scenario and roughly 40% under an extreme one. Even the least affected species in eastern Australia may lose about 10% of their range under moderate warming. Shifts will not simply be poleward; east-to-west range changes driven by changing rainfall patterns may be more significant in subtropical and mid-latitude regions.22PubMed Central. Eucalypts face increasing climate stress
Not all eucalypts will respond the same way. Seedling experiments comparing species with narrow and wide geographic ranges found that warming of 3.5°C generally stimulated growth, but the boost was larger for widely distributed species and for those from temperate rather than tropical origins. Narrowly distributed species showed some capacity to acclimate physiologically but were less able to adjust their leaf size and structure in response to heat.23PubMed. Pushing the envelope: do narrowly and widely distributed Eucalyptus species differ in response to climate warming? For a genus with hundreds of species that range from tropical Queensland to alpine Tasmania, this unevenness matters. The rare, range-restricted species are the ones least equipped to cope.
For the tallest flowering plant on Earth, Eucalyptus regnans (mountain ash), warming poses a particularly stark threat. Statistical modeling found that each additional degree Celsius of warming was associated with roughly a 25% increase in tree mortality rates and a 9% drop in the stand’s carrying capacity. A three-degree rise in mean annual temperature would translate to an estimated 24% decline in tree density and carbon stored in mountain ash forests.24Nature Communications. Global warming reduces the carrying capacity of the tallest angiosperm species (Eucalyptus regnans) These are forests that currently store some of the highest carbon densities of any ecosystem on the planet, so their decline has implications well beyond botany.
Indigenous Uses of Eucalyptus
Aboriginal Australians have used gum trees for thousands of years in ways that go far beyond kino. The Dharawal people, for example, traditionally used various local plant species, including eucalypts, in treatments for inflammatory conditions such as asthma, arthritis, fever, and eye inflammation. Modern laboratory testing of these plants has confirmed measurable anti-inflammatory activity in several of them.25PubMed Central. Medicinal Plants of the Australian Aboriginal Dharawal People Exhibiting Anti-Inflammatory Activity Eucalyptus bark was used for making shelters, canoes, and containers. Leaves were burned or crushed to release their oils for respiratory relief, a practice that early European settlers quickly adopted and eventually commercialized into the eucalyptus oil industry. The relationship between Aboriginal people and eucalyptus landscapes also extended to fire management. Indigenous burning practices, maintained over millennia, helped shape the very fire regimes to which eucalypts are so conspicuously adapted.