What it is
Not one tree. Over 700 species in the genus Eucalyptus, plus 100 more in Corymbia and Angophora. Together they are the eucalypts.
The range is staggering. Mountain ash (E. regnans) tops 100 meters, the tallest flowering plant on earth. Mallees stay under 3 meters, multi-stemmed shrubs scratching out a living in semi-arid scrub.
Eucalypts dominate every major Australian ecosystem. Rainforest margins. Temperate forests. Alpine woodlands. Coastal heath. Arid scrub. Each region runs on its own locally adapted suite.
The plantation debate hinges on three species: E. globulus, E. camaldulensis, and E. grandis. Judging the whole genus by these is like judging all grasses by industrial maize.
Why it works in Australia
Fifty million years of isolation. That is the timeline behind the genus.
Eucalypts hold ground from sea level to the snowline, from 150 mm of rain to over 3,000. Some tolerate salt spray. Some grow in waterlogged soil. Some survive on skeletal rock.
In Australia they are the structural foundation. Koalas eat only eucalyptus leaves. Sugar gliders and yellow-bellied gliders drink the sap. Cockatoos, parrots, and lorikeets feed on the nectar and nest in the hollows.
Those hollows are the catch. They take 100 to 200 years to form. No other Australian genus produces them.
Fire as a strategy
Most eucalypts are fire-adapted. Some are fire-promoting.
Oily leaves and peeling bark build a fuel load that carries flame into the canopy. The trees then exploit the burn. Epicormic buds under the bark sprout within weeks, re-leafing the trunk. Lignotubers, woody underground organs, push fresh shoots from below even after the crown is gone. Serotinous capsules release seed straight into the post-fire flush of nutrients and sunlight.
The loop is self-sustaining. Eucalyptus forest burns, eucalypts regenerate, fire-sensitive competitors are suppressed, and dominance holds.
Break that loop through land clearing, altered fire regimes, or Phytophthora dieback, and the cascade runs through every dependent species. This is why Australian restoration projects plant eucalypts. They are rebuilding the keystone, not establishing a plantation.
When it goes wrong
Outside Australia, the story flips.
From the mid-19th century, E. globulus and E. grandis went out as monocultures across southern Europe, South America, sub-Saharan Africa, and South Asia. Growth rates of 20 to 30 cubic meters per hectare per year. Straight trunks for pulp. Marginal land, minimal management. By the late 1900s, plantations covered over 20 million hectares.
The ecological bill came due.
Biodiversity collapse. Eucalyptus monocultures hold a fraction of the species the native vegetation did.
Water draw. A mature E. globulus transpires over 100 liters a day. Plantations in Portugal, South Africa, and India have measurably dropped water tables and cut stream flow.
Litter buildup. The fungi and invertebrates that decompose eucalyptus leaves did not travel with the trees. Litter accumulates and smothers ground flora. Allelopathic compounds in the leaves and roots block germination of understory species.
Fire. The 2017 Portugal fires killed over 100 people and burned hard through eucalyptus plantations. Volatile oils and shedding bark, useful in Australia, become accelerant in Mediterranean landscapes that did not evolve with them. Calls for restrictions on planting and a shift to native species have grown louder since.
Right species, right place
The frame is not good tree or bad tree. It is right context or wrong context.
In Australia, the question is which of 700 species fits which soil, rainfall, and ecosystem. Snow gum (E. pauciflora) belongs in alpine woodland. River red gum belongs on a riverbank. Site matching is the foundation of all restoration design, and the genus offers extraordinary precision of fit within its home continent.
Outside Australia, the case is narrow but real. Agroforestry can use selected eucalyptus species for windbreaks, shade, or biomass on four conditions: mixed planting with local natives, climate-appropriate species selection, active management against escape, and an honest water budget.
Some tropical restoration projects run eucalyptus as a short-rotation pioneer species. Five to ten years of timber and biomass income while native trees establish beneath, then the eucalyptus comes out as the native canopy closes. The line is clear. Transitional eucalyptus in service of native forest is one thing. Permanent monoculture that replaces native ecosystems is another.