What Is Mountain Ash? Two Very Different Trees

“Mountain ash” refers to two completely unrelated trees on opposite sides of the world. In Australia, the name belongs to Eucalyptus regnans, the tallest flowering plant on Earth, a forest giant that can top 100 meters. In Europe and North America, it means Sorbus aucuparia, a modest, berry-laden tree in the rose family that rarely exceeds 15 meters. They share a common name for reasons rooted in old colonial botany, but they share almost nothing else in their biology, ecology, or the roles they play in human life.

The Australian Giant

Eucalyptus regnans dominates the tall, wet forests of southeastern Australia, particularly the Central Highlands of Victoria and the wet valleys of Tasmania. Living individuals reach heights of 100 meters, making them the tallest angiosperm species in the world.¹ To put that in perspective, a 100-meter tree stands taller than the Statue of Liberty from base to torch tip. Only certain conifers, notably the coastal redwoods of California, grow taller, and those belong to an entirely different branch of the plant kingdom. Among all flowering plants, E. regnans has no rival for sheer vertical reach.

These trees are not just tall; they are massive accumulators of biomass. Intensive measurements of 27 standing trees ranging from about 61 to nearly 100 meters tall and 80 to 430 years old showed that the largest living individual carried roughly 215 tonnes of aboveground biomass, adding close to 0.8 tonnes per year even in old age. An even larger specimen, killed by fire in 2003, had an estimated aboveground biomass of about 270 tonnes and was around 480 years old when it died.² A single tree accumulating a tonne of new wood each year, decade after decade, century after century, is remarkable productivity for any organism.

The European Rowan

Sorbus aucuparia, commonly called rowan in Britain and Scandinavia, is a slender deciduous tree that grows across Europe and into parts of western Asia. It thrives at altitudes where many other broadleaved trees struggle, colonizing mountain slopes, moorland edges, and rocky hillsides. Its pinnate leaves, made up of paired leaflets along a central stem, superficially resemble the foliage of true ash trees in the genus Fraxinus, and that resemblance is the source of the “ash” in its name. The “mountain” part comes from its preference for upland terrain. Put together, you get “mountain ash,” a name bestowed by people who noticed ash-like leaves on a tree growing in hilly country.

Rowan belongs to the Rosaceae, the rose family, alongside apples, pears, and hawthorns. Its clusters of bright orange-red berries are one of its most recognizable features, ripening in late summer and hanging in heavy bunches through autumn. The berries are technically pomes, small fruit structures related to crab apples. They are mildly toxic when raw because of parasorbic acid, which can cause nausea, but cooking or freezing converts parasorbic acid into sorbic acid, rendering the fruit safe and mildly tart. For centuries, rowan berries have been made into jellies, wines, and preserves across Northern Europe.

Why Two Unrelated Trees Share a Name

European settlers arriving in southeastern Australia in the 19th century encountered vast forests of Eucalyptus regnans and needed a working name for the timber. The wood of E. regnans is pale, straight-grained, and relatively light for a eucalypt, qualities that reminded the settlers of European ash (Fraxinus excelsior). The trees also grew in cool, wet, mountainous country. So the logic followed the same path it had followed centuries earlier in Europe with rowan: ash-like wood, mountain habitat, “mountain ash.” The fact that neither tree is actually related to true ash did not trouble anyone at the time. Common names are practical labels, not taxonomic statements, and the same label got applied independently to two organisms that happened to share a few superficial traits.

This sort of naming collision is surprisingly common in botany. “Cedar” refers to entirely different trees in Lebanon, Japan, and the American Pacific Northwest. “Ironwood” applies to dozens of unrelated hardwoods worldwide. Mountain ash is just one of the more confusing cases because both trees remain widely known by the same name in their respective regions, creating real confusion for anyone reading across geographies.

The World’s Most Carbon-Dense Forests

One of the most striking findings about E. regnans forests concerns their capacity for carbon storage. An analysis of published global site biomass data from 136 primary forests found that the highest known total biomass carbon density, covering both living and dead material, occurred in Australian temperate moist E. regnans forests. Across 13 study sites, these forests averaged 1,867 tonnes of carbon per hectare.³ That figure exceeded the averages for sampled tropical rainforests, which many people assume hold the carbon-density record.

The combination of factors that makes this possible is relatively straightforward. Cool temperatures and moderately high rainfall produce fast growth rates but slow decomposition of dead wood, so carbon accumulates faster than it breaks down. Many of these stands are old-growth, with multiple age classes and canopy layers, and they have experienced minimal human disturbance, allowing centuries of biomass to build up without major losses.³ Before a stand-replacing wildfire in 2009, one of Australia’s tallest forest sites held a maximum aboveground biomass of over 1,500 tonnes per hectare and a carbon mass of about 706 tonnes per hectare.² That is an extraordinary amount of carbon locked up in living wood.

Fire and the Paradox of a Fire-Sensitive Eucalypt

Most people associate eucalypts with fire resilience. Many eucalyptus species resprout vigorously after burning, sending new shoots from specialized structures beneath the bark. E. regnans breaks this mold. It is one of the few eastern Australian eucalypts classified as fire-sensitive and an obligate seeder, meaning that mature trees are killed by intense fire rather than resprouting from their trunks.⁴ The species depends entirely on seed to regenerate after a blaze.

This reproductive strategy means the timing between fires is everything. After a major wildfire, E. regnans seeds germinate in large numbers within the first year. Research into seedling establishment found that seedling presence and abundance were strongly tied to the occurrence of fire itself, but not to variation in fire severity between moderate and high intensity.⁵ In other words, what matters is that a fire happens and opens the canopy, not whether the fire was moderately fierce or catastrophic. But if a second fire arrives before the new generation has matured enough to produce its own seed, the population can collapse. Research on logged and repeatedly burned mountain ash stands has confirmed that multiple fires reduced populations of species dependent on on-site seeding for persistence, including E. regnans itself.⁶

This vulnerability creates a conservation tension. A warming climate increases the frequency and intensity of bushfires in southeastern Australia, shortening the interval between burns. For a tree that needs decades to grow large enough to produce viable seed, more frequent fire is an existential threat, not a natural cycle it can ride out.

Hollow Trees and the Animals That Need Them

Old mountain ash trees eventually develop cavities in their trunks and limbs, and these hollows are critical habitat for a range of animals. In Victoria’s Central Highlands, researchers found that hollow-bearing E. regnans trees occupied by arboreal marsupials, such as Leadbeater’s possum and the greater glider, had significantly larger diameters than unoccupied hollow-bearing trees. The mean diameter of both occupied and unoccupied hollow-bearing trees was almost three times that of younger trees from the 1939 cohort (a generation that regenerated after the catastrophic Black Friday bushfires) that did not yet contain hollows.⁷

The problem is time. Hollows do not form in E. regnans until a tree is well over a century old, sometimes closer to two centuries. Meanwhile, large hollow-bearing trees in mountain ash forests are in rapid decline, lost to fire, windstorms, and industrial logging. The gap between the loss of existing old trees and the decades needed for younger trees to develop usable cavities creates a housing crisis for cavity-dependent wildlife.⁷ Protecting every remaining hollow-bearing tree has become a central goal in the conservation management of these forests.

Pre-Colonial Mountain Ash Forests

There is a common assumption that Australian forests before European colonization were open, park-like landscapes maintained by Aboriginal burning. For some vegetation types, particularly drier woodlands and grasslands, this appears to have been the case. But research into the historical structure of tall, wet mountain ash forests suggests they were different. These forests were not open or park-like; they were dense, multilayered, and structurally complex.⁸ At the same time, they were not untouched wilderness. They were places of significance to First Peoples, managed through traditional ecological knowledge over thousands of years. Understanding what these forests looked like before British colonization is considered essential for establishing meaningful baselines for restoration, particularly for reintegrating Indigenous land management practices after more than two centuries of disturbance.

Rowan Berries and Birds

Back on the other side of the world, the ecology of Sorbus aucuparia revolves around its fruit. Rowan berries are a staple food for thrushes and other frugivorous birds across Europe, and this relationship is far from passive. On the west coast of Norway, thrushes are important dispersal agents for rowan seeds, and passage through a bird’s gut actually improves the seed’s chances. Research found that gut treatment increased both the germination rate and subsequent seedling growth of S. aucuparia compared to seeds that had not passed through a bird.⁹ The digestive process strips away the fruit pulp and chemically scarifies the seed coat, priming it for faster sprouting.

Rowan also displays masting behavior, producing bumper crops of berries in some years and far fewer in others. Research has shown that masting increases seedling recruitment through two simultaneous mechanisms: it overwhelms seed predators (who cannot consume the entire crop, leaving more seeds to survive) and it attracts more frugivorous birds during big-crop years, which disperse seeds farther from the parent tree.¹⁰ This dual payoff helps explain why rowan invests in the energetically expensive strategy of variable fruiting rather than producing a steady, moderate crop every year.

Rowan in Folklore and Traditional Use

Few European trees carry as much cultural baggage as rowan. In Celtic, Norse, and Anglo-Saxon traditions, rowan was widely regarded as a protective tree, believed to ward off witchcraft and evil spirits. Sprigs of rowan were hung over doorways and livestock pens, and the wood was used for walking sticks, spinning tools, and protective charms. These traditions are documented across centuries of European folk practice.¹¹ The species name aucuparia comes from the Latin for “bird-catching,” a reference to the long-standing practice of using the sticky berries to bait bird traps.

Beyond folklore, rowan’s berries have a genuine culinary history. In Scandinavia, rowan jelly is a traditional accompaniment to game meats. In Scotland, rowan berry wine has been produced for generations. The key is that the berries must be cooked or frozen before consumption to neutralize the parasorbic acid that makes them unpleasant (and potentially nauseating) when eaten raw. Once processed, they contribute a sharp, slightly bitter flavor that works well in preserves and fermented drinks.

Sorbitol and the Chemistry of Rowan

Rowan has a small but legitimate claim to chemical fame: the sugar alcohol sorbitol was first isolated from rowan berries in 1872, and the compound takes its name directly from Sorbus. Sorbitol is now one of the most widely used sugar substitutes in the world, found in sugar-free gum, diabetic foods, and pharmaceutical formulations, though commercial production today uses corn starch rather than rowan berries.

The chemistry of rowan shifts noticeably with the seasons. Research on S. aucuparia found that autumn samples showed a pronounced accumulation of polyols, particularly sorbitol, whose concentration rose from about 7% to nearly 22% as the plant prepared for winter. Fatty acids, including palmitic acid and various unsaturated fatty acids, also accumulated in autumn tissue.¹² This seasonal ramp-up of sorbitol and other polyols is a cold-protection strategy. Polyols function as biological antifreeze, lowering the freezing point of cellular fluids and helping the tree survive harsh mountain winters without ice damage to its tissues.

How to Tell Them Apart at a Glance

If you somehow encountered both trees side by side, confusion would be impossible. But since they live on different continents, the real confusion is verbal: someone says “mountain ash” and the listener pictures the wrong tree. A few key distinctions help keep them straight.

• Size: E. regnans is a forest skyscraper that can exceed 90 meters. S. aucuparia is a modest tree, typically 8 to 15 meters tall.
• Leaves: E. regnans has the classic eucalyptus leaf, long, narrow, leathery, and evergreen. Rowan has compound pinnate leaves with paired leaflets, deciduous in winter.
• Fruit: E. regnans produces small, woody seed capsules. Rowan produces clusters of bright red-orange berries.
• Bark: E. regnans has smooth, pale bark that sheds in long ribbons on the upper trunk, with a rough fibrous “stocking” at the base. Rowan bark is smooth and silvery-grey.
• Family: E. regnans is a myrtle (Myrtaceae). S. aucuparia is a rose (Rosaceae). Their last common ancestor lived hundreds of millions of years ago.

The naming confusion matters most in practical contexts: forestry, ecology, conservation policy, and even casual reading. A study about “mountain ash forest decline” from Australia is describing a completely different ecosystem, with completely different threats and conservation needs, than a European discussion of declining rowan populations. Being aware that the name spans two trees is the first step toward reading either body of literature clearly.

Global Warming and the Tallest Flowering Plant

Climate projections for E. regnans are not encouraging. Research has indicated that global warming is expected to reduce the carrying capacity of these forests, meaning the maximum height and biomass they can sustain over time.¹ The mechanisms are interrelated: higher temperatures increase evaporative demand, raise fire frequency, and shorten the intervals between stand-replacing blazes. For a species that needs long fire-free windows to reach reproductive maturity and centuries to achieve its full stature, a warming climate threatens not just individual trees but the structural character of the forests themselves. The 100-meter giants that exist today grew during cooler centuries. Whether the species can still produce trees of that caliber in a hotter, drier future is an open and sobering question.

Rowan faces its own climate pressures in Europe, though they are less dramatic. As a cold-adapted species, warming temperatures push its viable range uphill and northward. In some lowland areas where rowan was once common, it is being outcompeted by faster-growing broadleaved species that thrive in milder conditions. Still, rowan is adaptable, tolerant of poor soils, and widely planted in urban and suburban settings across Europe. Its survival as a species is not in serious doubt, even if its wild distribution shifts.