The Australian species spreading all over the world and exacerbating bushfires

If you’ve been around the world a couple of times or maybe more, you’ll have seen the sights of gum trees on almost every foreign shore.

But climate change bringing hotter, drier conditions means the trees – especially plantations of Eucalyptus globulus or Tasmanian blue gum grown for paper – are contributing to bushfires from California to Portugal.

“With climate change, drying vegetation is problematic regardless of what species it is,” said Alen Slijepcevic, an internationally experienced firefighter and deputy chief officer of the Country Fire Authority in Victoria. “Eucalyptus brings that extra level of complexity because it wants to burn, and when it burns, it is quite volatile and produces a lot of embers.”

Eucalyptus trees burning in Spain in 2023.Credit: Europa Press via Getty Images

Hundreds of species of Eucalyptus or gum trees are native to Australia and a handful to Papua New Guinea, Indonesia and the Philippines. Elsewhere, Eucalyptus is an exotic weedy tree introduced in the late 19th or early 20th century.

Research from Western Sydney University reveals Eucalyptus trees are grown in plantations in nearly 100 countries and planted as city trees on every continent besides Antarctica.

Just as rabbits, blackberries and other introduced pests have devastated ecosystems in Australia, Eucalyptus trees are blamed for undermining biodiversity and increasing fire risk overseas.

Adjunct Professor Rick McRae, a bushfire expert at the University of NSW Canberra, said Eucalyptus trees had exacerbated large bushfires in Algeria, Portugal, Spain, Greece and California in recent years.

Eucalyptus is now the main tree species in Portugal, covering 845,000 hectares or 26 per cent of forests, including abandoned plantations. Much of the original forests were cut down during the Renaissance to build ships.

The Australian imports have played a big role in bushfires in Portugal, most notably in 2017 when 45,000 hectares burned in three days, including swaths of blue gum plantation, and dozens of people died. This prompted the Portuguese government to ban the establishment of new Eucalyptus plantations.

Eucalyptus forests are increasingly burning in Portugal and Spain in megafires larger than 500 hectares, researchers at the University of Lisbon wrote in a 2021 article in European Journal of Forest Research. Eucalyptus globulus is also self-seeding and the researchers found the wildings had a good chance of survival.

Dr Manuel Esperon-Rodriguez at Western Sydney University collected the figures on Eucalyptus plantations and was a co-author on a 2020 paper on urban Eucalyptus trees published in Global Ecology and Biogeography.

Smoke from an approaching fire rolls over Eucalyptus trees on a mountainside in Portugal in 2005.Credit: Associated Press

He said Eucalyptus camaldulensis – or the river red gum – was the most popular species for an urban tree, planted in at least 46 cities globally.

“It has a very high tolerance to heat and drought, so that could be one of the reasons why it has been broadly used in cities,” Esperon-Rodriguez said.

In North America, Eucalypts were blamed for the intensity of the 1991 Oakland fire in Northern California. The US National Park Service identified dense vegetation as a major contributor, especially Eucalyptus trees overhanging homes.

Los Angeles City firefighters fighting a brush fire under Eucalyptus trees in 2007.Credit: AP

In the recent Los Angeles fires, observers report that palm trees were more of a problem.

Professor Stephanie Pincetl, director of the California Centre for Sustainable Communities at the University of California Los Angeles, said Eucalyptus were brought to California in the 19th century for timber, especially for the railroads.

They did not take off as a plantation tree, she said, but became popular because they were “attractive, fast-growing and drought resistant”.

The CFA’s Slijepcevic has fought fires in Australia, New Zealand, Montana and Idaho in the US, and worked in forestry in his native Croatia. He would prefer to fight any other forest fire than Eucalyptus.

“You will have a propagation by ember in pine forests of 200 to 500 metres or maybe up to a couple of kilometres, whereas the recorded spotting … in Australia for Black Saturday was 35 kilometres,” Slijepcevic said.

A firefighter works to extinguish a fire in Eucalyptus forest in Portugal in 2024.Credit: AP

He said Eucalyptus also released volatile oils, though so do Chaparral shrubs in California.

Associate Professor Rachael Nolan at the Hawkesbury Institute for the Environment at Western Sydney University said the main reason Eucalyptus was so flammable was the “detachable bark”, which could catch fire and fly off in the wind.

“During drought they will also often drop their leaves or their bark, which creates a lot of dead litter on the forest floor [for fuel],” she added.

McRae, from UNSW, said a Eucalyptus plantation was a greater fire hazard than the same trees growing naturally in a forest community.

“When a gum tree is happy, there’s a big shrub layer underneath, and what that does is it reduces the wind flow underneath the canopy, and that changes the overall fire behaviour,” he said.

“But if you’ve got a blue gum plantation, then there’s nothing under the canopy for 30 metres, the wind just barges through, and it’s a very different fire behaviour.”

Professor Jason Sharples at UNSW Canberra said plantations also had the added danger of being high-density planting.

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