A forest full of arsonists

Eucalyptus is the most dangerous tree in the world. The entire plant is pumped with highly flammable oil. This is one reasons for the current megafires in Australia.

In Australia, the largest forest fires in fifty years are raging. The causes include persistent drought and, possibly, incorrect forest management. However, there is another reason for which an entire continent is on fire. This is the eucalyptus – probably the most dangerous tree species in the world.


The eucalyptus' species number around six hundred and make up the largest part of the Australian tree population. This is not a coincidence as these trees get rid of their competition using a brutal element: fire.


It sounds bizarre, but eucalyptus plants do everything they can to burn their forest. The most drastic measure is the production of tons of eucalyptus oil. The whole continent smells of it. In fact, they produce so much that the oil evaporates from the leaves and spreads in the air as a bluish mist, which is the reason behind the name given to the 'Blue Mountains' west of Sydney.


This essential oil has it all. Its most prominent characteristic is that it is more flammable than diesel. Imagine a Swiss beech tree when its leaves are sprayed with fuel – a spark is enough for the whole tree to become an oversized torch.


"Originally, eucalyptus oil [was] probably developed as a defence against predators such as beetles and caterpillars", says Michael Kessler, botanist and scientific director at the botanical garden at the University of Zurich. The oil is toxic to most animals and – in quantities beyond a few drops – to humans. A few animals, including the koala, have adapted to the toxins in the course of their evolution and can eat the leaves. "Presumably, the eucalyptus additionally enriches the essential oil to further fuel any fire", says Kessler.


The eucalyptus builds its own pyre.

But that's not enough for this tree; it has to be no less than an inferno. To do this, it builds its own pyre, so to speak. In addition to the leaves that fall to the ground all year round, eucalyptus trees also continuously shed smaller and larger branches. Long pieces of bark hang from the trunk from top to bottom, and when they are completely detached, they fall to the ground where, together with the oil-soaked leaves and branches, they form a perfectly prepared fireplace.


With the years, up to one and a half kilograms are added to every square meter. That is fifteen tons of dry tinder per hectare. And almost none of it rots. "It's way too dry for most of the year", says Kessler. "In addition, eucalyptus leaves are full of poisonous oil that also works against bacteria and fungi. Nothing can break it down".


A forest prepared in this way only needs a spark. As the eucalyptus cannot light a fire, it has to wait for lightning to strike or for the next cigarette butt to fall.


But once it begins, it makes life as difficult as possible for the fire brigade. The pieces of bark hanging from the trunk ignite and fall off, which makes it seem as if it's raining fire. When the wind blows, these pieces can be blown for up to 30 kilometres in front of the fire wall and can trigger a new forest fire there. In many cases, fighting a fire in a forest that repeatedly ignites itself is hopeless.


However, the eucalyptus survives its homemade apocalypse. Though the parts above ground level die of very hot fires, the roots survive. "If you cut a spruce, it's dead. If you cut a eucalyptus, it sprouts again", says Kessler. Finally, new trees sprout from the charred tree stumps.


Its seeds have it even better. They come in fireproof packaging. "Only when there is heat or smoke does the fruit open and release the seeds", says Kessler. They now fall on a soil that has been freshly fertilised by ashes and are completely unrivalled.


"There is also chemical warfare in his arsenal."

You may think that the people outside Australia would be afraid of this tree, but the opposite is true. They love it. There are huge plantations of eucalyptus trees in China, South Africa, Portugal, Brazil and California, among other places. They grow very quickly for hardwood and are thus popular for the production of construction and energy wood and as a raw material for paper. Beekeepers get a wonderful kind of honey from its flowers and eucalyptus oil is used to make cold ointment and cough drops. Their versatility makes them the most planted tree species in the world.


However, this plant also has the tendency to ruthlessly eliminate its competition, which is why it is now considered to be a rampant weed in many places. "On the one hand, eucalyptus is very thirsty", says Arne Witt, biologist and expert for invasive species at the Centre for Agriculture and Bioscience International (CABI). "A large tree can draw 200 litres of water from a neighbouring pond per day". Thus, it can cut off the water supply for entire ecosystems. 


"There is also chemical warfare in its arsenal", says Witt. "Through its roots, it releases substances into the soil that inhibit the growth of other plants".


Finally, of course, there's its pyromaniacal disposition. Many eucalyptus trees are planned to be felled in Portugal and California because the authorities are noticing a connection between their presence and the increased intensity of forest fires. They could be right about that.


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