Eucalyptus and fire

Eucalyptus
Eucalyptus tereticornis’ buds, capsules, flowers and foliage, Rockhampton, Queensland. Photo by Ethel Aardvark.

Wildland firefighters in Australia and in some areas of California are very familiar with eucalyptus trees. They are native and very common in Australia and are planted as ornamentals in the United States. The leaves produce a volatile highly combustible oil, and the ground beneath the trees is covered with large amounts of litter which is high in phenolics, preventing its breakdown by fungi. Wildfires burn rapidly under them and through the tree crowns. It has been estimated that other than the 3,000+ homes that burned in the 1991 Oakland Hills Fire in California, about 70 percent of the energy released was through the combustion of eucalyptus.

Eucalyptus is one of three similar genera that are commonly referred to as “eucalypts”.

Jon Henley, a reporter who covered the numerous large bushfires a year ago in Australia, has written a book about fire down under, titled “Firestorm: Surviving the Tasmanian bushfire”. Below is an excerpt:

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“…Gum trees, as eucalypts are known, ‘are like weeds that come up on bombed-out blocks’, adds Jamie Kirkpatrick, professor of geography and environmental studies at the university. ‘They’re fantastically fast growers and great colonisers, but not great competitors.’

Eucalypts typically let through a lot of light, allowing other vegetation types such as scrub and grass to grow beneath them. They can live for maybe 700 years. But they won’t regenerate, Kirkpatrick explains, if what is growing beneath them over the years becomes too dense. Most eucalypt species, therefore — there are more than 600 in Australia, between 30 and 40 in Tasmania — have evolved traits that allow them to survive and prosper in the fires that will clear that undergrowth.

Some, like the mighty, 100-metre-tall Eucalyptus regnans — also known as the mountain ash, stringy gum or Tasmanian oak — hold their seeds inside small, hard capsules; a fire will instantly trigger a massive drop of seeds to the newly fertilised ground.

The myriad bright green buds that sprout spectacularly from the trunks of other eucalypts in the aftermath of a big fire are another kind of regeneration mechanism, bursting through the scorched and blackened bark within weeks of a blaze.

Within five or six years, ‘a burned forest will be looking pretty good’, Kirkpatrick says. ‘And a large proportion of Tasmania’s flora fits into this fire ecology. Pea plants, wattles — their germination is stimulated by heat and smoke. Fire is really, really important in Tasmania.’

At the centre of it all, though, is the eucalypt. Because these trees do not just resist fire, they actively encourage it. ‘They withstand fire, they need fire; to some extent, they create fire,’ Bowman says. ‘The leaves, the bark, don’t decompose. They’re highly, highly flammable. And on a hot day, you can smell their oils.’

The bark and leaves of eucalypts seem almost made to promote fire. Some are known as stringyor candle-barks: long, easily lit strips hang loosely off their trunks and, once alight, whirl blazing up into the flammable canopy above, or are carried by the wind many kilometres ahead of a fire to speed its advance.”

This is an edited extract from Firestorm: Surviving the Tasmanian bushfire by Jon Henley (Guardian Shorts £1.99 / $2.99)

Get it from Amazon Kindle or directly from Guardian Shorts.

20 years later, potential for another Oakland Hills fire?

It was 20 years ago today that a rapidly moving fire in the Oakland Hills east of San Francisco ravaged a community. Here is the way we describe it in our Infamous Fires Around the World document:

The “Tunnel Fire”, commonly referred to as the Oakland Hills fire or East Bay Hills fire, occurred on Sunday October 20, 1991. The fire killed 25 people (23 civilians, 1 police officer, and 1 firefighter), injured 150, and destroyed 2,449 single-family dwellings and 437 apartment and condominium units. Eleven of the fire victims died in traffic jams on Charing Cross Road while evacuating. Eight others died on narrow streets in the same area. The economic loss has been estimated at $1.5 billion.

1991 Oakland Hills fire progression mapThe fire started when an ember from a grass fire the previous day blew beyond the fire hoses that were still on the fire perimeter and started a new fire. Houses, like the vegetation, have grown back and some of the residents that lived through the 1991 fire are worried when they look around and see that some of their new neighbors are not doing as much as they could to prevent another disaster.

Here is an excerpt from an interesting article in the Mercury News:

As autumn returned and the mercury hovered in the 90s in the Oakland hills, Milt Brown started to feel anxious.

Twenty years ago, on a scorching, wind-whipped day, he lost two houses in one of the nation’s deadliest and most destructive urban wildfires, an inferno that jumped two freeways, destroyed more than 3,800 homes and killed 25 people, including the Browns’ former baby sitter.

Although he tries not to dwell on the horrible memories — or the chance of another devastating blaze — Brown and other survivors of the Oakland hills fire worry that the painful lessons of that day are being forgotten. Or worse, they are being ignored by the many newer residents who didn’t experience firsthand the hell of Oct. 20, 1991. Even the subtlest signs of danger make him nervous.

“I’m looking at the two houses below me and the branches are touching the house,” Brown said from his perch on Buckingham Boulevard — less than a minute’s walk from where the fire erupted on a hot Sunday morning. “I’m in a box canyon. If someone throws a match in there it will set the whole block off.”

But it isn’t just those who lived through the Oakland hills fire who are anxious about what they fear is a growing complacency that has built up alongside the stately homes in these steep, once-woodsy enclaves. Fire officials say that time has not only given rise to dense stands of fast-growing and fire-susceptible eucalyptus on public lands, it has also given vegetation on private property throughout the hills 20 years to mature. It often takes a second notice before residents take heed and clear a defensible space around their homes to protect it from fire.

Thanks go out to Dick

Have some plants evolved to promote fire?

Olivia Judson, an evolutionary biologist, recently traveled to Australia where she learned a little about fires in eucalyptus forests. In an article she wrote for the New York Times, she wonders if some plants have specific characteristics that make it more likely that fires will burn intensely. Here is an excerpt from her article.

….It’s common knowledge that plants regularly exposed to fire tend to have features that help them cope with it — such as thick bark, or seeds that only grow after being exposed to intense heat or smoke. But what is less often remarked on is that the plants themselves affect the nature and severity of fire.

For example, dead branches burn more readily than living branches, so a tree that keeps dead branches (rather than letting them fall) makes it easier for a fire to climb into a forest canopy: the dead branches provide a ladder for the fire. Deadwood also allows fires to get hotter. Leaves that are high in cellulose, or that contain oils, also stoke the flames. Resins and gums are highly flammable. And as any girl scout knows, twigs catch light more readily than branches, so a twiggy sort of plant can catch fire more readily than its non-twiggy sister.

But here’s the odd thing. Many plants that live in places prone to fire are highly flammable — more flammable than plants that live elsewhere. This has led some to speculate that these plants have actually evolved to cause fires: that they “want” fire, and have evolved features that make it more likely that a spark will become a flame, and a flame will become a fire. I call this the torch-me hypothesis.

The argument goes like this. Many plants depend on fire for their propagation. Indeed, without fire, these plants disappear. If, for example, longleaf pine forests do not burn regularly, the pines will be replaced by water oaks and other species. So — runs the argument — fires are desirable because they kill the competition. Plants that enhance fires may thus have an evolutionary advantage: they murder the competition while creating the right circumstances for their own seeds to sprout.

This idea has sparked a heated debate. The problem is, showing that a trait has evolved because it enhances fire is difficult. Yes, oily leaves are more flammable; but perhaps the real advantage of oily leaves is that insects don’t enjoy eating them. Then, their flammability may be a by-product of tasting terrible.

The best evidence that some plants may have evolved to promote fire comes from pines. Some species of pine keep their dead branches; others tend to self-prune. As you would expect under the torch-me hypothesis, the more flammable species — the ones with the dead wood — also tend to have seeds that are released by fire. In short, the two traits go together….

The author seems to think she is the first person to consider this concept.

Lodgepole pine, photo: N. C. Heywood

One example of this is the lodgepole pine. It has shaggy bark and does not self-prune its limbs readily, so a fire at the base of the tree can, under dry conditions, run up the trunk of the tree and become a crown fire. It has a fire return interval of about 300 years, and fires tend to be of the stand replacement type, leaving nothing but snags. The serotinous cones open and disperse the seeds after the fire, promoting the resurgence of another lodgepole forest.

Lodgepole pine infested with dwarf mistletoe. Photo: D. Johnson

UPDATE: Chuck Bushey wrote to us about this.

Bob Mutch actually wrote his MS thesis on this topic at the University of Montana in the late 70’s. It was later published in the Journal of Ecology and I think he was the first to formally express the concept in the scientific literature.

Chuck later said the actual citation is:

Mutch, Robert W., 1970. Wildland Fires and Ecosystems – A Hypothesis.

Ecology, Volume 51(6):1046-1051.