Peter Meecham

Why do some trees fight fire, and others spread it?

We know that some plants are less likely to catch ablaze than others. Can we use them to help slow or stop wildfires?

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On Valentine’s Day, ecologist Tim Curran was out on Kaitorete Spit when he saw a column of smoke rising from the Port Hills. It grew, turned darker, and spread over Banks Peninsula. “That’s not good,” he thought.

Seven years ago, in 2017, a fire started in roughly the same place, almost on the same day—February 13. It burned for more than two months. Afterwards, Curran, who is an associate professor at Lincoln University, helped guide the local council on how to replant the area.

Curran studies how easy it is for different trees and shrubs to catch fire—he’s been systematically testing New Zealand plants for their flammability for years, and he’s found dramatic differences between them. A video of his plant barbecue shows gorse igniting like a torch while broadleaf/kāpuka doesn’t even smoulder. (He’s now tested 550 plant species.)

Based on his research, Curran suggested which species the council could plant to install a “green firebreak” where the 2017 fire began, near the junction of Worsley Road and the Summit Road. Locals turned out to help in such force that the council ran out of plants: low-flammability species such as five-finger and karamū as well as broadleaf.

The last time Curran visited the spot, he was pleased to see the seven-year-old trees were now so tall that the canopy was starting to close over. That meant a moister microclimate, which would help leaf litter decompose more quickly. “Which makes it less likely to carry a fire,” he says.

Now it was being put to the test.

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As anyone who’s tried to light a fire under challenging circumstances (damp wood, personal incompetence) knows, there’s a lot more to it than striking a match. Fires the size of the Port Hills blazes require a set of specific ingredients: hot weather, low relative humidity, high winds, fire-friendly topography (hills are ideal), and the right fuel—meaning the right plants.

Christchurch Adventure Park, pictured, burned in the 2017 Port Hills fire—and the 2024 fire also reached some of its ziplines. The full extent of this year’s damage to the park isn’t yet known.

The best plants for spreading fires are dry, dead ones. The 2017 Port Hills fire was partly fuelled by forestry slash, and afterwards, a local business was prosecuted for leaving slash on its land. Christchurch Adventure Park left its chairlifts running as the fire approached, hoping this would prevent any single component of the chairlift from overheating and failing. But the plastic seats melted and dripped on the slash piled below, which ignited, creating a new branch of the fire. It was this blaze which spread to a residential area and destroyed homes.

Afterwards, 80 property owners successfully sued the Adventure Park, the Appeals Court writing that the park should have “long ago” removed the slash “which they must have known was a fire risk”.

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If there are the right plants to fuel a fire, then there are also the wrong ones. “Happily, a lot of our New Zealand forest species fall into that category,” says Curran, “simply because they’re essentially rainforest species.”

Our forests don’t have fire as part of their lifecycle, as many North American forests do, and were rarely set alight before human arrival. That’s because New Zealand has an unusually low rate of lightning strikes, which are the main natural cause of forest fires elsewhere.

Port Hills residents were evacuated on Valentine’s Day as firefighters battled the blaze. One couple had recently moved into their newly built home; their previous house had burned in the 2017 fire. They’d spent five weeks in their new place.
On the second day of the fire, a swathe of destruction remains on the upper reaches of Worsley’s Road on Worsley’s Spur, close to where the 2017 fire began. The cause of the 2024 blaze is being investigated, and a source was never identified for the 2017 event.

Internationally, there’s not a lot of research into green firebreaks, and until recently, most of it was inaccessible to Curran: it had been conducted in China and published in Chinese. It wasn’t until one of Curran’s PhD students, Xinglei Cui, translated a series of journal papers and government reports on green firebreaks that Curran was able to examine exactly how the firebreaks were being used. Cui translated one study describing a green firebreak that had stopped a blaze equivalent in strength to the most intense phase of the 2017 Port Hills fire.

“The Chinese, if they’re on track, by next year plan to have half a billion kilometres of green firebreaks throughout their country,” says Curran. “And they’ve also probably done most of the testing of green firebreaks.”

Testing is difficult, as there are so many factors in wildfire spread and intensity. “It’s hard retrospectively to go, ‘Okay, well, that was stopped definitely by this strip of low-flammability vegetation’,” says Curran. “Under given circumstances, any plant will burn.”

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As the Port Hills burned, fire scientists were on the ground predicting where the blaze might go next, and collecting samples of grass, leaves, branches and twigs from areas about to burn.

Scion fire ecologist Shana Gross plans to measure the moisture content in the vegetation she collected in an attempt to figure out how those moisture levels affected the fire’s behaviour. Did it leap ahead or stutter when it reached those plants? Should they update their models?

The last Port Hills fire made some odd moves—it burned downhill at one point, and at another, created a superheated fire tornado. Gross has been making her own fire tornados in the lab (actually, in a paddock) in an attempt to figure out what conditions produce them. There’s a particular signal that one is forming: smoke suddenly starts rising straight up rather than drifting in the wind.

The Valentine’s Day fire burned through pine, such as this area of Christchurch Adventure Park. Christchurch mayor Phil Mauger promises to reconsider forestry activities in the Port Hills.

Gross moved from California to New Zealand in 2021 partly to escape wildfires—more specifically, to escape the smoke from them. “The continual months on end of really bad air quality,” she says.

Climate change is creating more fire weather—“a combination of when the vegetation is really dry and you have high winds”—but New Zealand has perhaps a better chance of controlling outbreaks than other places, says Gross. “The difference in New Zealand is that it is all manageable in the sense that 99 per cent of the ignitions are from humans. Sometimes it’s intentional, but mostly accidental.”

Gross is optimistic that more public awareness might results in fewer fires. Curran points out that, out of all the wildcards that influence fire behaviour, plants are the only one we can easily control.

Currently the Port Hills are stacked with species that are on the highly-flammable end of Curran’s plant-barbecuing tests—but this may change. On the third day of the fire, Christchurch mayor Phil Mauger told RNZ that there needed to be “a wider community conversation” about whether pine forestry was still a sensible use of the land.

Now, scientists wait for the fire to be extinguished; after that, they’ll start figuring out how the flames moved, what stopped them, and what hurried them on.

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