This story was originally published on Newsroom.co.nz and is republished with permission.

New Zealand's old-growth forests, grasslands and one billion trees programme are a leafy basket of carbon sequestration. But is it wise to put all our eggs in one flammable basket? Eloise Gibson and Farah Hancock report on the risk fire poses to our above and below-ground carbon sinks

In coming years it could be New Zealand's own fires, not smoke drift from Australia creating apocalyptic afternoon skies.

In just 20 years it's expected the number of days this country faces very high or extreme fire risk will increase by 71 per cent.

In dry areas, the forests sequestering our carbon will face a risk of going up in flames and our carbon sinks could flip to become carbon sources.

In just three months, Australia's fires are estimated to have released 350 million tonnes of carbon dioxide into the atmosphere – around two-thirds of the nation's normal annual industrial emissions budget.

While New Zealand won't face the same level of extreme fire risk as Australia, dry conditions look set to increase in some of the same parts of the country where forest planting is also likely to accelerate.

With trees firmly ensconced in New Zealand's emissions trading scheme as an offset to greenhouse gas emissions, planting could be a cost-effective way to reach our climate goals. But many of the trees planted are likely to be relatively-flammable plantation pine – prompting researchers to warn that we need to be careful what species get planted, where. Forest scientists also warn we'll need to defend our existing native forests, which lock up astonishing carbon stores.

TIMELY WARNING

Before Australia's fires brought the risk to everyone's attention, Parliamentary Commissioner for the Environment Simon Upton was warning that increasing fire hazards in dry parts of New Zealand will raise the risk of forests going up in smoke. Ironically, the forests being planted to avert climate change will become more vulnerable to destruction as climate change worsens.

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Upton was concerned that giving fossil fuel emitters an unlimited ability to offset emissions with tree planting would let them off the hook from making actual emissions cuts. Not only that, Upton's report noted, many of the hot-spots for new tree planting – particularly Canterbury and Otago – would also experience extreme fire risk.

Historically, clearing native forest has been New Zealand's single biggest contribution to climate change. Massive forest clearance to make way for farming has contributed more to climate change than any ongoing emissions from farming, and about seven times more than New Zealand's fossil fuel emissions. More than three billion tonnes of carbon were shifted to the atmosphere by historical forest-felling, Upton's report said.

To put that in context, New Zealand's forests currently lock away an estimated 1.8 billion tonnes of carbon. Most of this is contained in old-growth native forests, mostly on conservation land. About 0.1-0.2 billion tonnes is tied up in exotic plantations, usually in the form of Pinus Radiata. That means it's vital to defend old-growth forests from fire, wind and pests that might release their carbon.

The good news is that established native forests tend to be less flammable than plantation pine and other younger species, such as gorse, says plant ecophysiologist Cate Macinnis-Ng, from the University of Auckland's School of Biological Sciences. In the right conditions, New Zealand's old-growth forests can even act as a natural fire break, provided the fire is not too large or intense.

Australia's long chain of dry spells has made it peculiarly vulnerable to widespread fires, even by Australia's fire-prone standards. "The reason it's so bad in Australia is because of the extensive droughts. When vegetation dries out, it becomes more flammable," says Macinnis-Ng. "Comparing it to New Zealand, the intensity of fire in Australia is more severe."

But increasing dry spells are projected in the future for some already-dry parts of New Zealand – Canterbury, the East Coast, and Northland, for example. "We need to be thinking about how to prepare for fires," says Macinnis-Ng. "We need better data on fire risk, carbon uptake and water use (by different tree species) so we can find that balance."

"If we have a dry spring and a hot, windy summer, those are the conditions that we need to watch out for."

UNLEASHING OLD BURIED CARBON

The potential fire risk to trees is one reason why scientists have started focussing on locking carbon away in the soil, where it's less likely to be touched by fire, says Macinnis-Ng.

Upton's report estimated an impressive 2.6 billion tonnes of carbon was stored in our soil already, and researchers at Waikato University and elsewhere are studying how to increase this.

But even carbon stored below the ground isn't necessarily safe.

Auckland Institute of Technology Rutherford Postdoctoral Fellow Nicola J Day is studying the effects of fire on carbon stored in soil. She spoke to Newsroom about the effect bigger and more frequent fires are having on soil carbon which had been locked away for millennia.

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This 'legacy' carbon beneath the ground is made up of old branches, leaves and roots, long fallen and locked beneath more recent leaf fall.

"Boreal forests store about a third of carbon globally on the planet and most of that is in the soil. In many places it's fire adapted, for thousands of years it's burned and regenerated and come back."

What's changing is the fire risk. Climate change means more frequent fires and more intense fires.

Usually fire only burns what's above the ground. In extreme fires, or when fires happen too often for a protective upper-layer to form, this isn't the case. The fire burns deeper, going beyond recent leaf litter and into the soil below. What was trapped is released into the atmosphere.

"This means we're seeing older carbon that has never been released in a fire before being released in the atmosphere now after an extreme fire event."

Day's research has previously focused on an extreme event in Canada in 2014. Using soil radiocarbon dating, she and other researchers found the wildfire had unlocked carbon which hadn't been affected by previous fires.

"Fires are becoming more frequent and the ecosystem is not recovering in the way it used to. In some of these areas the carbon that's very old and would usually be trapped in the soil for hundreds or thousands of years, it's burning now."

The forests most affected were the forests younger than 60 years old.

"We can liken it to thinking about boreal forest soils as being like money on the bank. It's like carbon in the bank. With this increased frequency of fires and this old carbon being released it is like we're going into debt."

In New Zealand it's the east coast of the South Island which faces the biggest risk of fire. This area is predominantly covered by tussock grassland. Previous research has shown native plants in tussock grasslands burn better than exotic plants.

"That's a bit of a concern, because it suggests that if we're going to see increased fire activity in these areas and the native plants are burning better than the exotic plants, then what are the implications for New Zealand native plants and animals and ecosystem recovery and carbon storage? We just don't know."

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Day has returned to apply the knowledge she gained overseas to New Zealand. As part of a three-year project she's leading a team looking at how tussock burns and recovers, including looking at soil carbon.

"In tussock grasslands most of the carbon is in the soil. When these areas burn, if it's the soil that's burning, that's where the greatest carbon loss is.

"When you get more carbon in the atmosphere, then you get more climate change and then you get more fires. It's quite worrying."

Part of her research will include looking at the 4664 hectare burn near Middlemarch which the Otago Daily Times reported surprising firefighters with its "extreme" behaviour.

Day said it's been concerning seeing what's happening in Australia.

"All of those climate change reports which came out are coming into fruition. We really need to start paying attention to those a lot more in New Zealand and think about how we're going to manage that in the next couple of decades."