For some reason, one of New Zealand’s biggest wildfires in 70 years stopped here, on the edge of Eves Valley Reserve, and Dan Chisnall’s not sure why. “We’d done some planting along here,” says Chisnall, a Department of Conservation (DOC) ranger, “and the fire stopped at the edge of that.”
In February 2019, a contractor’s disc harrow struck a stone in a paddock just over the hill, at Pigeon Valley near Nelson’s Waimea Plain. The spark fell on land eager to burn: drought-seared and loaded with fuel. It ignited surrounding pine forests, and launched the biggest aerial firefight in the country’s history. For a month, 22 helicopter pilots hurled bucketloads of the Waimea River at an inferno that rampaged across 2400 hectares, forcing 3000 people to flee.
Chisnall saw the night sky bright with fire and smoke. Even now, he can’t quite believe these precious 29 hectares of native forest were spared. Incredibly, the saplings here are still wearing their plastic protectors.
Eves Valley Scenic Reserve is all that remains—little more than a copse now—of the vast beech-podocarp forest that once covered these floodplains. This type of forest was lost to other blazes set hundreds of years ago, and it never returned, because native forests have no answer to fire.
Unlike their Gondwanan counterparts in Australia, there is nothing in the DNA of mataī, tōtara and beech that codes for thick, flameproof bark, underground tubers, or phoenix-like buds and seed capsules waiting for liberation by fire. Why should there be? Holocene charcoal deposits tell us that just 12 per cent of pre-human Aotearoa experienced fires. Lightning strikes were rare, and always came with rain. The few fires they started soon fizzled in the cool damp.
“The West Coast and the mountains probably only burned every one or two millennia,” says Tim Curran, a plant ecologist at Lincoln University. That’s according to the charcoal record. Even in the eastern dryland forests, he adds, “fire intervals were several centuries apart”.
Then, in the 13th century, humans showed up—perhaps as few as 100 people. Within 10 generations, they had burned down some 40 per cent of forest cover. Between them, Māori and Pākehā have left Tāne’s cloak in ashes. Before human arrival, 90 per cent of Aotearoa was covered in forest: today, that stands at 25 per cent.
“I would argue that fire has probably been the transformative disturbance in the ecological history of New Zealand,” says Curran.
Few native plants relish a fire. Once air temperatures hit 50 or 60ºC, plant cells generally start to die. Native bracken survives because it can regenerate from underground rhizomes, which are insulated from fierce topside temperatures. Pollen records show it has thrived since human colonisation.
Mānuka, kānuka and matagouri burn readily, but their fiery deaths are merely a precursor to procreation. Mānuka and kānuka, for instance, play a trick called serotiny—when fire sweeps through, mānuka trees release millions of seeds from heat-resistant woody capsules. “Those seeds,” says Curran, “get dropped into the post-fire landscape, where there are virtually no other competitors.” These winners share something in common: they originated in Australia.
Among indigenous flora, losers abound.
The dryland forests—kōwhai, mataī and mountain toatoa—that once carpeted most of inland eastern South Island were perhaps the biggest losers of all: just one per cent of these remain. Mataī and mountain toatoa vanished from the drylands entirely and may never return, even if we intervene.
In 1980, a fire destroyed 300 hectares of beech forest at Mount Thomas, not far from Oxford in north Canterbury. Despite people sowing thousands of seeds, and fertilising those that germinated, the restoration project was stifled by a hard fact of fire ecology: introduced plants are much better at filling the vacuum left by a blaze. Agrostis capillaris, a Eurasian grass you probably know as browntop, quickly invaded Mount Thomas. Back in its homeland, browntop gets burned all the time, as do many of the 2000 or so exotic species that have naturalised in New Zealand. Many of them carry adaptations that allow them not only to survive a blaze, but prosper from it. A conflagration is hardly the end of their world—more the start of a new order. In Aotearoa, fire favours the foreign.
“Weeds like gorse, and pines, and hakea,” says Curran, “have all come from fire-prone landscapes overseas. They’re much better adapted for fire, and often, they’re more flammable than native plants. So after a fire, you get these weeds invading the landscape, and that increases the likelihood of more frequent, more intense future fires.”
Ecologists call this “fire begets fire”, and its implications for Aotearoa are profound. Left alone, plant communities normally develop over centuries, evolving from fast-growing pioneers, through shrubs and low trees, into the sort of tall, less flammable rainforest that was once ubiquitous here. Fires, says Curran, send that succession back to square one: “They kill a lot of plants, remove a lot of biomass. The process is halted, and the community goes back to the start.”
Then, fire-loving invasive weeds monopolise such blank slates. “The plant community gets stuck in this early, highly flammable state.”
Here at Eves Valley, you can clearly see what Curran means. Dan Chisnall points across the valley floor to the slopes beyond, a cemetery of charred tree trunks, where the thermodynamics of fire supercharged the blaze. “The fire pre-heated everything above it, which was pine with shrub weeds, so it just raced up that slope,” says Chisnall.
In the gullies, māhoe and five-finger survived. Around them, the ground is a uniform green: “That’s gorse coming back.”
DOC has little option but to leave it to grow. “We’ll just let natural succession happen,” says Chisnall. “It’ll probably take 50 years.”
But the odds of five decades passing without another fire are looking increasingly slim. In just 20 years, fire seasons in Aotearoa are predicted to lengthen by 70 per cent. Climate change keeps tacking on more and more “severe” fire days, even in vegetated districts like Manawatū, Whanganui and coastal Otago, where severe fire weather is tipped to double and maybe treble.
It leaves Nicola Day wondering where it will all end. “What I want to know is how our ecosystems will look in 100 years,” says the fire ecologist from AUT University. “But we just don’t know.”
Day and colleagues are sifting the ashes from the October 2020 wildfire at Lake Ōhau that burned 50 homes and 5500 hectares of Mackenzie Country tussockland. They hope to find out which plants come back first.
“Given the implications of climate change—the increased frequency of fire—we’re asking whether native systems can recover to what they were,” says Day. “We genuinely don’t know, and that’s the knowledge gap we’re trying to fill here.”
The study is in its infancy, so Day can’t make any predictions. “But I can tell you that the majority of plants resprouting in the first few months have tended to be exotic
That observation matches succession patterns from two other conflagrations, the Deep Stream fires in Otago in November 2019, and the Pukaki burn in September 2020.
“In all these cases,” says Day, “exotic species seem to be coming back pretty well, compared to the natives, and that’s a concern for our biodiversity. Lizards and invertebrates and birds rely on that habitat.”
She’s also investigating whether underground fungi might help shape the future plant communities of Ōhau. “We know that certain micro-organisms are really good at surviving adverse conditions. They can form really robust dormant spore structures, and those get selected for under fire.”
Some fungi form symbiotic relationships with particular plant hosts. Subterranean mycorrhizal networks colonise the root systems of the plant, providing it with water and nutrients. In return, the plant gives photosynthesised sugars to the fungi. Day’s work in the forests of Canada has shown that some mycorrhizal fungi readily survive fire there, but it isn’t known if heat-resistant fungi exist in New Zealand.
Even if they do, it’s possible that they might promote invasive species over indigenous. “So knowing which fungi might survive fire could determine which plants survive,” says Day.
At Eves Valley, Don Chisnall is putting mycorrhizal fungi to work in an effort to heal the reserve. “We’re doing a few trials here,” he says, pointing out newly planted mānuka and kānuka on a distant slope. They seem a curious choice, given their willingness to burn. “We wouldn’t normally plant them here, but they share the same mycorrhizal association with beech trees, so we wondered if they might help to get some beech established.”
We walk between other recent plantings, either side of the access road, of pittosporum, five-finger, cabbage trees, fuchsia, broadleaf, putaputawētā and karamū. The reserve’s neighbours, Tasman Pine Forests, have moved 10 metres back to facilitate this experiment, called a green firebreak. In fact, green firebreaks have been a thing in China since the Song Dynasty around AD 960, and aboriginal Australians have used them for millennia. It’s a simple enough idea: create barriers of fire-resistant plants to protect more vulnerable or valuable assets—like the Eves Valley Reserve, or timber plantations, or people.
Work by Curran and his students has shown that resprouting—a plant’s ability to throw out new buds and shoots after being burnt—is a cosmopolitan life skill in fiery places like the United States, Africa and Australia, but patchy among New Zealand natives.
Most canopy trees—the podocarps, the beeches—have no talent for it, but some smaller trees and shrubs do, even though fire has been the least of their problems for millions of years. It’s likely, says Curran, that resprouting evolved in some native plants as a defence against other trauma, such as browsing, wind shear or slips, but just happens to be handy for dealing with fire, too.
Earlier this year, Curran, as part of a team led by Canterbury University’s Ana Teixeira, took the first good, hard look at the resprouting ability of 20 native species in the wake of the Port Hills blaze. Many were mediocre resprouters, and some—hebes, tree daisies, ribbonwoods—never even tried.
But the species in this firebreak at Eves Valley are known, capable resprouters and might blunt the advance of a future fire, fending off radiant heat and quelling embers raining ahead of the fire front.
Nearly two years after the Pigeon Valley fire, the surrounding hills are still desolate. Yet for all its might, the blaze barely encroached on this cool forest remnant. Inside, Chisnall points out a few hotspots where embers fell, ignited the undergrowth, but soon sputtered out.
Green firebreaks, says Curran, should be front and centre of our Billion Trees aspirations: “Billion Trees is long overdue, and very necessary, but we need to have a really good think about what we’re trying to achieve—what threats we’re trying to mitigate—and I would argue that fire should be near the top of the list.”
Our choices will determine, he says, “the amount of fuel, the biomass available, and the flammability of different systems, and that will change fire regimes, so we really need to think about the implications. Green firebreaks should be a part of that thinking.”
For 85 million years, Aotearoa scarcely knew the searing brush of flame. But climate change heralds a fiery future, and if we leave it to natural selection, it will weave Tāne’s cloak into a mantle of weeds.