Department of Conservation ranger Paul Cashmore, right, inspects one of the wilding pines cut on Rainbow Mountain by Wiremu Redmond, left. Photo / Alan Gibson
Tourists heading past Rainbow Mountain, south of Rotorua, might have once been puzzled at its name.
For years, the colourful ridges that adorned the 743m-high geothermal landmark had been swallowed up by dense stands of ugly green pine.
Paul Cashmore remembered the scruffy-looking forest that dominated the mountain back in the 1980s and 1990s.
"The geothermal slopes were covered in conifers - and now you have to look quite hard to see them," the local Department of Conservation biodiversity ranger said.
"The mountain has regained its colours."
That hasn't come easily.
For nearly two decades, chainsaw gangs have negotiated the mountain's hazardous terrain to cut away the wild pines, whose seeds had blown in from a surrounding sea of plantation forest.
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• Conservation Comment: Wild pines a fire risk
What the workers couldn't reach was sprayed by helicopters.
Cashmore said native vegetation had reclaimed the mountain, and the tourists had returned.
But the war against the wilding pine scourge hasn't been won, and might not ever be.
"I don't think we are ever going to finish this job, and it's always going to be an ongoing battle," he said.
"But once you've got the back of it broken, the maintenance of it is certainly manageable.
"Stop doing it, and you're up against the wall again - and you've got an expensive job."
Despite the gains at Rainbow Mountain and nearby thermal wonderland Wai-O-Tapu, wilding pines remain the most dominant pest plants in the Central North Island's cherished geothermal areas.
GNS Science volcanologist Brad Scott said he'd noticed the invasion during his time in the region, but pointed out the hot ground at least managed to keep the pines to the margins of hot springs and mud pools.
The bigger concern is the impact the pines are having on local native plant species, many of which are found nowhere else.
One recent report that looked at vegetation within 15 geothermal fields found the vast majority had wilding or planted pines present.
Wilding conifers - particularly radiata pine and pinaster - formed between one and 25 per cent of plant cover at most sites at that time.
DoC currently controls wilding conifers in the Lake Rotokawa Conservation Area, and Broadlands Road Geothermal Scenic Reserve.
DoC and Ngāti Tahu–Ngāti Whaoa are also considering a control programme at Ngatamariki Scenic Reserve.
But this represents just a tiny proportion of the threat the menace posed to our wilderness areas.
About one quarter of all conservation land is highly vulnerable to wilding conifer invasion.
That includes three quarters of New Zealand's 70 naturally rare habitats; among them, alpine herb fields, dry tussock lands, geothermal areas and the volcanic plateau.
Also at risk are the South Island's mineral belt, Coromandel scrub lands, gumlands in Northland, coastal dunes, frost flats and seasonal wetlands.
"If we do nothing, then wilding pines will spread to 7.5 million ha of vulnerable land over a period of 15 to 30 years," Conservation Minister Eugenie Sage said.
Alarmingly, that represents well over a quarter of New Zealand's land area.
The origin story of the wilding pine scourge in New Zealand isn't unlike that of stoats, possums, or any other misguided introduction that has swiftly become a conservation nightmare.
Few people understood how pines and other conifers would thrive when they were brought here in the 1800s.
Since then, wilding trees of some species have spread from forests, erosion and amenity plantings and farm shelterbelts out into the surrounding countryside.
Pine and other conifer seeds can be blown many kilometres by wind, and have travelled into areas such as farmland, the high country and public conservation land.
Seedlings can quickly infest an area, and if they aren't removed, can grow into dense, impenetrable forests in as little as 10 to 15 years.
Sometimes it isn't until many years after the original plantings that the problem and extent of seedling spread become apparent.
By this time though, things are often already out of hand. Seedlings have grown into trees that spread their own seeds further across the countryside.
Infestations of wilding pines and other conifers have grown significantly in recent decades, particularly in the high country of the North and South Islands – where extensive areas are vulnerable to wilding spread.
Species such as contorta pine are particularly spread-prone in the high country, as their seeds are light and carried far by the wind.
New Zealand isn't alone in this crisis: South Africa, Australia, Chile and Argentina all introduced spread-prone conifer trees for forestry and erosion control, only for them to become invasive weeds there, too.
Today, it is estimated wilding pines and other conifers cover at least 1.6 million hectares – almost 6 per cent of New Zealand's land area – and that is increasing by 5 per cent each year.
The country's nature isn't just on the line, but also what our economy reaps from it.
Biosecurity New Zealand long-term programmes manager Sherman Smith says the spread could leave less water available for irrigation and hydro-power.
High-country farmland, potential tourism draws and assets with immense cultural importance to mana whenua are under threat.
Economic analysis suggests that, if left to spread over the next 50 years, wilding pines and other conifers would cause the loss of 537,000ha of productive land - and a cumulative loss in productivity of about $5.3 billion in GDP.
That includes some $739 million of lost primary industry production, costs of $1.9b and $955m for reduced water for irrigation and hydro-generation respectively, and a conservative $331m from lost biodiversity.
Added to that is an extra $1.3b that would have to be spent on controlling wildfire - a problem expected to intensify under climate change.
But, because wilding pine and other conifer seedlings are easily seen, and their seeds only last in the soil for up to five to six years, it is hoped control of the pest is as achievable nationally as it has proven at Rainbow Mountain.
Over the past three years, results from the first phase of the multi-agency National Wilding Conifer Control Programme have suggested the goal is possible.
"Ultimately, the aim is to reduce infestations to the point where the affected land can be handed back to local land holders to manage," Smith said.
The programme's focus has been protecting the most at-risk land from the most spread-prone pine and other conifer species.
About half of New Zealand's known wilding infestations fall within the programme's current operational areas in Central North Island, Marlborough, Canterbury, Otago and Southland.
Across these regions, agencies had treated more than 500,000ha of scattered infestation, along with another 40,000ha where the wildlings were growing more densely.
The programme and its partners had also searched more than 1.5 million ha of surrounding land for any remote, outlier trees - an area about the size of Northland.
"So far, we've made good progress," Smith said.
"But the programme is now at a key point, where we need to maintain and build on the hard-won gains of Phase 1, by working with communities, landowners, councils and other partner government agencies."
"A Phase 2 priority must be to maintain these previously cleared areas and work to secure these by removing or containing remaining seed sources."
Where a wilding forest is removed, the land can be put back into more productive use, as grassland or planted trees.
"Where decisions are made to plant in trees, the One Billion Trees fund may be able to assist in planting natives, or low-spread-risk exotic trees."
Over coming weeks, the programme's leaders will be looking at what areas to focus on throughout the next year.
"We aim to have at least a broadly agreed approach to this by mid-July."
The Government has been pumping an estimated $11 million into control each year, on top of an extra $16m invested for the first phase back in 2016.
In this year's Budget, Agriculture Minister Damien O'Connor announced another $21m would be thrown at the problem.
But critics argue what's being spent isn't enough - and Federated Farmers has slammed the amount ear-marked in the Budget as "woefully inadequate".
The lobby group's arable and biosecurity spokeswoman, Karen Williams, said the "extremely modest" increase locked in meant work would be going backwards.
"It's a $250 million problem growing at 20 per cent per year," she said.
"By ignoring the problem all the Government is doing is pushing the responsibility further down the line for someone else to fix.
"If the country is not going to take pest plant wilding conifers seriously then it's about time it was just said out loud."
Sage said the Government had to weigh up a number of other biosecurity and environmental priorities – "striking a balance between what is affordable over the next two years and providing funding to scale up further".
And she pointed out that funding granted over the next two years had doubled present levels, meaning the workforce could be built for future phases.
At its peak, 20 fire engines were on-site battling the blaze.