The unpopular tree sucking carbon from our air

Pinus Radiata grows like a weed, which is why it’s so fast at sequestering carbon. But since many people prefer native trees, forestry scientists are proposing an unconventional solution to get the best of both worlds.

To measure how much carbon is in a tree, you first have to kill it.

You slice up the trunk, branches, twigs, leaves and roots and dry the dismembered tree parts in an oven. Then you weigh them.

“It takes a long time,” says Euan Mason, a professor at the University of Canterbury’s School of Forestry. “I did some in 2012 with two students, and in six weeks I think we did 25 trees.”

Sacrificing trees like this is expensive, but researchers need these measurements.

Typically, about half a tree’s dry weight is carbon, which you can multiply by roughly 3.7 to work out how much carbon dioxide the tree has sucked from the atmosphere.

Once enough trees of different ages and species have been dissected, the results are used to help build computer models estimating how much carbon is in a hectare of living forest, or an entire country’s worth of trees.

Forest owners can use models like this to see how much money they can claim for carbon credits under the Emissions Trading Scheme. Similar estimates tell the Ministry for the Environment that New Zealand’s forests removed 24 million tonnes of carbon dioxide equivalent from the atmosphere in 2017, enough to offset 29 percent of the country’s greenhouse gas emissions.

Most of this CO2 was absorbed by Pinus Radiata, a species much-loved by commercial foresters for its astonishing rate of growth, but seemingly little-loved by anyone in the general population.

Radiata became the nation’s wood crop after most of our ancient Kauri forests were destroyed by indiscriminate logging in the 1880s. (“I wouldn’t call it forestry, because it was just pillaging,” says Mason).

Permanent indigenous forest still covers a much larger area than pine - almost quarter of the country, compared with 6.6 percent in wood plantations. But old-growth forests on conservation land are excluded from the tallies of New Zealand’s carbon sinks and emissions. (This sounds less insane after you find out that mature forests often reach a steady state, sucking about the same amount of CO2 they are losing from dead wood.)

Radiata has a superpower: It is adept at growing fast and hoovering carbon dioxide while it's still young, leaving other trees in a cloud of wood-chip.

For such peaceful beings, trees have sparked some heated arguments lately: how many we should plant, where and what kind. One point on which no one disagrees is that New Zealand needs to hold on to its old, indigenous forests: mature forest in the conservation estate holds about twice as much carbon per hectare as tree plantations do. After all, our ancient forest has centuries to hoard it.

But the question of what to plant in the next few decades is different, and even forestry scientists can’t agree. The basic points are common ground. We face a climate emergency. The Government, like others around the world, is committed to being carbon neutral by 2050. Trees can help.

But do we want maximum carbon-sucking, fast, or do we value other attributes more, or is there some way to have it all?

Already, plans to expand forests have fallen foul of some pastoral farmers, who see wholesale forest conversions as threatening their way of life.

Their main target is pine. People think it sours the soil, destroys biodiversity and turns the landscape into a boring monoculture. (The monoculture part is true but Mason – who studies pine a lot – maintains most of Radiata’s other alleged sins are actually the fault of the clear-felling regime that's usually used to harvest it).

But Radiata has a superpower. It is adept at growing fast and hoovering carbon dioxide while it's still young, leaving other trees in a cloud of wood-chip. Its facility for growth was only fully unleashed after it left its original homeland of Monterey, California. Nobody knows exactly why it thrives here.

Some tree species hunker down and stop growing when the days get shorter, but Radiata grows throughout the winter in northern New Zealand, taking full advantage of the Mediterranean climate, says Mason. It also seems to benefit from a phenomenon researchers call the ‘exotic species effect’. “If we take a species out of its natural range, it becomes an exotic and grows faster,” says Mason. “There’s only a small amount of evidence, but we think it probably has to do with endemic pests that we don’t bring with the exotic species, particularly soil organisms.”

Understorey regeneration in 34-year-old central north island radiata pine plantation. Photo: Adam Forbes

Trees grow in similar patterns to people, says Gerard Horgan, a trustee of Tāne’s Tree Trust, a non-profit dedicated to fostering indigenous tree species. “They grow slow to begin with, and then they become teenagers and grow like a stick and eat you out of house and home and then eventually they get old and senescent and they might be putting on a little bit of weight, but they are losing a bit too.” Radiata gets to the teen phase quicker, he says, and though some natives can also grow quickly once going, they take much longer to get started. The trust has charts showing Radiata pine outstripping the carbon sequestration of Rimu, Totara, Kauri and native shrubs over periods of up to 80 years. Horgan refers to it, wryly, as “the weed”.

Radiata not only takes off faster, it sequesters carbon at a rate at least double – maybe three to four times – the average rate of native forest, says Mason. “If we have a target to get to greenhouse gas neutral by 2050 and we really want to get there, one of the things that will hold us back is if we plant native species,” he says. 

But this is where carbon may clash with culture and other things people may want from trees. Mason – like every forestry scientist Newsroom spoke to – personally prefers native forest. “It’s my heritage,” he says.

He's not alone in this. Newly-announced findings from the Waro project, a research effort by Motu and researcher Pia Pohatu, surveyed Māori landowners on the East Coast and found native regeneration and planting sat better with them than planting exotics on their land.

“I am concerned that the apparent focus on carbon sequestration ... is simply an excuse for us to continue to emit CO2.”

Pohatu’s research found carbon credits were not the main driver for planting by these landowners – any money they might earn from farming carbon was considered a bonus, at best. People’s reasons for planting trees included making their land more resilient to climate change, improved water quality and restoring rongoa and other customary resources.

David Hall, a climate and forestry policy researcher at The Policy Observatory at AUT, who wrote a 2016 report calling for 1.3 million hectares of new planting, says valuing native species solely for their carbon is “incredibly narrow”. What about biodiversity, resilience to climate change, and cultural values, he says. He and Horgan point out there's a lot less research on indigenous species, so sweeping comparisons may be unfair. 

“If you were purely wanting nothing but carbon sequestration now, you’d go for a fast-growing, medium-rotation age exotic,” says Horgan. “But that is not the only reason you possibly want to plant trees.”

“What makes New Zealand New Zealand?” he says. “We have some obligation to protect and look after our native flora and fauna. If we don’t, who the hell is going to?”

The not-billion trees

The question of what to plant is pressing, not only due to climate change, but because the Government is well under way with a plan to get a billion trees planted by 2027.

A new tree-boosting agency, Te Uru Rakau, has $120 million from the Provincial Growth Fund to give grants to landowners who plant trees, and a goal of funding two-thirds natives. Native trees are paid for at a higher rate than pines and other exotics, reflecting their higher planting costs.

Yet, almost immediately, the programme was accused of subsidising too much Radiata pine.

To understand how the split between species works, you need to know that “one billion” is a catchy, but wholly inaccurate, slogan. Only half a billion new trees, at most, will be planted because of the programme. The other half a billion would have been planted anyway by commercial foresters, who habitually replace their pine crops after harvesting, without government assistance.

The Government is keen to put the brakes on forest clearance, which continued apace until at least 2015, according to Ministry for the Environment figures. But while it plans to count "business as usual" planting by forestry companies, it won’t be giving any grants for these trees. Te Uru Rakau says its focus is helping farmers add trees to their land, not supporting forestry companies or wholesale forest conversations.

Because the re-planted pines on existing forest land will count towards the “one billion” total, Te Uru Rakau estimates the total mix of species planted during the life of the programme will be 70 percent exotic and 30 percent indigenous, despite its own goal of funding two-thirds natives (which applies to only half the trees).

Te Uru Rakau’s acting head, Oliver Hendrickson, told Newsroom the One Billion Trees programme would sequester a projected 384 million tonnes of CO2, but the real total would depend on the mix of native and exotic trees. The more natives planted, the less carbon impact. That's because the agency estimates that every hectare of exotic pine sequesters more than twice as much carbon as a hectare of indigenous forest. It takes an average newly-planted native forest 100 years to overtake the carbon stored by planting Radiata, he says. (Radiata stays ahead on carbon even after harvesting, so long as new trees are replanted. There's a lag while the new trees get going, but not enough to squelch pine's lead.)

Carbon is not the planting programme's only goal: creating employment, mitigating climate change, supporting Māori values and aspirations, and protecting the environment are all in the mix, and it's not clear which one is meant to take primacy.

Meanwhile the Zero Carbon Bill looks set to foster carbon farming by imposing progressively tougher emissions targets, while parallel tweaks to the Emissions Trading Scheme will allow the price of carbon to rise. 

Mason estimates that planting half a billion genuinely new trees will get us less than half way to being carbon neutral by 2050. “I’m not disparaging it, it’s a step in the right direction. But we’re going to have to do more,” he says.

“We have some obligation to protect and look after our native flora and fauna. If we don’t, who the hell is going to?”

There have been various estimations by commissions and policy think-tanks of the number of new hectares of forest New Zealand needs to reach carbon zero. (Trees are generally considered to be the only way to get there by 2050, it could take until around 2080 without new forests). Estimates range from about 1.2 million to 2.8 million hectares, with a couple coming in at around 1.3m. Mason believes we could get that area down to just over a million hectares by planting pine - but only if we never harvested it.

Forestry scientists have been debating among themselves the best way forward, says David Norton, a forestry professor who works with Mason at the University of Canterbury’s School of Forestry. 

“My own bias is very much pro-native, but not just from a carbon sequestration perspective - for all the benefits that native trees can provide to society: biodiversity, cultural, soil and water conservation,” he says. “I am concerned that the apparent focus on carbon sequestration ... is simply an excuse for us (from individuals to companies) to continue to emit CO2.”

Norton and Hall, from AUT, each told Newsroom they think government policies focus too much on storing carbon to help avoid climate change, and too little on making sure our forests and economy can survive the inevitable changes that are coming. Monocultures of any sort - pine included - may be more vulnerable to new pests, diseases, and climate extremes that arrive as the climate shifts, says Hall. “Diversification is important.”

Information imbalance 

Currently our system of allocating carbon credits assumes the carbon from every forest harvested is totally lost back to the atmosphere, regardless of whether the wood is going into a long-lived building, or fast-degrading toilet paper. Getting better data on where wood is going is one of many information gaps researchers want to fill as the price of carbon rises, making such details more important.

But there’s another, more gaping, hole in our tree-related information.

Research on the real sequestration rate of trees, like most scientific research, tends to follow the money. As a result, native species have been neglected in favour of studying the main cash crop, Radiata.

Right now, it's hard for someone wanting to invest in planting natives to know what they are truly in for in terms of growth rates around the country, says Hall. “We’ve got ourselves locked into one kind of forest system,” he says. “One of the problems is the fact that there is a huge amount of knowledge gaps and what knowledge and data there is scattered about the place.”

The lack of long-term studies on indigenous forest make it hard to get rigorous comparisons on carbon, says Horgan. There is also the question of whether the native species that are studied have been planted on poor land, since people sometimes plant natives on land too scrappy to plant anything else. “You haven’t got a representative sample,” says Horgan.

Speedily sucking carbon does not mean everything is wonderful in the world of pine forests.

People are working to flesh out the estimates. Hall is starting a project with Ngāti Whātua o Ōrakei to plant native species in a reserve near Ōrakei train station in Auckland and monitor their climate and other benefits, for example.

Until more studies are done, “we don’t have all the information we need to make informed decisions about natives versus pine," says Cate Macinnis-Ng, a plant ecophysiologist and associate professor at the University of Auckland. Macinnis-Ng says the more diverse understory of healthy native forest captures carbon may not be well captured in the tallies, and indigenous forests are generally more fire-proof. Other aspects need exploring, too, she says - like how much carbon is stored in soil under different forests, and which species are most water-efficient.

Other new research may also boost the climate credentials of native species. Climate scientist Jim Salinger told a Parliamentary select committee recently that research he’s been working on shows that, compared with native forest, exotic pine produces more monoterpenes from its leaves. Monoterpenes absorb hydroxyl radicals that would otherwise decompose methane, potentially prolonging (by a few years) the heating effect of the methane burped by New Zealand's cows and sheep. The exact difference between tree species needs exploring, and Salinger is promising to publish his findings.

But, while several studies are ongoing, it would take something extraordinary to eclipse the climate benefit that government data currently attributes to Radiata’s carbon-slurping.

Take these figures, from tables published by the Ministry for Primary Industry. MPI made the tables to allow small forest owners to calculate their carbon credits more easily, albeit pretty crudely.

By age 27, when a typical plantation pine tree in New Zealand will be harvested, a hectare of Pinus Radiata will on average have sequestered 488 tonnes of carbon dioxide. That is in Canterbury, the slowest-growing region for pine. In the fastest region, Gisborne, a hectare of pine will slurp 779 tonnes by age 27 - more than twice what a hectare of indigenous forest sequesters even by age 50.

The tables are rough estimates. Because of the lack of comprehensive data on native trees, MPI has lumped all native species, in all regions, together. It’s almost certain that native trees in a good area – a top Totara, or a hectare of Manuka on the North Island’s East Coast, for example – would outperform the tables.

A native conifer forest in central North Island. Photo: Adam Forbes

On the other hand, one forestry carbon consultant has said that actual field measurements he has taken for his clients show natives doing worse than the tables suggest in some areas.

Mason says the tables almost certainly under-estimate pine’s growth. He cites New Zealand’s international emissions accounting figures, which imply that between 2008 and 2012, an average hectare of Radiata sequestered about 34 tonnes of CO2 a year. That is higher than MPI’s tables suggest, possibly because it reflects actual measurements from foresters with more than 1000 hectares, who are not allowed to rely on MPI's tables. “The highest I’ve seen from native forest – and only particular species, in particular circumstances – was around 15, 16 tonnes (of carbon dioxide removed) a year, and much of the research I’ve seen puts it down between one and 10 tonnes,” says Mason.

“From a personal perspective, I would be delighted if we discovered we should do this in natives, but as an academic I also have a role as a critic and conscience of society.”

Trees on farms 

Speedily sucking carbon does not mean everything is wonderful in the world of pine forests.

The environmental risks from clear-felling pine plantations in the wrong place, at the wrong moment, came to national attention last year, when intense rain sluiced down pine debris that had been left on steep hillsides near Tolaga Bay, destroying homes and choking beaches.

Plantation pine forestry, as currently practiced, almost always uses clear-felling, creating a risky window when the soil is bare and vulnerable to rain and erosion. Clear-felling also destroys the forest understory that can provide homes for native creatures, since it is all flattened when the pine is removed.

As researchers look for places to put millions of new trees, they are discussing different ways of doing things: planting smaller tracts of forest that are never to be harvested, or ways of making money from higher-value, selective logging, or farm/forest mixes, or all three.

“If you have very steep areas or difficult to harvest areas…you may want something you plant and leave,” says the tree trust's Horgan. “You might want to get a helicopter in and take a few logs out (on terrain like that) but they need to be high value logs. It’s hardly likely to be pine."

“It’s a temporary fill, because we know the trees are going to become reservoirs, eventually.”

Norton, one of the professors from Canterbury, fears that if pine is planted solely for its carbon, it may end up in places unsuitable for logging – only to be logged, regardless, when the income from carbon credits runs out. “I worry that a focus on plantation forests will result in future perverse outcomes with exotic plantation trees being planted in the wrong places,” he says. “This would not be an issue if the plantation trees were left in the ground as permanent forests ... but my concern is that when investors stop receiving an income from carbon credits, they might look to harvesting.”

These fears – and the need to find suitable homes for trees – have led researchers such as Tim Payn, a Research Leader on forest systems at Scion, to look at the potential to mix farming with forestry, putting trees on marginal or unprofitable bits of farmland and managing them alongside animal farming.

Agroforestry, as it’s known in Europe, is popular in France and Portugal, but New Zealand has tended to pursue the two things separately, he says. It's not just that Europe has subsidies, he says. “I would say (the separation) is very cultural,” says Payn.

Horgan agrees: “We inevitably look for the silver bullet: the solution is dairying, everywhere, the solution is Radiata pine grown on a clear-felling regime, everywhere, the solution is goats. Unfortunately, the world is not like that."

Internationally, Payn says, the trend is to explore mixed land use to spread risk, and reduce environmental damage. “This is where I see us going, where all the different land uses complement each other.”

Mixing farms and forestry can boost financial, as well as climate, resilience, says Payn. “You are working your agricultural systems on a smaller area, which is potentially less expensive in costs like fertilisers, and you could get a win from both. If sheep prices go up and tree prices go down, or vice versa, you’ve got a diverse portfolio of products.”

“What the pine canopy will do ... is moderate the microclimate, to mimic the shelter of a natural forest and to allow native tree species to establish."

Some farmers will decide to forgo harvesting the timber, preferring to keep farming the carbon and enjoying the environmental benefits of trees, says Payn. “You lose your timber crop, but looking at the economics it could work out in the longer term. Based on the forest-owner’s values, they might say ‘I’ll take less carbon in the shorter term because in the long term I’m going to get a more valuable (native) forest, or save costs from roading.”

“You get far more than just carbon or timber out of a forest,” says Payn. “Thinking about trees on farms, riparian planting could get you carbon but it could also get you cleaner water and better stream biodiversity, and a potential wildlife corridor if you plant it near to good block of natives."

Payn adds: “If you planted a higher-value timber you could also potentially get a higher value (crop), maybe Totara, and in the meantime you'll be protecting your stream water.”

Leave them in the ground?

But what if there was a way to harness pine’s speedy carbon sucking and still end up with native forest?

Researcher and consultant Adam Forbes has been studying what would happen if pine was planted and left in the ground, eventually to be replaced, gradually and naturally, by native forest.

Forbes’ PhD research showed that Pinus Radiata can be used as a nurse canopy, nurturing baby native forest through its vulnerable early years. Many native seedlings scorch and die if they attempt to grow in the open. In nature, they rely on mature trees to keep them safe until they’re bigger.

On the right site, says Forbes: “What the pine canopy will do ... is moderate the microclimate, to mimic the shelter of a natural forest and to allow native tree species to establish."

Having shown that it can work, Forbes is now researching how to get maximum carbon benefit from the pine, while managing the native understory to help big, long-lived species, such as Totara, get established.

Native and exotic plantation forests. Photo: Adam Forbes

How well it works, says Forbes, depends on the age of the pine forest (it needs to be tall enough to let sufficient light through) and the availability of a good source of native seeds nearby, most likely from stands of native forest. There also needs to be enough rain and warmth in the area, as well as sufficient native birds to spread the indigenous tree seeds.

“It’s an excellent forestry choice for large land areas of regions such as Gisborne, where clear-felling pines on erodible hill country has proven to be an extremely risky proposition,” Forbes says. “Other areas would be more suited to Radiata pine plantations, or even better, more ecologically interesting … native species.”

Norton and Mason - who both know Forbes’ work - are very enthusiastic.

Mason says pine planted near native forest already sometimes nurses baby natives, but typically the native undergrowth gets killed when the pines are clear-felled. “If you go to northern Kaingaroa, where it is warm and wet, you will see native species under the canopy of old plantations. There is ponga, there are ferns and if there is podocarp in the vicinity you’ll start to see them regenerating. And then they cut the whole lot down.”

For the plan to have a real chance, the forest must be permanent, says Forbes. Norton suggests landowners could sign a covenant before getting carbon credits or cash for the planting, committing them to keeping the forest in perpetuity.

“You could do it on eroding farmland or marginal farmland where people hardly make any money at all,” says Mason. “As part of that policy, we could probably say that for every 10 hectare of Radiata pine we put in for carbon sequestration, we should plant or at least identify one hectare nearby of native species that is going to supply a seed source.”

Even without any new incentives, Mason says the price of carbon would only need to rise by roughly $10 a tonne, to $35, for it to become economic to leave some pine forests in the ground. “It would depend how close you were to a port, and what the price of wood was at the time,” he says.

Hall, from AUT, thinks the nurse crop plan could work. But he also thinks its proponents might be underestimating landowners' antipathy to pine trees. “So many landowners are just going to refuse upfront to plant pine,” says Hall. “If you talk to iwi, they have a preference for planting native trees upfront because it better aligns with kaitiakitanga principles. A lot of Pākehā landowners also don’t want anything to do with pine.” He is considerably keener on exploring diverse planting from the outset, and finding ways to make it work.

There's also big, looming reckoning coming when it comes to greenhouse emissions. Even the perfect forest is only a temporary fix. At some point – whether it’s 2050 or 2080 or some other year - we will need to address the sources of our greenhouse gas emissions.

Like New Zealand's old-growth forests, even the most vigorous pine or well-nurtured Totara will stop sequestering eventually. Nothing stays a teen, or even a young adult, forever.

“It’s a temporary fill,” says Mason. “Because we know the trees are going to become reservoirs, eventually.”

On the positive side, all going well, there will still be an upside to today's pre-occupation with planting. “Our great-grandchildren will be able to enjoy native forest,” he says. 

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