Rob Suisted

How to fix the Waikato peatlands

Don’t call them swamps. Bogs soak up and store more carbon than forests do, but when they’re drained and used for agriculture, that immense amount of carbon is slowly released.

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The entrance to one of New Zealand’s most unusual wild places is surprisingly unremarkable. At the edge of a Hauraki Plains dairy paddock, a plank across a drainage ditch leads onto a rutted track through a stand of gangly mānuka.

There’s no sign to welcome visitors to the one of the last remnants of a unique waterscape. No botanical notes to introduce the rare plant which dominates vegetation across a hundred square kilometres. No graph to illustrate that this ecosystem stores 24 million tonnes of carbon—the rough emissions equivalent of New Zealand’s entire team of five million driving vehicles for a year. And no warning that the ground may give way at any moment.

As I follow my guide, Waikato University wetland scientist and carbon researcher Dave Campbell, into the heart of the Kopuatai bog, I manage to find every weak spot in the surface to sink thigh-deep into the cold mire. It sucks my leg in tightly. I wish I hadn’t just learned that the water-logged soft black peat under my feet drops down to depths of ten metres or more.

By the time we cross another drainage channel, the mānuka scrub gradually gives way to a low-lying tangle of sedges and rushes. Here, the bog holds me up more reliably, sending out circles of ground waves with each stomping step, quivering like a gigantic jelly.

The Kopuatai bog is New Zealand’s largest remaining peat dome, built almost single-handedly over thousands of years by a plant you won’t find in any other ecosystem. The leafless stems of the jointed wire rush (Empodisma robustum) are just one or two millimetres thin and stand about half a metre tall, with the tiniest of cotton-tuft flowers emerging straight from the stalks. This low canopy covers most of the ground, as far as the eye can see, occasionally interrupted by stands of the taller, rarer bamboo rush (Sporadanthus ferrugineus). This bog is its last stronghold.

Campbell tells me that the wire rush is an ecosystem engineer, like beavers or corals elsewhere, changing and maintaining the environment for its own benefit. Its fresh growths emerge from deep layers of fallen stems. When Campbell reaches through the litter, his lower arm disappears as he grabs a clump of moss-like cluster roots from the peat surface. “The roots are actually growing upwards,” he says, “to get the first access to rain and any nutrients. They strip everything out before it can get to the roots of any other plant.” He squeezes the water out of the fistful he gathered.

Empodisma is able to conserve water, which keeps the bog saturated and free of oxygen, or anaerobic. Microorganisms that require oxygen can’t get a foothold, so as plants die, they decompose only partially. Combined with the peat’s high acidity, this restricts weeds and allows Empodisma to outcompete all other plants.

“We call it a wet desert because the action of this vegetation is to hold on to moisture,” says Campbell. “It might be a key factor that allows peatlands to form in our near-subtropical climate with frequent drought.”

The incomplete decomposition produces peat the colour of dark chocolate, full of carbon. Intact peatlands serve as near-permanent carbon storage—better than forests. At Kopuatai, the carbon stored in the dead plant material is higher than the carbon emitted by the plants during their lives and as they decay. At a rate of one millimetre per year, the peat builds up. “You end up with a pile of partially decomposed matter 10 to 12 metres deep,” says Campbell.

Our excursion’s main destination is a small research tower in the middle of the bog. As we walk on, single file, a fernbird calls. It sounds close, within a few metres at most, but we never spot it through the tousled mesh of vegetation.

The Kopuatai peatland is vast in extent and what so much of the lower Hauraki Plains and the Hamilton Basin once looked like before being drained for pasture.

The research station is festooned with monitoring instruments that have been continuously tracking carbon fluxes and water flows since 2011. The data reiterate the important role peat bogs play in the carbon cycle. The peat at Kopuatai draws down about 200 grams of carbon per square metre each year—which adds up to 18,000 tonnes across the whole bog.

But this carbon-storing power applies only as long as a bog remains wet. Once it’s drained, oxygen enters the system, turning once-submerged carbon into carbon dioxide that escapes into the atmosphere. When drained peatland is farmed, fertilisers accelerate the carbon loss, turning a sink into a significant source.

Almost half of New Zealand’s peatlands are in the Waikato—a total of 89,000 hectares—followed by Southland, Northland and the West Coast. In the Waikato, only 19,400 of those hectares remain in a natural state, including the Kopuatai bog and the Whangamarino wetland, two of New Zealand’s six sites listed by the Ramsar Convention as wetlands of international significance.

Kopuatai is surrounded by dairy farms operating on drained peat. Of the almost 67,000 hectares of drained peatlands in the Waikato, most are used for dairying. (A few smaller peat patches produce much of New Zealand’s blueberry harvest.)

The Waikato’s drained peatlands produce between 10 and 33 tonnes of carbon-dioxide-equivalent emissions per hectare each year, depending on how the land is used. These emissions are from the peat itself as it decomposes once it’s exposed to oxygen.

“The total amount of carbon held in peat in New Zealand is equivalent to the amount that’s in forests,” says Waikato University soil scientist Louis Schipper.

What happens if we continue to farm the peatlands currently used for pastoral agriculture? “It is equivalent to burning down all forests and not replacing them.”

What we know

At 10,000 hectares, Kopuatai is New Zealand’s largest unaltered restiad bog, named after the Restionaceae plant family that includes both of its peat-building rushes. The bog formed some 11,000 years ago, but it could do so only because the Waikato River had changed course between 7000 and 9000 years earlier—it stopped flowing across the Hauraki Plains that it had built.

The plains remained wet and boggy, and once accommodated all kinds of wetlands—from grassy marshes and kahikatea swamps to mired fens and peaty bogs. The difference between a swamp, a fen or a bog lies in the source of water. Swamps get water from rivers nearby, bringing in nutrient-rich sediment, and plants within usually grow and decay quickly. A bog develops over millennia, accumulating organic matter. As its peat surface rises, rain becomes the only source of water. Any remaining swamp plants, such as kahikatea, mānuka or harakeke, die out, and restiads take over.

Data collated from soil surveys over the past 50 years shows the fate of the majority of the peat soils in the Waikato Region. These would once have all been wetlands, but today few remain, with most having been converted to some form of pastoral agriculture. DATA: LINZ, JUSTIN WYATT/WAIKATO REGIONAL COUNCIL, LANDCOVER DATABASE Manaaki Whenua/Landcare Research

Drained peat shrinks and sinks. Across Aotearoa, peatlands cover around 240,000 hectares, but about two-thirds of this has been drained for livestock grazing. Once drainage channels have been dug, gravity does most of the hard work reducing the water level on many farmed Waikato peats, except for the Hauraki Plains, where peatlands lie lower and change with the tides. When tides are high or rivers flood, water has to be pumped out. (Rising seas will only exacerbate the need to pump water.)

When peat is drained, it shrinks, compacts and starts to subside. The strongest slump happens soon after the initial drainage, but subsidence continues long term, its height falling at an average rate of two centimetres each year, for as long as the water table is kept artificially low.

The subsidence makes drainage channels and pump stations less efficient, and keeps the land wetter for longer. Because the compacting peat remains acidic, plant roots struggle to access deeper water layers during dry periods, which makes peatlands much more susceptible to drought.

Drained peat becomes a source of greenhouse-gas emissions. Campbell has set up monitoring stations like the one in Kopuatai on two neighbouring peatland dairy farms. Scientists have long thought that on drained peatlands, it’s mostly the water table that determines how much peat is exposed to decay, so lower water tables lead to higher carbon-dioxide emissions. But Campbell is taking a more nuanced, farm-scale look at how emissions change with the seasons and land management practices.

One farm has shallow surface drains while the other has deeper drains along paddock edges. Campbell has found that carbon-dioxide emissions are similar when soil is moist, but very different during the dry months of late summer. On the land with deeper drainage channels, almost five more tonnes of carbon dioxide escape from each hectare. “It’s in dry years, which we’re getting more frequently,” says Campbell, “when the really big emissions are happening.”

Peat bogs are home to rare plants and birds. In the northern hemisphere, where sphagnum moss is the main peat builder, bogs are often associated with moorlands in cooler and wetter climates. The unusual traits of the wire rush are the main reason big peat deposits have established in New Zealand’s drier conditions—and for Campbell, that alone is enough reason to keep the Kopuatai bog intact as the plants’ main refuge. Rushes could be transferred from here to help restore other peat mines and retired farmland. But peatlands are also home to threatened birds and invertebrates, such as Fred the Thread (Houdinia flexilissima), a skinny caterpillar found feeding only on bamboo rushes.

 

Studies suggest there are a number of strategies to reduce peat subsidence and carbon loss, but they’re not equally practical or effective. While they need testing in the New Zealand context, mitigation techniques broadly adhere to a mud map like that above—reducing cultivation provides more benefit and is easier to implement than surface amendments. At the 180-hectare Gamman’s mine, opposite, harvesters collect and bag peat for sale at garden stores.

How we fix it

The Climate Change Commission’s draft advice to the government notes that emissions from organic soils are among the most significant land discharges not yet included in the accounting towards New Zealand’s targets under the Paris Agreement. The commission recommends all human-caused emissions—sources and sinks—should be included.

This would give farmers incentives to retire peatlands, but Louis Schipper warns that the full restoration of drained peatlands would take longer than several lifetimes. “The peatlands in the Waikato and elsewhere have accumulated beyond the reach of river flooding, and that gives them the distinct low-nutrient and low-mineral content—and that becomes a real challenge to recreate.”

He says restoration would require planting different species in a particular sequence, but it may be possible to short-circuit the process. “For example, what we’ve done on cutover peat mines in the past, where it’s just a black desert of peat: if you bring in a truckload of peat and create a mound, put some mānuka branches on top and walk away, the mānuka takes off.” This creates an environment for the rushes to get a foothold.

 Accounting for emissions from peatlands is a good start, but it’s more about avoiding carbon losses.

Even if carbon sequestration by intact peatlands was used to give farmers credit and encourage them to retire peatland farms, any gains would be small compared to the carbon losses of using peatlands in agriculture. The worst-case scenario for farmed peat, says Campbell, is 33 tonnes of carbon-dioxide-equivalent emissions per hectare, compared to the four tonnes per hectare that an intact bog soaks up.

In other words, it’s more important to figure out how to use drained peatlands in ways that reduce emissions, or to let them regenerate, than it is to count on existing peatlands compensating for the ones we’re farming.

Muggeridge’s Pump Station, near Ngatea, is on the government’s list of “climate resilience” projects as it helps prevent the area from flooding, even though it also enables the drainage and therefore the emissions from 1100 hectares of decomposing peatland. Campbell suggests it’s perhaps time to decide “the days of farming have gone” from the land covered by the pump station. “We’re just doing more damage.”

 Farming on wet soil is possible.

When Manaaki Whenua—Landcare Research soil scientist Jack Pronger suggested farmers on peatland could graze water buffalo instead of cows, the room of agricultural advisers he was addressing responded with a collective chuckle.

But paludiculture, the practice of wet agriculture, could well become part of New Zealand’s future. Several European countries are already developing it. In the Netherlands, farmers are pumping water through subsurface irrigation pipes to rewet peat fields, with the aim of reducing emissions while still being able to grow certain crops. But the practice is expensive and not always successful. It may not be suitable for all peatlands, but Pronger and others are keen to scope out its potential for New Zealand.

For Louis Schipper, how we deal with peatlands is part of a broader question. “What do we do with land that’s being used, where it’s not fit for purpose? Because there are multiple consequences of that use—we need to have big pumping stations, we’re going to end up below sea level in some places, we’ve lost biodiversity and there are greenhouse-gas emissions. We need, as a society, to figure out whether that is what we want.”

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