Rakatu Wetlands. By Alan Liefting - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=5767826

The climate-saving potential of our vanishing wetlands – Expert Q&A

They’ve been called a “secret weapon” in the fight against climate change. So what exactly do New Zealand’s wetlands offer, especially when it comes to absorbing carbon?

Aotearoa has lost around 90 percent of its wetlands in the past 150 years and this trend is continuing, as highlighted in the recently released Our Land 2024 report. The National Wetland Trust is currently holding its first symposium since the Covid-19 pandemic, which will cover the role of wetlands in providing life-supporting ecosystem services across the country and the threats they face.

The SMC asked experts to explain the science behind New Zealand’s wetlands potential.

David Campbell, Associate Professor in School of Science Te Aka Mātuatua, The University of Waikato, comments:

What are the opportunities and risks in wetland conservation and restoration in NZ, specifically when it comes to their ability to sequester carbon?

“Aotearoa / New Zealand has suffered one of the greatest amounts of wetland loss of any country. Since European settlement, 90% of our lowland wetlands have been drained and destroyed and converted to farmland and settlements. And we are still losing wetlands. Unchecked development will further impact on wetlands with consequent loss of their biodiversity and ecosystem services beneficial to freshwater quality and carbon storage.

“There are diverse types of wetlands, and they are not all equal when it comes to carbon storage. Rain-fed and nutrient-poor peat wetlands, classified as bogs, store the largest amounts of carbon. Remnants of these still exist, mostly in Northland, Waikato, Southland and the West Coast. In their intact state, they gradually sequester carbon from the atmosphere and store it away as peat over thousands of years. Kopuatai Bog is a 100 km 2, largely intact bog in the Hauraki that has been well studied and compared to other peat wetlands globally. Over the last 11,000 years, it has accumulated peat as deep as 14 m and with an average carbon stock of 2,400 tonnes per hectare. Overall, it has low carbon accumulation rates compared to forests, but our research shows Kopuatai to be surprisingly resilient to drought and climate-related warming. We have almost no information about carbon storage rates in other types of NZ wetlands.

“When wetlands are drained, the carbon they store is lost – mostly as carbon dioxide. The current best estimate of the total greenhouse gas emissions by peatlands drained for farming in Aotearoa / New Zealand is 4.2 to 5.4 million tonnes of carbon dioxide-equivalent emissions every year, equivalent to 8–10% of the country’s net emissions. Yet these super-emitters of carbon dioxide only make up 1.3% of our total area of farmland. These huge emissions will go on for hundreds of years until all the peat has gone or we decide to do something about it.

“To slow down or reverse these emissions, we need to protect our remaining wetlands from further drainage and development and start rewetting and restoring our drained peatlands. Peatland restoration is challenging, and much more work needs to be done on the best way to go about it and on understanding the trade-offs compared to the loss of valuable farmland.”

Why should NZ wetland conservation and restoration be prioritised, or neglected?

“Aotearoa wetlands are diverse and globally significant. Seven of our wetlands are listed as wetlands of international significance under the Ramsar Convention. We should think of these as our “National Park of wetlands”, yet their future is far from assured. For instance, Whangamarino Wetland in the lower Waikato River Valley is threatened by sedimentation and nutrient enrichment from farmland erosion and a flood control scheme. Awarua Wetland in Southland has suffered large fires that have destroyed habitat. Under the government’s Fast-track Approvals Bill, wetlands, including our Ramsar sites, may be at risk from development for agricultural production or mining.

“The large areas of former peat wetlands from Northland to Southland that were drained over the past century are hot spots for CO 2 emissions, equivalent to around 8–10% of the country’s net emissions. The vast peatlands of the Chatham Islands are heavily degraded by farming and wild stock, yet we still don’t account for their emissions. In some places, New Zealand’s drained peatlands have lost so much peat that they are no longer economic to farm, so should be rewetted and restored. This is likely to be expensive and we still lack the knowledge and experience to do this at scale to address this huge risk to our emission reduction targets.”

Conflict of interest statement: No conflicts of interest.

Olivia Burge, Senior Ecologist, Manaaki Whenua Landcare Research, comments:

What are the opportunities and risks in wetland conservation and restoration in NZ, specifically when it comes to their ability to sequester carbon?

“The term ‘wetland’ is much broader than you might think! Wetlands are characterised by wet ground – but this means they can vary enormously, just as non-wetlands (dry-lands) do. Coastal lagoons filled with seagrass, like Waituna Lagoon in Southland; coastal mangrove areas like Waikare Inlet in Northland; rich wetland swamp forests, filled with kahikatea and swamp maire; intermittently wet marshy areas of now-farmland that contain grass species that plainly ‘look different’ to other areas of pasture; areas of willow-invaded wetland that now function differently to the past due to the water-hungry nature of willow; naturally more dry peat bogs with shorter stature plants with a bog-forest edge.

“Different wetlands provide different kinds of protection against climate change. Wetlands don’t just protect us from the negative effects of climate change, such as flood protection, by absorbing excess water, or by absorbing storm wave energy in coastal wetlands. They also regulate climate, due to their stored carbon. Per hectare, wetlands are the most dense ecosystem store of carbon in the world. In many wetlands, this is due to carbon in the soil. But where does this carbon come from? It builds up and is stored in wetland soils because the wetness of the soil means that plant and other organic matter does not decompose (i.e. if it were a compost, it would be a very bad one), and instead builds up over time, because organic material builds up quicker than it decomposes.

“Thus, draining and drying out wetlands can reverse carbon storage, leading to carbon emissions (and therefore exacerbating climate change). This is a consequence of failing to conserve existing wetlands. On the other hand, rewetting drained wetlands can lead to the cessation of carbon emissions – and restoring the original vegetation can help re-start carbon sequestration into the soil. New Zealand has unique peat-forming species that can survive in the relatively warm and dry climates of the Waikato (as well as in cooler areas, including Southland).

“But it’s not just soil carbon. Before human arrival, New Zealand had a far greater extent of forested and woody wetlands. Ninety per cent of New Zealand’s wetlands have been lost, but a disproportionately high amount of forested and woody wetlands have been lost. Forest and woody ecosystems do not contain as much total carbon as the wetlands discussed above, but contain vast amounts of carbon in the trees and wood that live above the soil. New Zealand wetland forest types include swamp forest, bog forest, and marine forest (mangroves – or ‘blue carbon’). Restoring forest wetlands is therefore an opportunity to combine forest restoration with wetland restoration. Manaaki Whenua – Landcare Research has recently undertaken some work for the Ministry for the Environment to quantify the carbon found in wetland plants. While data were sparse and thus uncertainties around estimates high, it allows the impact of both soil and vegetation carbon to be accounted for in New Zealand’s international carbon reporting. We found that New Zealand peat bogs contain twice the amount of vegetation carbon found internationally, while mangrove vegetation is particularly carbon-rich. Quantifying carbon impacts of wetland change is a first step towards quantifying the costs of wetland loss with respect to carbon.

“There are very few risks with conserving existing wetlands. Many wetland types are complex and slow to restore, and therefore not losing more wetlands is one of the most important steps we can do for wetland carbon. Rewetting wetlands has been found to cause increases in methane emissions in the short term (also bad for climate change) – so research is critically needed into how to ‘fast-track’ restored wetlands from being a methane source, to a carbon store! Recent research by Manaaki Whenua – Landcare Research does indicate that after 50 years, restored peatland wetlands will be a net sink for greenhouse gas emissions (i.e. net positive after accounting for methane and carbon).

“Another issue we probably don’t know enough about is how to help our native plants that are adapted to low-nutrient conditions, when we try to restore them in post-agricultural landscapes that may be highly enriched with nutrients – or even, whether they need our help! We are also currently working on revising the pre-human wetland extent map of New Zealand using new data such as LiDAR and updated soils mapping. This will improve our ability to prioritise wetland restoration at the catchment and even sub-catchment scale.”

Why should NZ wetland conservation and restoration be prioritised, or neglected?

“New Zealand wetland conservation should be prioritised for the reasons given above – carbon storage and the ability to help New Zealand meet emissions targets, flood protection – but also because of their unique biodiversity and associated benefits to people. New Zealand’s peat bog plant species are unique and persist in warm, dry areas of northern New Zealand (primarily the Waikato), due to their special physiology. We continue to lose wetlands across the country – even though we know far more now than we did in previous decades. If we only conserve wetlands on public land, then many of the benefits that accrue from wetlands being near people (e.g. ecosystem services) will be lost. So conservation – and restoration – of wetlands on private land is incredibly important for biodiversity preservation, but also because of the benefits of wetlands to people.

“I would really encourage careful thought about what type of wetland people want to restore, rather than just ‘wetland’ restoration. Some of the different types of wetlands are listed above. Different wetland types will need different water sources, and as a result, have different levels of nutrients and as a result of both, different types of naturally occurring vegetation. It can be useful to know what vegetation or wetland type used to be on a site (palaeoecologists are super useful here) – but there is no need to be a slave to history! Sometimes it might be more appropriate to restore a different type of wetland, or perhaps it is no longer possible to restore the past wetland type. Often, ‘open water’ wetlands are restored – but this is just one type of wetland, and it would be a shame if all new wetlands were open water wetlands – they also do not maximise carbon storage!

“The loss of wetlands historically in New Zealand has hit mana whenua particularly hard, because of their long-standing cultural association with, and use of, wetlands in New Zealand. Facilitating wetland restoration that also restores cultural connections to wetlands is one way of mitigating the harms associated with the extensive drainage of wetlands that occurred with the spread of European agriculture in New Zealand.”

Conflict of interest statement: “I have worked, and continue to work, on contracts relating to wetland carbon and wetland ecology more generally.”