Kauri dieback threatens more than trees: it could change how much carbon forests store

New research shows forests affected by severe kauri dieback can shift from accumulating carbon to losing it, with mature trees particularly affected.

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Fiona Goodall/Getty ImagesKauri are among Aotearoa New Zealand’s tallest and longest-lived trees – and their place in our cultural landscape looms equally large.

Centuries-old kauri such as Tāne Mahuta and Te Matua Ngahere are taonga, with deep cultural and spiritual significance to Māori.

Also remarkable is the amount of carbon these trees hold. Large kauri have gradually accumulated enormous amounts of biomass, leaving great quantities of carbon stored in their trunks, branches and roots.

Over the past two decades, however, these giants of our forests have faced a growing threat from kauri dieback.

The disease is caused by the soil-borne water mould Phytophthora agathidicida, which infects kauri through their roots and disrupts their ability to take up water and nutrients. As it progresses, trees can lose foliage and branches and eventually die.

The toll has been significant. Hundreds to thousands of kauri across New Zealand’s upper North Island have died, with some of the heaviest losses recorded in Northland and Auckland’s Waitākere Ranges.

As dieback spreads and affected trees decline, some of the carbon held in their living biomass is lost from that store – with direct implications for carbon sequestration and the broader health of these forest ecosystems.

Our latest research sheds some new light on these important impacts.

Tracking a decade of change

To build a clearer picture, we visited study plots in kauri forests in the Waitākere Ranges that had first been surveyed between 2011 and 2014.

Our study co-author, Toby Elliot, revisited the plots to measure the trees again, assess the condition of their canopies and record which had died or become established. We then used the size of each tree, along with the typical wood density of each species, to estimate how much living biomass the plots contained.

This allowed us to see how the amount of carbon stored in living vegetation had changed over about a decade.

We found that the more severe the kauri dieback, the less carbon forests gained through tree growth – and the more they lost as trees declined and died.

The difference was notable. For forest stands starting with an average 169 tonnes of carbon per hectare, healthy stands gained about 0.8 tonnes per hectare each year. Those showing signs of dieback instead lost about 0.1 tonnes, while severely affected stands lost around one tonne per hectare each year.

That amounts to a shift of about 1.8 tonnes of carbon per hectare per year between healthy and severely affected forest.

The reason is relatively simple. Healthy trees keep growing and accumulating carbon. Dieback slows that growth and, when trees eventually die, the carbon they contained is no longer part of the forest’s living vegetation.

When the biggest trees are lost

The largest changes occurred in forests that initially contained the most carbon – mature kauri forests with large trees that have accumulated carbon over many decades or centuries.

In our study, mature forest plots held an average of about 248 tonnes of living carbon per hectare, compared with about 88 tonnes in younger, regenerating forest.

Kauri themselves accounted for a particularly large share of this store. Where kauri were present, our modelling estimated they contributed, on average, about half of the living carbon, with their contribution becoming greater in more carbon-rich forests.

This means the impact of kauri dieback depends on which trees die, as well as how many. When a large kauri dies, the amount of carbon removed from the living vegetation pool can be significant.

Our modelling also suggests severely affected, carbon-rich stands can shift from accumulating carbon to losing it. These forests may not regain their previous carbon stocks, instead eventually stabilising with less living biomass and a reduced capacity to store and accumulate carbon.

This makes protecting mature kauri forests particularly important for maintaining the carbon stored in these ecosystems.

Where does the carbon go?

Our study measured carbon only in living vegetation, capturing just one part of the larger carbon cycle.

Because we did not directly measure carbon in dead wood, litter and soils, we cannot determine how much of the carbon lost from living vegetation ultimately remains stored elsewhere in the ecosystem, or how quickly it is returned to the atmosphere.

Longer-term measurements of these other potential stores will be needed to track where this carbon goes – and how that changes as kauri mortality increases.

Protecting kauri from dieback therefore has implications beyond the survival of individual trees.

Management approaches designed to slow disease progression and reduce symptoms, such as phosphite treatments, are being investigated and applied as part of broader kauri protection efforts.

In the meantime, preventing the spread of dieback disease remains something visitors can help with: staying on marked tracks and thoroughly cleaning footwear when entering and leaving kauri forests.

Doing so helps protect kauri as taonga – and the ecological functions of forests that have taken centuries to develop.

The authors acknowledge the support of Te Kawerau ā Maki in this research.

Pin Jia Chan received research funding for this work from the Ministry of Business, Innovation and Employment and support from the Lucy Cranwell Student Grant for Botanical Research.Luitgard Schwendenmann received research funding for this work from the Ministry of Business, Innovation and Employment.

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