The Kauri killer and the three soil types — Looking for answers underground to stop a deadly fungus in its tracks

This past February, I saw my first kauri tree. I had just arrived in New Zealand from the United States and was moving into my office at Lincoln University. There was a small potted plant on my officemate Alexa’s shelf, which was labelled with the name tag “Eric”. Eric seemed to be your typical houseplant — aesthetically pleasing, but otherwise unremarkable. But Eric is remarkable. Eric is a kauri.

Meet my officemate, Eric. Photograph by Alana Thurston.

If you’re from New Zealand, you might understand my surprise in learning that this pint-sized plant is a kauri tree. If not, buckle in for some rapid-fire kauri facts.

Kauri trees can reach heights of just over 50 m and are New Zealand’s largest tree by volume. They’re so colossal that it’s really difficult to find a photograph that does them justice. You can take a picture of the canopy or the base of the trunk, but they’re too large to capture in a single frame.

Two people hug the base of a kauri tree on a boardwalk. Between the two of them, they only cover a quarter of the circumference of the tree. The kauri is so tall it extends past the top of the image.
Here’s a few people at the base of a kauri for scale. Photograph taken by Chase Cheviron (CC BY-NC 2.0)

To help put their sheer magnitude into context, you can look at the names that some of these trees have been given. The largest living kauri is named Tāne Mahuta after the Māori god of forests. Another formidable giant is called Te Matua Ngahere, which translates to “Father of the forest”. Oh, and did I mention that these trees are beyond ancient? Kauri trees can live for over a thousand years. My office kauri is only two years old and just beginning its journey.

But now, kauri trees are in grave danger. Resin oozes from the base of their trunks while their roots rot away and leaves drop from their canopies. All that remains is a ghostly pale skeleton in an otherwise vibrant forest. And the culprit behind it all? A ruthless, microscopic soil fungus called Phytophthora agathidicida (which sadly, does not have a common name, but we’ll abbreviate it to “PTA” from here on out).

The leftmost and centre image were taken by Onco p53 (CC BY-SA 4.0). The rightmost image was taken by Cristel Veefkind (CC BY-NC-ND 2.0).

With no known cure available, Conservation Minister Eugenie Sage declared last year that “the best way to protect our kauri is to slow and stop the disease from spreading“. The subsequent response was sweeping and swift — tracks were closed. From Northlands to Tauranga, access to dozens of tracks was restricted.

Some restrictions have been temporary, with tracks closing just long enough for additional boardwalks to be installed above vulnerable tree roots. Others, however, are closed permanently, either due to the imminent risk of PTA spreading or lack of funds required for the upgrades.

To find the most current status on track closures, check out Keep Kauri Standing.

While the scientific community has largely supported these efforts to reduce the spread of kauri dieback, the decision has also seen a fair amount of public pushback. Some dismiss the claims that kauri trees could be extinct in as few as 30 years without action. Others deny that the loss of kauri trees will affect other plants and animals that live in the forest. Track volunteer Ross Jackson went so far as to say “The science is unbelievably flawed“.

I often find that scientific scepticism is rooted in a discomfort toward uncertainty. We want to know definitively that something is going to work, or we want the assurance that one thing causes another. With kauri dieback, there are, unfortunately, just as many (if not more) unknowns than knowns.

We know that this fungus causes dieback in kauri trees, but we don’t know which other plants it infects. We know that PTA can hitch a ride on the soles of our shoes and spread throughout a forest, but we’re not sure if feral pigs and goats act to spread the disease in a similar manner. We know that the disease has a near 100% rate of mortality in kauri trees, but we don’t know if certain trees possess natural resistance.

One of the larger areas of uncertainty lies in trying to figure out how different environmental conditions might affect PTA. Specifically, are there soil qualities can impact the growth, survival, and spread of the fungus? Soil quality could be determined by the abundance or deficit of key nutrients, such as carbon, nitrogen, or phosphorus. Perhaps the presence of water may play a role — would a damp or dry environment be more favourable to the fungus? Microscopic bacteria and fungus also live in the soil. Some of these microbes might compete for resources with PTA and create a more challenging environment for the pathogen to thrive in.

This line of research is already well under way. Just last year, a graduate from Lincoln University — Kai Lewis — published his master’s thesis. You can read the paper in its entirety here* if you’re interested, but since the content and language isn’t necessarily geared toward a public audience, I’ll give a quick summary.

Kai’s project aimed to investigate the growth of PTA across different soils in and around the Waipoua Forest.

The image shows a raised boardwalk through a lush green forest. The forest here is the Waipoua Forest, which is located in the northern part of the North Island of New Zealand.
The Waipoua Forest is located in the northern part of the North Island of New Zealand. This photograph was taken by itravelNZ© (CC BY 2.0).

Kai collected soil from different environments — natural kauri stands, forestry managed pine groves, and grazed pastures — with a simple research question in mind: are there measurable differences between these three soil types that influence PTA?

To answer this question, you need to do two different things. First, you take soil from each environment and run a whole bunch of tests to characterise it. This characterisation can include the quantification of basic nutrients (again, carbon, nitrogen, and phosphorus), measurements of soil acidity, and the determination of soil texture, among other things.

Then, you need to see how well the pathogen grows in each soil type. To do this, you infect the soil with a small amount of PTA in a lab setting. After a few days, you count fungal spores under a microscope to see how well the pathogen has grown.

Here’s what fungal spores can look like under a microscope. Image by Dr. Sahay (CC BY-SA 4.0)

Once you know the properties of the soils and how well PTA grows in it, you can see if there is any correlation between the two.

Kai found that none of the soil characteristics he tested for affected PTA‘s growth. But, PTA still had different growth rates across the three soil types. PTA grew more rapidly — and therefore thrived better — when it was in soil from pine forests and pastures, rather than the soil from underneath the kauri trees. What this means is that there are differences between the soil types that influence the growth of PTA, but that they’re caused by factors that weren’t specifically studied in this project.

This research is just the tip of the iceberg in terms of soil studies involving PTA. Alexa — aka my officemate, aka Eric’s (the kauri tree) guardian — is studying differences in microbial communities between diseased and healthy kauri stands. Next year, our lab group will add two more PTA based projects into the mix. I’ll embark on the research portion of my master’s program to conduct a study similar to Kai’s, but using sites on Great Barrier Island. And my other officemate — Jacque — will study chemicals secreted by plant roots to see if they have an impact on the growth and survival of PTA.

And this is just the research of one group at Lincoln. There are others across New Zealand that are also working toward mitigating kauri dieback and putting to rest some of the many unknowns of this disease. And just maybe, one day we’ll even be able to plant Eric back in his natural habit where he can grow in the wild for the next millennium.

Alana Thurston is a postgraduate student in the Master of Science. She wrote this article as part of her assessment for ECOL 608 Research Methods in Ecology.

*Lewis, K. S. J., Black, A., Condron, L. M., Waipara, N. W., Scott, P., Williams, N., and O’Callaghan, M. (2019). Land‐use changes influence the sporulation and survival ofPhytophthora agathidicida, a lethal pathogen of New Zealandkauri (Agathis australis). Forest Pathology, DOI: 10.1111/efp.12502.

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