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The Great Dieback

Published
November 28, 2019
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’ve touched on the issue of native forest dieback diseases in a couple of blog posts already (The Sanctuary Mountain and Outside Adelaide). Australian eucalypts and New Zealand kauri are being killed off by a disease in which the top of the tree dies, followed by the rest of the tree.

The disease is known as ‘eucalytus dieback’ in Australia and ‘kauri dieback’ in New Zealand, on the basis of the trees most commonly affected in each country.

A little while back I was visiting Cape Brett, or Rākaumangamanga, which I’ve got another post about here.

The author at Cape Brett

If you liken the North Island of New Zealand to a stingray with a long tail, the tail of the stingray is the Northland peninsula, which extends about 330 kilometres north of Auckland.

The Northland Peninsula north of Auckland, New Zealand. Locations of Cape Brett / Rākaumangamanga and Waitākere Ranges added for this post, along with the red circle around the Waipoua Forest. All other map data ©2019 Google.

Cape Brett’s on the east coast of the peninsula. Around the same time, I decided to visit the Waipoua Forest on the west coast, where there are a lot of big kauri trees. The Waipoua Forest is inside a red circle on the map above. A map that also shows the location of Cape Brett / Rākaumangamanga and, to the south near Auckland, the Waitākere Ranges. I’ll be talking about those a little further along.

The west coast of the peninsula is a bit poorer than the east coast. It was good to see the things that were being done to overcome local underdevelopment and unemployment. Young people were being trained as baristas to serve the local tourism industry.

A large part of the tourism industry on Northland’s west coast depends on the primeval kauri forests.

Certain giant specimens of the species are especially popular with tourists. Trees like Tāne Mahuta and Te Matua Ngahere, each of which has about a quarter the volume of wood of the largest tree still standing in the world today, the giant redwood known as General Sherman. Tāne Mahuta dates back to the time of Jesus. Some still larger kauri felled in the past were presumably of Old Testament vintage.

Slide from a lecture I attended at the Arataki Visitor Centre in the Waitakere Ranges west of Auckland I talk about this lecture further on). Kairaru contained about half the volume of wood of General Sherman, a California giant redwood which is the largest single-stem tree in existence.

As a type, the kauri dates back to the dinosaur age. It’s an ancient conifer, which means that it is distantly related to the pines and redwoods of the northern hemisphere. It’s more closely related to South American conifers: the araucarias, of which the most famous is the Chilean monkey-puzzle tree. But a mature kauri’s far bigger and more massive than any monkey-puzzle tree.

In colonial New Zealand, the kauri was of enormous commercial value. The kauri was the ‘forester’s dream tree’, with each mature specimen containing enough wood for two houses. Wood that was straight, close-grained, honey-coloured, rot-proof and as almost as easily carved as butter. Younger specimens could be used for sailing-ships’ masts. And the resin or ‘gum’ of the kauri was of great value as well.

From the Arataki Visitor Centre lecture

At any rate, the kauri used to be the forester’s dream tree.

For it’s protected these days, of course.

Interestingly enough, the whole New Zealand forest ecosystem in which the kauri lives pretty much dates back to the dinosaur age, as a package.

In an earlier post about a place called the Catlins, I mentioned a forest at the southern end of New Zealand, on the exact same spot where you can see the remains of a forest petrified by a volcanic eruption in the Jurassic. There’s not much difference between the trees of the petrified forest and the trees of the Catlins today.

The forests are similarly primordial at the warmer, northern end of New Zealand where the kauri grow (kauri don’t like the cold).

The sort of forest the kauri inhabits

The dinosaur forests of New Zealand have survived largely unchanged for all these millions of years for one simple reason. Namely, that New Zealand is a long way from anywhere else.

In these forests, there are lots of things that are quite weird by the standards of other countries. Things like fruit-bearing conifers: trees similar to pine trees but with fruit instead of cones! A good example is the tānekaha, a Māori word meaning ‘strong man’. For the wood of this species is also of excellent quality.

All in all, New Zealand’s forest ecosystems, with all their ancient weirdness, are very precious. We have to make sure that they don’t vanish in our time, after all these millions of years of having survived precisely because they weren’t being disturbed.

Precisely for that reason, I was quite shocked by the amount of kauri dieback I saw. Many of the small kauri around the famous ones had died off, with branches fallen on the roads. Even the most famous trees were at imminent threat of infection.

A kauri with advanced dieback disease

A bit over a year later, I went to a talk on the subject of kauri dieback at the Arataki Visitor Centre in the Waitākere Ranges west of Auckland. I met people who were organising community initiatives to try and fight the disease on private land. That was in addition to the efforts being made officially, in public forests.

The Waitākere Ranges, with a freshwater reservoir at right and Manukau Harbour at left

I was shocked once again to learn that kauri dieback was almost certainly spreading to other species in the forest ecoystem such as the tānekaha. Perhaps, even, to a quite unrelated herb that grows under the great old trees, called kauri grass.

The Destroyer of Plants

The dieback disease is caused by fungus-like organisms called phytophthora, almost certainly brought in from other countries, to which local forest species have no immunity.

Right around the world, ancient forests are at risk of being wiped out by introduced phytophthora. There are slight differences between the individual species of phytophthora that attack eucalyptus trees, kauri trees and other species, but basically it is the same problem in every case.

Kauri dieback science, from the Arataki Visitor Centre lecture

More kauri dieback science. Notice the rapid expansion of diseased or potentially diseased trees (red and purple) between the two dates. Detail from on-screen lecture slide, Arataki Visitor Centre.

From the same lecture

The word phytophthora means ‘destroyer of plants’. Phytophthora live in the soil, where they mainly attack the roots of plants. The results are often virulent and fatal. For instance, the great Irish potato blights of the 1840s, and the ensuing famine, were caused by a strain of phytophthora.

With large forest trees like eucalyptus and kauri, the classic symptom is the withering and death of the top of the tree, also known as the ‘crown’. This also explains why phytophthora disease is called dieback when it affects forests.

The crown is not normally infected. But it dies back because the roots are being slowly killed and can’t supply the tree with enough water to get all the way to the top, anymore.

Whether the affected plant is small like the one that yields potatoes, or a giant of the forest, it is quite likely to die completely in the end. At that point the phytophthora will multiply in huge numbers in the decaying roots. It will then spread to the next susceptible plant the next time the soil is waterlogged.

Phytophthora can also be spread further afield by wild pigs, which root around in the soil and eat things that are covered in dirt. The pigs will consume the disease organism in one place and then deposit it somewhere else via their droppings.

And phytophthora can be spread further still by muddy boots, even if the mud has dried. So, in New Zealand, the problem was first addressed with boot-cleaning stations like the one in this photo.

But after a while it became clear that that wasn’t enough. And that more drastic action was required.

Losing our Tracks

In my book A Maverick New Zealand Way, first published in 2017, I included a map showing the Hillary Trail in the Waitākere Ranges west of Auckland City. Here it is:

Unfortunately, most of the Hillary Trail was then closed, along with a whole lot of other tracks in the Waitākeres. This was done in order to stop kauri dieback from being spread by muddy boots. The Aucklanders could hardly access the great green parkland to the west of their city anymore.

Some of the places that are still accessible are really overcrowded. For instance, I saw about 400 people at Kitekite Falls, a popular waterfall. Lack of access to the parks could have major implications for Aucklanders’ wellbeing.

I also noticed that the tānekaha at the top of Kitekite Falls had turned black.

Now there are plans to reopen some of the trails, but not all of them, after upgrades to make them less muddy. The link in the first sentence of this paragraph leads to documents put out for public consultantion in June 2019 that include a map of proposed track re-openings. The map is reproduced here. The tracks scheduled to be re-opened, after upgrades, are shown in red.

June 2019 Waitākere Ranges track re-opening consultation map

Everybody who’s looked into the matter seems to agree that the dieback problem has really caught New Zealand on the hop. And that everything we’ve been doing about it has been verging on too little, too late.

It seems as though conservation agencies that had faced years of cuts didn’t want to take on a whole new programme. So, for years and years, officialdom pretended that a disease spreading like wildfire and threatening some of New Zealand’s most iconic native forests, wasn’t spreading like wildfire.

A Patchy Response, still

There’s a reasonable amount of action going on now in the Waitākeres. But that’s actually quite a small area, close to a big city.

In Northland, many tracks have been closed there as well. But otherwise, in this area, which is a lot more rural and where the biggest and most important kauri in New Zealand are found, less is being done.

Wild pigs — an introduced pest in New Zealand — are being actively culled in the Waitākeres. But in Northland they still roam free in great numbers.

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