Kiwi scientists have been astonished to find how kauri stumps can keep themselves alive by feeding off water from neighbouring trees.

The AUT researchers behind the ground-breaking discovery say it should mean we view trees not as individuals, but members of a forest ecosystem that's essentially a "super-organism".

Further, their findings could have big implications for tackling the disease killing kauri across the upper North Island.

In the new study, published in iScience this week, AUT's Dr Martin Bader and Associate Professor Sebastian Leuzinger described how trees surrounding kauri stumps offer them a form of life support, possibly in exchange for access to larger root systems.


It was an insight the pair stumbled across while hiking in West Auckland, and spotting an unusual-looking stump.

"It was odd, because even though the stump didn't have any foliage, it was alive," Leuzinger said.

They decided to investigate how the nearby trees were keeping the tree stump alive by measuring water flow in both the stump and the surrounding trees belonging to the same species.

They found that the water movement in the tree stump was strongly negatively correlated with that in the other trees.

These measurements suggest the roots of the stump and surrounding conspecific trees were grafted together, Leuzinger said.

Root grafts can form between trees once a tree recognises that a nearby root tissue, although genetically different, is similar enough to allow for the exchange of resources.

"This is different from how normal trees operate, where the water flow is driven by the water potential of the atmosphere," Leuzinger said.

"In this case, the stump has to follow what the rest of the trees do or else use osmotic pressure to drive water flow, because since it lacks transpiring leaves, it escapes the atmospheric pull."


But while root grafts are common between living trees of the same species, the pair were interested in why a living kauri tree would want to keep a nearby stump alive.

"For the stump, the advantages are obvious— it would be dead without the grafts, because it doesn't have any green tissue of its own," Leuzinger said.

"But why would the green trees keep their grandpa tree alive on the forest floor while it doesn't seem to provide anything for its host trees?"

AUT scientists have described how a kauri stump is keeping itself alive by holding onto the roots of neighbouring trees. Photo / Supplied
AUT scientists have described how a kauri stump is keeping itself alive by holding onto the roots of neighbouring trees. Photo / Supplied

One explanation, Leuzinger said, is that the root grafts formed before one of the trees lost its leaves and became a stump.

The grafted roots expand the root systems of the trees, allowing them to access more resources such as water and nutrients.

They also increased the stability of the trees on the steep forest slope.

When one of the trees lost its leaves and stopped providing carbohydrates, this could go unnoticed and so the "pensioner" was able to continue its life on the backs of surrounding, intact trees.

"This has far-reaching consequences for our perception of trees – possibly we are not really dealing with trees as individuals, but with the forest as a superorganism," Leuzinger said.

During a drought, for example, trees with less access to water might be connected to those with more access to water, allowing them to share the water and increase their chances of survival.

However, this interconnectivity could also allow for the rapid spread of diseases such as kauri dieback, which has so far killed thousands of kauri and prompted the Department of Conservation to list the species as threatened.

"The common root system provides sort of like a 'motorway' for pathogen transmission among grafted trees," Bader said.

"This means that Phytophthora agathidicida, the causal agent for kauri dieback, can easily spread among connected trees by travelling through the water-conducting tissue within the common root system.

"So, this exacerbates the situation in terms of pathogen spread tremendously."

But on the other hand, he added, scientists might be able to use this knowledge to our advantage for future disease management.

"Currently, there is no cure for kauri dieback disease but researchers are working feverishly to develop a remedy," he said.

"Our findings indicate that such a remedy - once it has been found - may not necessarily have to be administered to each individual tree as it could be distributed from treated trees to untreated trees via the common root network."

To better understand how root systems are formed between kauri stumps and living trees, Leuzinger hoped to find more instances of these types of stumps and to explore root grafting in intact trees, which will help expand their scope of research.

"This is a call for more research in this area, particularly in a changing climate and a risk of more frequent and more severe droughts," Leuzinger said.

"This changes the way we look at the survival of trees and the ecology of forests."