A tree stump that refuses to die

This stump of a kauri tree (Agathis australis) should be dead, and the fact that it isn’t is more than just a novelty.

It may mean we have to start seeing trees not as individuals, but as part of a superorganism.

New Zealand researchers have found that the stump keeps itself alive by holding onto the roots of neighbouring trees, exchanging water and resources through the grafted root system. 

Writing in the journal iScience, they suggest the trees keep it alive in exchange for access to larger root systems. 

Sebastian Leuzinger and Martin Bader from Auckland University of Technology stumbled upon the stump while hiking near Auckland and were surprised to notice that it was alive but didn’t have any foliage.

When they measured water flow in the stump and in surrounding trees belonging to the same species, they found a negative correlation, suggesting that the roots of the stump and trees were grafted together.

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 says. 

“In this case, the stump has to follow what the rest of the trees do, because since it lacks transpiring leaves, it escapes the atmospheric pull.” 

But why would healthy kauri trees want to keep a stump alive?

One explanation, Leuzinger says, is that the root grafts formed before the tree became a stump. This expanded the root systems of the surrounding trees, giving them access to more water and nutrients, and increasing the stability of those on the forest slope. 

The others didn’t notice when the stump stopped providing carbohydrates, allowing it to continue its life with their support.

And that, Leuzinger suggests, “changes the way we look at the survival of trees and the ecology of forests”.