Zombie tree stump may have uncovered existence of vast ‘superorganism’

26 Jul 2019

The Kauri tree stump in the study. Image: Sebastian Leuzinger/iScience

The discovery of a tree stump that should be dead, but isn’t, could radically alter our understanding of trees merely as individuals.

In an unsuspecting New Zealand forest, scientists from the Auckland University of Technology (AUT) stumbled on what looked like a typical dead tree stump. However, Sebastian Leuzinger, co-author of a new paper published to iScience, found something very different.

“It was odd, because even though the stump didn’t have any foliage, it was alive,” he said. Leuzinger and his colleague Martin Bader discovered that the stump was being kept alive by holding on to the roots of neighbouring trees, exchanging water and resources through the grafted root system.

In effect, the tree had turned itself into a ‘zombie’, where a tree that should have been long dead was in fact still alive by feeding off the living. This, the scientists said, could radically alter our perception of trees as individuals and that they actually exist as ‘superorganisms’.

They began their research by measuring water flow in both the kauri tree stump and the surrounding trees belonging to the same species. This showed that the water movement in the tree stump was strongly negatively correlated with that in the other trees.

The discovery suggested that the roots and stumps of the surrounding trees were grafted together, a process that occurs when trees recognise nearby root tissue that, while genetically different, is similar enough to share 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 because since it lacks transpiring leaves, it escapes the atmospheric pull.”

What benefit is there for the living?

While root grafts are common between trees of the same species, the scientists were curious as to why living kauri trees would want to keep the dead stump alive.

“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?” Leuzinger asked.

One possible explanation is that the root grafts formed before one of the trees lost its leaves and became a stump. These grafted roots expanded the whole root system of the trees, allowing them to access more water and nutrients as well as increase their stability on a steep forest slope.

“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.

Despite the positives, such as better survival during drought, the scientists said one potential drawback of this superorganism is that it could allow for the rapid spread of diseases – in this case, kauri dieback.

“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 added. “This changes the way we look at the survival of trees and the ecology of forests.”

Colm Gorey was a senior journalist with Silicon Republic