Why Would Trees Keep a Nearby Stump Alive?

Kauri trees, Waipoua Forest, New Zealand
Kauri trees tower above the ground in Waipoua Forest on New Zealand's North Island. (Photo: riekephotos/Shutterstock)
kauri tree stump in New Zealand
This image shows the Kauri tree stump from the new study. (Photo: Sebastian Leuzinger/iScience)

A tree stump without leaves shouldn't be able to survive on its own. In a New Zealand forest, however, two researchers recently found a leafless stump defying death.

"My colleague Martin Bader and I stumbled upon this kauri tree stump while we were hiking in West Auckland," says Auckland University of Technology professor Sebastian Leuzinger, who co-authored a new study about the stump, in a statement. "It was odd, because even though the stump didn't have any foliage, it was alive."

The stump had callus tissue growing over its wounds, and it was also producing resin, a sign of living tissue. While this might leave a casual observer feeling ... stumped, Bader and Leuzinger are ecologists, and they quickly figured out what was going on.

This stump wasn't surviving on its own; it was surviving with help from nearby trees.

I get by with a little help from my friends

Kauri trees, Waipoua Forest, New Zealand
Kauri trees tower above the ground in Waipoua Forest on New Zealand's North Island. (Photo: riekephotos/Shutterstock)

Trees in a forest are often connected by vast underground networks of symbiotic soil fungi, whose subterranean internet helps the trees exchange nutrients and information. Trees of the same species also sometimes physically graft their roots together, blurring the line between individual trees to the point that an entire forest could be considered a "superorganism," sort of like an ant colony.

Bader and Leuzinger decided to investigate further, hoping to shed new light on this stump's relationship with its benefactors. By measuring water movement, they found a strong negative correlation between water flow in the stump and in surrounding trees of the same species (Agathis australis, a conifer known as kauri). That suggests their root systems were grafted together, which can happen when a tree recognizes that nearby root tissue is similar enough to establish an 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 a news release about the study. "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."

Root grafts are common between living trees of the same species, and while it may be rarer, they have been found sustaining leafless stumps before. The phenomenon was first reported in 1833 for European silver fir, the researchers note, and has been documented several times since. Still, they wondered about the details of the arrangement, specifically what's in it for the intact trees.

"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 says. "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?"

The root grafts might have formed before this tree became a stump, allowing it to live on as a "pensioner" even after it stopped producing carbohydrates on its own, the researchers explain. But it's also possible they formed more recently, because regardless of how the connection came about, it could still be more mutually beneficial than it seems on the surface.

The root of the matter

ferns in a kauri forest in New Zealand
Ferns grow on the forest floor beneath kauri trees in New Zealand. (Photo: Bildagentur Zoonar/Shutterstock)

Linking with neighbors lets trees expand their root systems, providing more stability when growing on a slope — which could be a significant perk for a species known to grow more than 50 meters (164 feet) tall. The stump may be a shadow of its former self aboveground, but it presumably still has a substantial root system underground, and can thus offer some additional stability to its neighbors.

Plus, because a combined root network lets trees exchange water as well as nutrients, a tree with poor access to water could boost its chances of survival in a drought by withdrawing water from the community's shared roots. Yet there could also be drawbacks to that, the researchers point out, since it might enable the spread of diseases like kauri dieback, a mounting problem for this species in New Zealand.

Leuzinger plans to look for more kauri stumps in this kind of situation, hoping to reveal new details about the roles they play. "This has far-reaching consequences for our perception of trees," he says. "Possibly we are not really dealing with trees as individuals, but with the forest as a superorganism."

He also says more investigation is needed into shared root networks in general, especially as climate change tests the adaptability of forests around the world.

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