Nearby trees keep tree stump alive belonging to same species

Washington: A recent study has found how surrounding trees keep tree stumps alive, suggesting a shift from the perception of trees as individuals towards understanding forest ecosystems as 'superorganisms.' Within a shrouded New Zealand forest, a tree stump keeps itself alive by holding onto the roots of its neighbouring trees, exchanging water and resources through the grafted root system.

"My colleague Martin Bader and I stumbled upon this kauri tree stump while we were hiking in West Auckland. It was odd because even though the stump didn't have any foliage, it was alive," said corresponding author Sebastian Leuzinger in the study published in the journal of iScience.

Researchers 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. What they found is that the water movement in the tree stump was strongly negatively correlated with that in the other trees.

These measurements suggested that the roots of the stump and surrounding conspecific 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. 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," Leuzinger said.

But while root grafts are common between living trees of the same species, researchers 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. 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?" Leuzinger said.

One explanation 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, as well as increase the stability of the trees on the 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. 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.

To better understand how root systems are formed between kauri stumps and living trees, researchers hope 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. These changes the way we look at the survival of trees and the ecology of forests," added Leuzinger.