Tag Archives: Superorganism

This tree stump shouldn’t be alive — but it’s fed by its neighbors

A tree stump in New Zealand manages to keep itself alive by grafting its roots to those of other trees, exchanging water and nutrients through this system. Researchers say this should convince us to think of forest trees less as individuals, and more as “superorganisms”.

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

Tree society

It all started when two scientists were hiking in the outskirts of Auckland, New Zealand. They came across something extremely unusual: a tree stump. It wasn’t surprising to find the stump itself, which belonged to a kauri tree, a common species in the area. Rather, it was surprising that it was still alive without any leaves.

“My colleague Martin Bader and I stumbled upon this kauri tree stump while we were hiking in West Auckland,” says corresponding author Sebastian Leuzinger, an associate professor at the Auckland University of Technology (AUT). “It was odd, because even though the stump didn’t have any foliage, it was alive.”

Photosynthesis is a vital process which helps trees to live and grow, but without any foliage, they can’t undergo photosynthesis, and therefore can’t (theoretically) survive. Leuzinger and Bader measured the water flow in both the stump and the surrounding trees (all of which belong to the same species). They found that when water was flowing in the tree stump, it was flowing out of the other trees.

This indicates that the trees are grafted together, and the surrounding trees support it.

“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.”

From the stump’s perspective, it makes a lot of sense: it’s probably the only way it can survive. But why would the other trees do it?

“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?”

Superorganisms

There’s no clear answer to this, but Leuzinger has a theory. He suspects that the grafts were formed before the tree became a stump. This would have a couple of advantages for all the trees. For starters, it would provide improved anchorage for all the connected trees — which is important for the steep slope where the kauri trees are located. Secondly, it would enable the trees to share nutrients with one another, creating a more balanced resource distribution.

As one of the trees stopped playing its part in supporting nutrients, this may have just gone unnoticed, or the trees may have continued to support it (it’s unclear exactly how they could stop this from happening).

If this is indeed the case, it opens up an intriguing discussion: are trees more than just isolated individuals? In other words, can we talk about a tree “society”?

“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 says.

The question is not straightforward, because the association also comes with downsides. For instance, while this interconnectivity could help some trees cope with drought or insufficient nutrients, it also allows the rapid spread of diseases. The truth is we still don’t know enough about this to draw a definitive conclusion.

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

The study was published in iScience https://www.cell.com/iscience/fulltext/S2589-0042(19)30146-4

.

Ant colonies behave as a single superorganism when attacked

Ant colonies are incredibly complex systems — the tightly knit, intensely cooperative colonies are closer to a single superorganism than to human societies. Researchers form the University of Bristol wanted to know how this single mind of the hive reacted to distress, and subjected colonies of migrating rock ants to differing forms of simulated predator attack to record their response.

Led by Thomas O’Shea-Wheller, the researchers subjected ants to simulated predator attacks to investigate the extent to which colonies of rock ants behave as a single entity.
Image via phys

By studying the ants responses, the team observed different reactions depending on where the attack was performed. When targeting scouting ants, that stay primarily at the periphery of colonial activity, the “arms” of foraging ants were recalled back into the nest. But when they targeted the workers at the heart of the colony, the whole body of ants retreated from the mound, seeking asylum in a new location.

The team was able to draw some pretty interesting parallels with human behavior. The first attacks could be compared to burning your hand on a hot stove, while the ones centered on the workers were more dangerous, kind of a ‘house on fire’ scare. And in each scenario, the ants reacted surprisingly similar to any animal with a nervous system — an involuntary reflex reaction to retreat from the damaging element in the first case, and a flight response from a predator that can’t be defended against in the later simulations.

“Our results draw parallels with the nervous systems of single organisms, in that they allow appropriate, location dependent, responses to damage, and suggest that just as we may respond to cell damage via pain, ant colonies respond to loss of workers via group awareness,” said Thomas O’Shea-Wheller, a PhD student in Bristol’s School of Biological Sciences and one of the authors of the study.