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Undead tree stump is kept alive by neighboring trees



The Kauri tree stump (left) and a living tree of the same species (right).
Image: Sebastian Leuzinger

A tree stump in New Zealand lives thanks to an interconnected root system that benefits both the tree stump and the neighboring trees. Scientists claim that this unusual symbiotic arrangement could change our idea of ​​what it means to be a tree.

We tend to view trees as individuals, but the roots of some species merge to allow the sharing of resources such as water, carbon, minerals, and microorganisms.

This tree stump in New Zealand is very much alive.
Image: Sebastian Leuzinger

Sometimes these elaborate root systems are a seemingly dead tree stump, an observation first made in the 1830s. Why living trees should devote resources to support leafless cohorts is not fully understood, nor is it clear to what extent these resources are being distributed to living trees and tree stumps.

New research published today in iScience, based on the discovery of a living Kauri ( Agathis australis ), is found in a New Zealand stump.

The authors of the new study, Sebastian Leuzinger and Martin Bader of Auckland University of Technology, say the discovery points to a previously unknown relationship between living trees and not-so-dead tree stumps. The physiological interactions between living trees and tree stumps, they write in the study, "may be much more complex than previously thought."

Fresh resin on the Kauri tree stump.
Image: Sebastian Leuzinger

Leuzinger and Bader stumbled across the tree stump on a hike. The woody stump caught on them as callus tissue grew over its dead and rotting parts. Resin was also produced, indicating the presence of living tissue. This resulted in a more thorough study, in which the researchers measured the flow of water through the stump tissue and also the rate of respiration corresponding to that of the surrounding trees.

"We measured water flow with heat ratio sap flow sensors," Leuzinger told Gizmodo. "They recognize the movement of water in the tissue by emitting very small heat pulses and measuring how quickly this heat is dissipated."

These measurements showed that the cowry swamp is inactive during the day when living trees transpire. But during the night and on rainy days, the tree stump becomes active, circulating water – and presumably carbon and nutrients – through its tissues. As the authors write in the new study, these results

indicate that such symbioses are far more complex than previously thought: through physiological exploitation of "downtime" of transpiring trees during the night or rainy days with high water potential in the living stumps The root network seems to be partially autonomous and strategically access resources instead of simply becoming part of the extended root network of neighboring trees.

A stump alone can not perform these functions. Trees need leaves for gas exchange and photosynthesis, which allow the production of carbohydrates. Without carbohydrates, trees and plants lack the energy and building blocks needed to grow. However, this leafless kauri tree stump is very much alive, as its roots are grafted on those of its living neighbors.

Heat Ratio Juice Flow Sensors at Stump.
Image: Sebastian Leuzinger

These grafts occur when a tree detects nearby biocompatible root tissue. hydraulic coupling. "In this case, the grafts probably formed before the stump lost its green foliage, but the researchers are not sure.

For the stump, the benefits of this arrangement are obvious – it can stay alive even though it can not produce carbohydrates. But as the authors point out in the study, this arrangement may actually be symbiotic in nature.

For example, living trees have improved access to resources such as water and nutrients. This arrangement also increases the stability of the trees on the steep forest slope, with the solid, healthy roots acting against erosion. On the other hand, these root-links could promote the spread of the disease, namely kauri death ( Phytophthora agathidicida ).

All this can only be speculated on the "exact mechanisms and evolutionary benefits of it at this time," said Leuzinger. More work is required, as the authors themselves admit in the study:

It is clear that only the observation of a single live Kauri stump prevents us from drawing more comprehensive conclusions. Although we personally have not seen a second occurrence of a living tree stump belonging to this iconic New Zealand species, we know that this phenomenon appears to have been noticed in the past and that the formation of natural root grafts was suspected to be present at Kauri 80 years ago.

This surprisingly complex interaction of living trees and stumps could change our perception of trees.

"If lateral water transport between trees turns out to be a common phenomenon, we need to rethink our definition of a 'tree," Leuzinger said. "In fact, we may regard the forests as superorganisms redistributing water between genetically different individuals."

The old adage that you can not see the forest for the trees is only partially correct. Scientists could not see the forest for the trees and stumps, as this new investigation shows wonderful.


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