Tree stumps that should be dead can be kept alive by nearby trees

A tree stump that should have died is being kept alive by neighbouring trees that are funnelling water and nutrients to it through an interconnected root system. The finding adds to a growing understanding that trees and other organisms can work together for the benefit of a forest.

Sebastian Leuzinger at the Auckland University of Technology, New Zealand, and a colleague were hiking through a forest track west of Auckland when they noticed a single tree stump with living tissue growing from it.

Curious about how it was surviving without green foliage, they decided to put several continuous water monitors in the kauri (Agathis australis) stump and in two nearby adult trees of the same species.

Over the following weeks, they found a relationship between the water flow in the trees and the stump. This meant that when the neighbouring trees would evaporate water through their leaves during the day, the water movement in the stump remained low. But when the trees were dormant during the evening, the water would begin circulating through the stump.

Similarly, when it became overcast or rainy and the water flow dropped in the trees, it picked up in the stump. In healthy trees, water flow is largely driven by evaporation, but without leaves the stump’s water flow was bound by the movements of its neighbours.

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Along with a growing awareness of the way fungi help trees exchange carbon and other nutrients, this relationship undermines the notion of trees as individuals or distinct entities. “And that dramatically changes our view of forest ecosystems as ‘superorganisms’,” says Leuzinger.

The networking of water among trees may make them more resistant to water scarcity, says Leuzinger, but it may also increase the risk of diseases spreading. This is a particular worry for kauri trees, which are being affected by a deadly disease called kauri dieback.

Foresters have reported living stumps as far back as the 1800s, but this is one of the first studies of how they survive.

There are several reasons why neighbouring trees could be supporting the stump. It could be that the stump and its roots offer the living tree more stability in the ground, or that a leafless stump just becomes part of the host tree’s broader root system.

Trees are “ruthlessly efficient” in maximising their resources, says Greg Moore at the University of Melbourne, Australia. “So the fact that this stump is being supported by nearby trees tells you they are getting a benefit,” he says.