Trees share water to keep this dying stump alive

Somewhere in the middle of New Zealand, there is a kauri tree stump (Agathis australis) that should not be alive. But it is, thanks to the root systems of surrounding trees, which have kept the almost-dead stump on life support by sharing water and nutrients.

For years, scientists have suspected such sharing networks exist, thanks to other living stumps. But such resource transfers have never been proved, and reports by other researchers are decades old and mostly anecdotal.

Researchers first found the stump in question (above) on a hike in a rainforest in the Waitakere Ranges on New Zealand’s North Island. They were surprised to see it was alive despite missing both branches and leaves. To see whether the surrounding kauri trees could be playing a role, they measured sap and water flow in both the stump and its neighbors.

The researchers found that the water flow in the stump increased as water flowed from the surrounding trees, meaning the stump was taking water from the trees around it, the researchers report today in iScience . This suggests, the scientists say, that the tree stump had formed connecting pieces, called grafts, with the roots of nearby trees to share water and other nutrients.

Many trees—nearly 150 species—form root grafts with other trees of the same species to exchange nutrients. But few have been found investing energy in keeping a stump alive. This is the first instance in kauri trees.

As for the reason it’s happening, researchers are still stumped. They speculate that the grafts formed before the tree became a stump, perhaps when it was still alive but missing leaves and unable to photosynthesize. Then, after the main part of the tree died, the stump continued to extract nutrients—and water—from its neighbors.