In a rain forest near Auckland, New Zealand, a leafless kauri tree stump rises a few feet off the ground. These trees can become giants: The country’s biggest, Tāne Mahuta, or the “Lord of the Forest,” has grown 168 feet high, with a 115-foot canopy.
But this stump is just a stump, so unassuming most would pass it by.
One day, two ecologists from Auckland University of Technology spotted it on a hike.
“A normal person would just think it’s dead. It looks dead to a point, but if you look a bit closer, you can see living tissue,” said Sebastian Leuzinger. “We both said to each other, ‘It’s clearly not dead. How does it live?’”
Naturalists have observed living tree stumps in New Jersey, the Sierra Nevadas, British Columbia and elsewhere. But for more than 150 years, how the stumps survived without leaves for photosynthesis was a mystery.
These types of relationships may be more common among trees in forests than once thought, adding evidence to the notion that while trees may appear solitary aboveground, they’re intimately connected underground.
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Most trees communicate through subterranean networks of symbiotic fungi referred to as the “wood wide web,” sharing nutrients, carbon and other information. But a tree needs roots for water, and leaves to sustain itself.
During photosynthesis, plants open pores on their leaves to allow carbon dioxide to flow in. Open pores also allow water the plant hasn’t used to be released into the atmosphere. This process, called transpiration, draws water up from the roots so the plant doesn’t wilt.
But a leafless stump needs another way to circulate water.
To figure how it was surviving, Dr. Leuzinger and Dr. Bader measured water moving in the stump as well as in a neighboring tree. They found that when the tree transpired on sunny days, it took in water, but the stump didn’t.
But during rainy days and at night, the stump drank and the tree shut down. The stump had somehow rerouted its circulatory system, and it seemed to take turns with the tree.
To confirm their hypothesis, the scientists will need to clear away soil to expose the roots. But their early finding strongly suggests the stump received and circulated water through grafted roots.
Why would a tree support a stump that can’t reproduce or make its own food? And for the stump, why bother?
“It’s a bit unlikely that the tree dies, and goes and knocks on the other trees’ doors and says, ‘Hey, can I get a little carbon off of you? I’m dead,’” said Dr. Leuzinger.
The stump may be an oddball that got lucky from a connection formed before it became a stump. And that could mean such connections are quite common, he suggested.
Natural root grafts have been reported in some 150 tree species. But how the roots fuse and the evolutionary reason for these grafts are buried mysteries.
Scientists think grafting could have developed between trees because it’s cheap and easy. Trees may not sense death, recognize dying connections, or spend energy cutting off little stumps with small appetites. A stump’s extra roots may also add stability to other trees.
The researchers also say their results have implications for how forests respond to disease and drought. In New Zealand, a root fungus thought to spread through human foot traffic is killing off kauris, which are among the largest trees in the world. But changing our view of forest ecology to include root grafts will take years of follow-up.
“We probably know more about the surface of the moon than how a tree internally functions,” said Dr. Leuzinger.