Read: Trees have their own song

How best to think about the living stump? Is it a vampiric parasite that sustains its undead existence by leeching the supplies of its fellow trees? Is it a beneficial partner that extends the root network of those other kauri in exchange for water? Is it even an individual entity anymore, or just a part of its neighbors? Without chlorophyll to harness the sun’s energy and make its own food, is it really much of a plant, or something more like a fungus or an animal—an organism that gets its nutrients from other living things? “I think this is really exciting,” says Franciska de Vries of the University of Amsterdam. “It poses so many questions.”

Credit: Sebastian Leuzinger

Underground, trees are intimately connected. The fungi on their roots can wire adjacent individuals to one another and ferry nutrients between them, creating what ecologists have come to call a “wood-wide web.” The roots themselves can also graft directly onto one another—a phenomenon that’s been documented in about 150 species, but is still mysterious. Why do it? Researchers have suggested that these natural grafts stabilize the trees, or allow them to share resources during times of hardship.

But what if one of the connected trees becomes a stump? Living stumps—no leaves, but intact roots—were documented first in 1833, and several times since. There’s a signposted one, for example, in Oregon’s Rogue River Gorge. Researchers have long assumed that their roots are connected to those of their neighbors, but that doesn’t fully explain how they survive.

Here’s the problem: Water isn’t pumped through trees, but pulled. The water that evaporates from leaves drags more water up through stems and from roots—a process called transpiration. But without leaves, that pulling force is absent. Water doesn’t flow, and neither do the nutrients dissolved within it. The innards of a leafless stump should be stagnant. Instead, they’re still on the move. Leuzinger and Bader proved that by inserting small needles into the water-carrying tissues, releasing small pulses of heat from one needle at intervals, and then detecting the pulses with the others. The stump’s water flows at a fifth the speed of its neighbors’, but it does flow.

The speed of that flow depends on what the surrounding trees are doing. If the neighbors’ sap flows faster, the stump’s sap flows slower. But if the neighbors reduce transpiration, whether at night or during heavy rain, the stump’s sap starts racing. This suggests that it isn’t just a passive part of its neighbors’ roots. Instead, it seemingly uses their downtime to gain more water.

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Why would the intact trees keep their connections to a partner that’s no longer contributing anything itself? They might benefit because the stump’s roots extend the range over which they can collect water, Leuzinger says. Or it could just be that they can’t identify freeloaders in their networks, or dissociate from them.