13 August, 2019
The Stump That Didn't Die
|Two botanists stumbled across this living kauri stump while on a walk in the woods. SEBASTIAN LEUZINGER
At some point, a kauri tree fell in a New Zealand forest, and no one noticed. Nor did anyone pay attention when the remnant of its trunk rotted away, leaving behind a stump that's barely even a stump-a chair-size, hollowed-out half cylinder, sticking up from the middle of a hiking trail, leafless and apparently dead. "It doesn't look spectacular at all," says Sebastian Leuzingerof the Auckland University of Technology. "Everyone would have walked past it for years."
But when Leuzinger saw the stump, on a walk with fellow botanist Martin Bader, his head turned. He saw that even though it had no leaves, stems, or greenery of any kind, it did still contain living tissue-and when he knocked on the stump, it sounded different from deadwood. All appearances to the contrary, it's still alive. How?
Leuzinger and Bader eventually showed that the stump is connected to one or more of the kauri trees around it, probably via its roots. They are hydraulically coupled: The water flowing through the full-size trees also drives water through the stump, keeping it alive. It will never green again, never make cones or seeds or pollen, never unfall, never reclaim its towering verticality. But at least for now, it's not going to die, either.
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."
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.
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.
How the stump keeps water flowing is still a mystery. "The vessels in a tree aren't built for this," Leuzinger says. "They're one-directional. Water goes from the roots to the crown. But if you're a living stump, you have to reorganize your pathways so water can enter and leave again. This is completely unknown."
It's also unclear exactly how the stump keeps water flowing to its neighbors, because the team didn't dig up its roots to check. (That would require a permit, because the kauri is a protected species.) "However, it's 100 percent that there is a root graft," says Annie Desrochers of the University of Québec at Abitibi-Témiscamingue. "Otherwise, how could the stump be living?"
So far, Leuzinger and Bader haven't been able to find any similar kauri stumps, and while local foresters told them they have seen examples before, no one could point them to one. "It would be really interesting to find out whether this is a regularly occurring phenomenon rather than a one-off observation," de Vries says. "That would have far-reaching implications for forest ecology."
Desrochers certainly thinks that it's common, especially because she has found evidence of natural root grafts in every forest where she has looked. "That means trees can share water, nutrients, and diseases," she says. If there's a drought or insect epidemic, connected trees are more likely to survive, because resources can flow from unaffected individuals to beleaguered ones. Yet if a forest is thinned, the surviving trees might be stuck supporting connected but unproductive roots.
If times are good, she suspects that "supporting a stump or a weaker connected neighbor doesn't make much difference." But harsher conditions could destroy an entire stand "because the few photosynthesizing trees cannot support a large root biomass, which they cannot ‘choose' to stop feeding, because they are connected with wood."
"That really changes our view on forest mortality," Leuzinger adds. If living, intact trees regularly share water through connected supplies, "we need to look at forests as superorganisms," he says.
Written by: Ed Yong
Source: The Atlantic