In an intriguing case of a tree stump that should be dead but is still alive, a team of scientists have chanced upon an arboreal mystery in a forest in New Zealand.

"My colleague Martin Bader and I stumbled upon this kauri tree stump while we were hiking in West Auckland," says corresponding author Sebastian Leuzinger, from Auckland University of Technology (AUT), in a statement. "It was odd, because even though the stump didn't have any foliage, it was alive."

Sometimes what is clearly visible is not always the entire story, they soon discovered. Instead, the answer could lie beneath their feet in the labyrinth of tree roots under the soil.

To investigate the mystery of the “should be dead but isn’t” stump, the team measured water flow in the stub and nearby trees. As the trees evaporated water through their leaves during the day, the stump received little if any nutrients. However, when the trees were dormant at night, the water began cycling to the squat stump. This subterranean plumping system is likely happening through an interconnected root system, whereby trees naturally graft, or fuse, their roots to each other.

"This has far-reaching consequences for our perception of trees – possibly we are not really dealing with trees as individuals, but with the forest as a superorganism," Leuzinger says.

But why would they seemingly take turns in water exchange?

"This is different from how normal trees operate, where the water flow is driven by the water potential of the atmosphere," Leuzinger says. "In this case, the stump has to follow what the rest of the trees do, because since it lacks transpiring leaves, it escapes the atmospheric pull."

"For the stump, the advantages are obvious – it would be dead without the grafts, because it doesn't have any green tissue of its own," Leuzinger says. "But why would the green trees keep their grandpa tree alive on the forest floor while it doesn't seem to provide anything for its host trees?"

The scientists theorize there are benefits to be gained by sharing resources via a vast root network. Such a chain could provide the trees with more nutrients and water as well as increase their stability if rooted on steep slopes. The stump may be the fortunate recipient of this system, with other trees unable to sense its possible death, especially when its peers tower as some of the mightiest on Earth, growing over 50 meters (165 feet) tall and 16 meters (52 feet) in girth. 

The connectivity could have its downsides, however, as diseases may spread more rapidly. This is particularly relevant for the kauri, a species that has recently been classified as threatened due to the spread of kauri dieback disease caused by the pathogen Phytophthora agathidicida. 

The team only observed one living kauri tree stump, which prevents them from drawing broader conclusions. “Although we personally have no yet seen a second occurrence of a living stump belonging to this iconic New Zealand Species, from talking to local foresters, we know this phenomenon has apparently been noticed in the past and the formation of natural root rafts in kauri was already suspected 80 years ago,” they write in the study published in iScience.

"This is a call for more research in this area, particularly in a changing climate and a risk of more frequent and more severe droughts," Leuzinger says. "This changes the way we look at the survival of trees and the ecology of forests."