NEW ZEALAND - Some tree stumps are dead. But some are alive.
Auckland University of Technology researcher Sebastian Leuzinger was hiking through a New Zealand forest when he noticed a tree stump with living tissue sprouting from it. But how could a tree stump survive without green foliage?
The answer may lie in what scientists are calling the wood wide web - a vast, fungi-enabled underground network that allows trees to communicate with each other and share resources. Leuzinger and a colleague found that the lone Kauri stump had grafted its roots onto the roots of a few neighboring trees, feeding at night on water and nutrients collected by the other trees in the day.
The Kauri stump shares resources from nearby trees.
"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," writes Leuzinger. "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?"
Leuzinger and other scientists aren't sure. One possibility is that the stump serves as a bridge to other nearby trees outside the direct influence of the helping trees. Another possibility is that the helping trees don't realize the stump has lost its greenery.
Leuzinger found that the Kauri stump and neighboring trees never drank water at the same time. The stump was never active during the day, only consuming at night when the other trees weren't at work.
Scientists don't yet understand the complexity of interactions between trees. Evidence of a single living stump is not enough to draw conclusions.
Photo by University of Plymouth
Just over 20 years ago, ecologist Suzanne Simard discovered that trees do communicate with each other. Simard studied how over the course of a summer, shaded fir trees received carbon from birch trees in the sun. The opposite happens in the fall, when birch trees receive carbon from fir trees as they start to shed leaves. She discovered this exchange takes place underground, through a "mycorrhizal network", a symbiotic relationship between a fungus and the roots of its host plant.
Both the fungi and trees benefit from the relationship. Since the fungi cannot photosynthesize (as they have no access to light), they receive a type of carbon and sugar produced during the tree's photosynthesis.
And in return for sugar and carbon, fungi release nutrients like nitrogen, phosphorus, and water to the trees.
But as scientists discovered, it goes deeper. Fungi actually connect trees to one another, allowing them to share resources, call for help, and even wage war on each other.
Remarkably, Simard and her team of graduate students found that older, or "mother" trees use the network to supply shaded seedlings with sugars. The mother trees are actually able to identify young, nearby seedlings as kin. If it identifies those nearby young trees as its own offspring, mother trees are more likely to share resources.
Scientists also discovered that dying trees may dump their resources into the network. And if a tree is attacked by a bug, for example, it can release chemical signals through their roots. Those signals travel through the network, warning other trees to activate defenses.
Other plans are more sinister. Certain types of orchids, for example, may hack the network in an attempt to steal resources from nearby trees. The black walnut tree spreads toxic chemicals to hurt or kill nearby plants.
Earlier this year, an international team of scientists created the first global map of the vast underground network. They did this by creating a computer algorithm to analyze a database from the Global Forest Inititiave, which includes 1.2 million trees in more than 70 countries.
The algorithm takes into account the different fungal species that associate with each tree species. It also takes into account local climate factors - which the scientists say has the biggest role to play.
"It's the first time that we've been able to understand the world beneath our feet, but at a global scale,” Thomas Crowther, an author of the study from ETH Zurich, told the BBC. "Just like an MRI scan of the brain helps us to understand how the brain works, this global map of the fungi beneath the soil helps us to understand how global ecosystems work.
"What we find is that certain types of microorganisms live in certain parts of the world, and by understanding that we can figure out how to restore different types of ecosystems and also how the climate is changing,” he said.
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