Beneath our feet, a vast microbial network dubbed the "wood wide web" allows trees to communicate and share resources with each other.
Most trees and plants depend on mycorrhizal fungi to get crucial nutrients Image: M. Gann/McPHOTO/blickwinkel/picture alliance
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Trees wouldn't be able to survive if it weren't for the massive networks of fungi operating below ground.
Out of sight, these microscopic fungal filaments pervade the soil, much like a subterranean internet, helping connect plants and trees to each other.
They can use this system — sometimes called the "wood wide web" — to exchange water, nitrogen, carbon and other nutrients, or even get advance warning of threats.
Mycorrhizal fungi networks have been around for more than 400 million years. And according to ecologist Thomas Crowther, they act as a type of "brain of the forest" that works to keep the entire ecosystem healthy.
"Mycorrhizal fungi are absolutely essential to the functioning of about 90% of the world's trees. One couldn't really exist without the other," says Crowther from ETH Zurich, who was among a team of scientists to develop the first ever global map of the wood wide web.
Mushrooms grow above ground, while networks of fungi filaments spread undergroundImage: Hans Reinhard/Okapia/picture-alliance
So how does it work?
Trees and plants have a symbiotic relationship with the mycorrhizal fungi that weave around and bore into their roots. The plants pass their fungal partners carbon, and in return they get nutrients such as phosphorus and nitrogen that the fungi take from the soil.
But beyond that, plants also use this extensive underground web of fungi to interact with each other, moving information, nutrients, sugar and water to other trees in the system that need it most.
"Trees that are struggling for nutrients will often be enhanced by the network because the fungi will redistribute nutrients towards those struggling trees or areas where there's been a lot of insect damage," Crowther said. "This connectivity is what keeps the whole system going."
The fungus collects nutrients from the soil and passes it to the plant through its root systemImage: Last Refuge/Mary Evans Picture Library/picture alliance
When seedlings get hooked up to the network, they can get an injection of nutrients and water from more mature trees. This helps them grow and develop resilience in the face of stress. Dying trees can also use the web to transfer their nutrients to neighboring plants.
Trees can receive early warning signals about threats via the network if a neighbor is under attack — for example, from aphids or caterpillars — and preemptively produce defensive chemicals to protect themselves from an assault.
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When trees die, so do the underground fungi
Mycorrhizal networks support ecosystems and make forests resilient. They are also massive carbon sinks, keeping heat-trapping CO2 locked away underground. But agricultural expansion, pollution from chemical fertilizers and deforestation are putting these microbial webs at risk.
According to the UN, around 178 million hectares of forest — about triple the size of France —have been lost over the past three decades. When trees are cut down, the fungi underground is also destroyed. Researchers have found that logging can slash the abundance of mycorrhizal fungi in the ground by as much as 95%.
Rising temperatures associated with climate change could also see fungi that lock carbon away for longer periods replaced by a fast-cycling variety that "could potentially drive carbon losses into the atmosphere," according to Crowther, leading to additional warming.
Mycorrhizal fungi have been underpinning life on Earth for millions of years. By disrupting the complex webs they form beneath our feet, we are also endangering the organisms we depend on to survive.
11 surprising facts about trees
Trees soak up carbon from the atmosphere, provide a home to wildlife and even improve our mental well-being. But did you know they can also "talk" to each other, and send out distress signals when under attack?
Image: picture-alliance/Goldmann
60,000 different species
There are around 3 trillion trees on Earth, according to a global study led by researchers from Yale University. That includes over 60,000 known tree species, more than half of which are endemic — meaning they're found in only one country. Brazil, Colombia and Indonesia are home to the most tree species. The bad news: there are 46% fewer trees today than at the start of human civilization.
Image: picture-alliance/Wildlife
Trees 'migrate' to escape climate change
Trees clearly can't uproot themselves and move, but their population centers can shift over time in response to climate pressures. A study looking at 86 trees species between 1980 and 2015 in the eastern United States found that 73% moved west, where rainfall is increasing. Others headed to the poles, apparently to escape heat. On average, they moved about 16 kilometers (10 miles) per decade.
Image: picture-alliance/blickwinkel/R. Linke
Keeping cities cool
Trees not only give us shade, they can also mitigate extreme temperatures by transpiring — absorbing the sun's radiation and releasing water into the air through their leaves. Urban areas can become sweltering "heat islands" in summer. But a 2019 study from the US found that tree canopy cover of 40% or more could lower summer temperatures in cities by as much as 5 degrees Celsius.
Image: picture-alliance/Loop Images
Sucking up pollutants
Trees draw CO2 from the atmosphere and are therefore crucial in the fight against climate change. They can also use their leaves to filter particulate matter and toxic gases like nitrogen dioxide and sulphur dioxide from the air. A recent UK study found that silver birch, yew and elder trees could reduce particles at rates of 79%, 71% and 70% respectively.
Image: picture-alliance
Healing power
Trees can reduce our stress levels and help us feel happier and healthier. Several studies have shown that spending time in nature, or even just looking at trees or flowers through a window, can lower blood pressure, boost the immune system, improve sleep, reduce depression and anxiety, and even speed up recovery after surgery.
Image: picture-alliance/dpa/C. Klose
Trees 'talk' to each other
Forests have their own communication systems — almost like an underground internet —that allows trees to swap nutrients and send warnings about drought or disease. They interact via networks of soil fungi, known as mycorrhizal networks. Research by ecologist Suzanne Simard has shown that paper birch (pictured) and fir trees use this system to send water, carbon and nutrients back and forth.
Image: picture-alliance/All Canada Photos
Sending signals in the air
Trees can't flee if their leaves are being devoured by a hungry herbivore. But what they can do is release chemicals — volatile organic compounds — into the air to warn nearby members of the same species there's a threat in the area. Studies show that other trees respond by boosting their own production of anti-herbivore toxins, which, in the case of acacias (pictured), makes their leaves bitter.
Image: picture-alliance/Anka Agency International
Call for backup
When besieged by bugs or parasites, some species, including apple trees, and tomato, cucumber and lima bean plants, release compounds into the air to alert the attackers' predator. Most often, these predators are insects. But a European study showed that trees infested with caterpillars also put out chemical signals to attract caterpillar-eating birds, such as the great tit (pictured).
Trees are the oldest living organisms on Earth. One individual can survive hundreds, even thousands of years. According to the OldList, an officially dated record of ancient trees, the oldest known living individual is a bristlecone pine in California's White Mountains. Named Methuselah, it's around 4,850 years old. Its exact location is kept a secret to protect it from vandals.
A photograph can't really do justice to the world's tallest trees: redwoods. The tallest known living specimen is a coast redwood called Hyperion measuring 115.85 meters (380 feet) — more than Big Ben or the Statue of Liberty. The giant, discovered in 2006 in California, is believed to be several hundred years old.
Image: picture alliance/ZUMA Press/B. Cahn
Other record breakers
California is also home to a giant sequoia named General Sherman, thought to be the biggest living tree in terms of volume. It stretches to a height of 83.8 meters and is 7.7 meters in diameter. The title of the world's widest tree goes to the Arbol del Tule (pictured), a Montezuma cypress in the Mexican state of Oaxaca. It has a diameter of 11.6 meters and circumference of 42 meters.
