Do trees communicate with each other?

Big old trees - dubbed 'mother trees' - are hubs in a mycorrhizal fungal network (Source: Smileus/iStockphoto)

Surprisingly, the answer is yes.

They might seem like the strong, tall and silent type, but trees actually communicate with each other.

Forest ecologist Dr Suzanne Simard, from the University of British Colombia, studies a type of fungi that forms underground communication networks between trees in North American forests.

Big old trees — dubbed 'mother trees' — are hubs in this mycorrhizal fungal network, playing a key role in supporting other trees in the forest, especially their offspring.

"If you're a mother and you have children, you recognise your children and you treat them in certain ways. We're finding that trees will do the same thing. They'll adjust their competitive behaviour to make room for their own kin and they send those signals through mycorrhizal networks," says Simard.

"We found that the biggest oldest trees had more connections to other trees than smaller trees. It stands to reason because they have more root systems," she says.

"So when a seedling establishes on the forest floor, if it's near one of these mother trees it just links into that network and accesses that huge resource network."

Sharing resources

Fungal networks don't just operate between related trees, but also between trees of different species in the same native community, says Simard.

In a landmark experiment, published in a 1997 issue of Nature, Simard used radioisotopes to trace carbon, nitrogen and water moving between a Douglas fir and a paper birch tree, which are both native to the inland forests of British Colombia.

When she shaded one tree, carbon-based sugars would flow into it from the other tree.
So rather than competing for resources, these two trees were using fungal networks to share them, says Simard.

In another study, Simard and her graduate student showed every tree in a 30 by 30-metre forest stand was connected to every other tree, with an estimated 250 to 300 trees being connected together in this single forest stand.

Defence signals

Other evidence shows trees use fungal networks to warn their neighbours about impending attacks from pests.

"When trees are attacked, they increase their defence against the invaders by upregulating their defence genes to make defence enzymes," says Simard.

"Research suggests they also send chemical signals down into their roots through their mycorrhizal networks to their neighbours, which then detect these signals and upregulate their own defence genes."

Lab studies have recorded defence signals travelling between trees in as little as six hours, says Simard.

She says when fungal networks are intact they allow a greater diversity of trees, each with their own strengths and weaknesses, to survive in the forest.

This diversity is the basis for forests that are resilient to disease, pests and climate change, says Simard.

Above-ground communication

Plant physiologist Professor Hans Lambers of the University of Western Australia says scientists have known for 20 or 30 years that plants communicate by giving off chemicals above ground.

The classic example is the release of volatile chemicals by plants that are attacked by pests. These chemicals are picked up by neighbouring plants which are then forewarned to defend themselves from the pests.

"The neighbours must sense those volatiles, and then respond by accumulating the chemicals that deter the attackers," says Lambers.

Above-ground chemicals can also attract predators that eat pests, and more recently this chemical communication has also been found to occur below ground, says Lambers.

But whether mycorrhizal fungi networks are used or not depends on the particular ecosystem.

Australian ecosystems

The Western Australian ecosystems, where Lambers works, are dominated by banksias, grevilleas and hakeas that on don't rely on mycorrhizal fungi.

However, eucalypt forests do have mycorrhizal fungal networks, says fungal ecologist Professor Ian Anderson of the University of Western Sydney, although no research has been done looking at their function.

"No-one's actually shown that an interconnected network is transferring carbon and nitrogen," he says. "It's a really under-researched area."

Anderson suspects fungal networks would be playing an even more important role in eucalypt forests than North American forests given their soils have much lower nutrients.

"I think these mycorrhizal networks have an even greater potential than what Suzanne Simard has shown," he says.

Implications for old-growth forests

Simard says her findings have implications for forestry practices that target old-growth trees.
"We need to leave these legacy trees and let them send their messages into the soil to surrounding plants," she says.

"This will help the recovery of forests following disturbance such as logging or fire."

Conserving fungal networks that help forests recover from disturbance could also prevent invasions by exotic species, which often compete with the endemic networks, she believes.

Dr Suzanne Simard, Professor Ian Anderson and Professor Hans Lambers spoke with Anna Salleh