Take a walk in the bush, and you'll find yourself immersed in a soundscape of chatter.
You might hear birds bantering to one another as they forage for food, or swarms of insects serenading potential partners.
But the quietest life forms are having some of the liveliest conversations.
Trees might seem like the type who prefer to keep to themselves, but beneath your feet they are busy forming secret relationships with vast networks of underground fungi.
Big, old trees rely on this "wood-wide web" to shuttle nutrients to their younger neighbours, while others use it to send chemical signals that warn nearby trees of looming threats, such as diseases and pests.
"The symbiosis is important for all aspects of plant growth, but also the diversity we see in our landscape," says Ian Anderson, a fungal ecologist at Western Sydney University.
It's a friends-with-benefits arrangement that's been around for millions of years, but researchers are only just beginning to unravel the secrets of how trees and fungi interact, particularly in Australian ecosystems, says Tom May, a mycologist at the Royal Botanic Gardens Victoria.
"It's just as challenging a frontier as deep space — it's deep in the soil."
Roots that wear socks
If you could shrink yourself down and dive beneath the forest floor, you would encounter a wonderland of miniature life forms working hard to keep the ecosystem running.
Among the busy mites and microbes, you'd be surrounded by dozens of tree roots wearing ghostly white "socks".
These sock-like coverings are known as ectomycorrhizal fungi, the most common type of symbiotic fungi in forests.
The friendly fungi reach into the soil with their hyphae — long, cobwebby filaments that are thinner than a strand of human hair.
In the northern hemisphere, these fungi "fingers" make up an estimated 30 per cent of the biomass in forests.
"There's kilometres of hyphae in just handfuls of soil," Dr May says.
These bundles of hyphae — or mycelium — are tasked with doing the food shopping for their tree hosts.
As they branch out into the soil, the hyphae release special enzymes that turn hard-to-access nutrients like nitrogen and phosphorous into forms that the tree can readily consume, Dr May says.
"They slobber over their food."
In exchange for their nutrient-scavenging efforts, host trees send their fungal partners a pay cheque of sugar and carbon, which they produce from sunlight via photosynthesis.
This gives the fungi the energy they need to grow and scour every crevice of soil for tree nutrients.
"There's almost like an economy," says Jonathan Plett, a molecular mycologist at Western Sydney University.
Trees are generous hosts, providing a home for dozens of species of mycorrhizal fungi that each have a role to play.
"Some of those fungi are connecting up to different trees of the same species, sometimes they're connecting up to different trees of different species," Dr May says.
"It's very complex."
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Australia's invisible friendships
Over the past two decades, most of the research exploring the relationship between fungi and trees has focused on northern hemisphere ecosystems, such as pine forests.
Studies from this side of the world have revealed that trees don't just rely on their mycorrhizal fungi to look after themselves.
One 2015 study found that drought-ravaged Douglas fir trees (Pseudotsuga menziesii) — which are native to North America — offload their carbon and send chemical stress signals to nearby ponderosa pine (Pinus ponderosae) trees.
But the conversations between Australian native trees and their mycorrhizal fungi are still largely shrouded in mystery.
Over the past few years, Dr Plett and his team have been digging into the nitty-gritty of how mycorrhizal fungi help eucalyptus trees thrive and communicate.
While Aussie tree-fungi relationships share some similarities with their northern hemisphere counterparts, Dr Plett has noticed some striking differences.
For one, his experiments have shown that Australian mycorrhizal fungi have evolved to take some heat.
Dr Plett has found that mycorrhizal fungi from the northern hemisphere don't cope well when they are exposed to temperatures above 25 degrees, leaving their tree partners vulnerable as a result.
"They either die or they go dormant and just kind of sit there ... they don't help the plant anymore," he says.
It's a different story for Australian fungi, which continue thriving at temperatures as high as 37 degrees.
"They love it," Dr Plett says.
"If you think of the Australian environment ... they've had that temperature extreme long enough that they've somehow adapted to that."
Dr Plett and his team have also found eucalyptus trees aren't quite as loyal to their fungi partners as northern hemisphere tree species, which tend to stick with the same type of mycorrhizal fungi for life.
As seedlings, eucalypts pair up with arbuscular mycorrhizal fungi — a group that usually favours crops and grasses over trees.
But like teenagers who drift away from childhood friends, eucalypts switch over to ectomycorrhizal fungi as they grow into adults.
"Eucalypts are one of only a very few hosts that do that," Professor Anderson says.
Because arbuscular mycorrhizal fungi typically sustain plants in nutrient-poor soils — a key feature of the Australian landscape — Professor Anderson suspects they give young eucalypts a good start in life until they're ready to meet their long-term partners.
"It is another peculiarity of the Australian native ecosystem."
Proteins as passwords
But not all underground fungi are interested in making friends with trees.
For instance, the Australian honey fungus (Armillaria luteobubalina) is the main culprit behind root rot in eucalyptus trees, and can spread from tree to tree like an underground wildfire.
So, how can trees tell the difference between friend and foe?
The answer lies in tiny molecular differences. Dr Plett has found that when mycorrhizal fungi encounter a tree's roots, they secrete special proteins that can unlock the tree's root cells without setting off its alarm bells.
If trading nutrients is like a conversation between trees and fungi, then proteins are the language mycorrhizal fungi use to "talk" to the tree's immune system, Dr Plett says.
"That's what allows that fungus to actually push into the root without the plant killing it off."
Some species of fungi will go even further to maintain a loving bond with their chosen trees.
Earlier this year, Dr Plett and his colleagues reported evidence that a fungus called Pisolithus microcarpus manipulates gene activity in the roots of flooded gum trees (Eucalyptus grandis).
As this fungus colonises the gum tree roots, it releases microRNA — small chunks of genetic material that control the production of certain proteins — a strategy that's more common among pathogenic fungi.
In the lab, Dr Plett and his team found that the fungus donated some of these genetic scraps to its host tree's roots, helping it form a lasting bond with its woody partner.
But it wasn't a one-sided relationship. The researchers also found that seedlings with the fungal microRNA in their roots pulled more nutrients from the soil than those without it.
The study showed that the fungus gave the trees some genetic tweaks that boosted their immune response and nutrient-processing capabilities.
The perfect match for regeneration
While eucalyptus seedlings are often inoculated with mycorrhizal fungi before they are planted in the wild, the results are hit and miss.
Some seedlings will grow up strong and robust, while others struggle to establish themselves, even if their roots are wrapped with the same type of mycorrhizal fungi.
Part of the problem is that not all fungi — even those within the same species — are willing to share the fruits of their labour, with some hoarding the nutrients they collect.
This could be due to subtle genetic and metabolic differences among individuals, Dr Plett says.
"Different types of fungi colonise different types of trees, much like we gravitate to certain people and not others."
By cracking the riddle of how trees rely on fungi to thrive and communicate, Dr Plett hopes to identify the most compatible tree-fungi partnerships — a boon for the nursery industry and bush regeneration efforts.
And while trying to play matchmaker is painstaking work, the pay-off could be huge.
Dr Plett estimates that inoculating eucalyptus and pine seedlings with the right mycorrhizal partners could cut fertiliser use by up to 40 per cent, and even help store more carbon underground.
Getting this delicate balance right is also important for restoring landscapes that are out of whack to begin with, such as cleared farmland.
For instance, in soil with abnormally high nitrogen and phosphorous levels, some trees dump their mycorrhizal partners as they get their nutrient needs met elsewhere, Dr May says.
But independent living has its downsides.
"[The trees] don't get all the other benefits, like disease protection," he says.
Given that roughly 50 species of Australian fungi are now on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species, cataloguing our native fungi and understanding their role in Australia's forests is more important than ever, Dr May says.
This means turning our attention to the underworld, instead of the heavens.
"It's interesting how people are really captured by cataloguing the stars … there's nothing going on out in space that we need to worry about," Dr May says.
"But if we don't crack on here and get an understanding of what we've got, in 50 years' time it could be gone."
