Untangling the deep-rooted relationship between trees and fungi to reverse dieback
Across the expansive Monaro plains and foothills of the Snowy Mountains in southern New South Wales, rows of grey ribbon gum skeletons haunt the landscape.
Dieback has killed millions of Australian trees, reducing biodiversity, habitat resources for native animals and degrading ecosystem health.
It occurs when trees lose their leaves and eventually die.
Across the Snowy Monaro region, during the drought years of the last two decades, the amount of dieback has been startling.
While drought, insects and climate change are all thought to be contributors, CSIRO researchers are investigating how the disruption and depletion of vital soil microbes like fungi contribute to dieback.
They are also investigating how increasing soil microbes can be used to power forest regeneration.
Getting to the roots of the relationship
The intimate relationship between trees and fungi is deep-rooted – both need each other to thrive.
Trees, and other types of land plants, rely on soil microbes such as mycorrhizal fungi – microscopic networks of fungi that form connections with their root system – for access to vital micronutrients and water, protection from pathogens, resistance and resilience to drought, and overall forest health.
Sweetening the deal, the host plant provides the mycorrhizal fungi with sugar it produces through photosynthesis in exchange for its services.
But while most research has been done in the northern hemisphere forests, the role those mycorrhizal fungi play in the health of Australian forest ecosystems, and their potential benefits for forest regeneration after disturbance, remain poorly understood.
Forming early bonds
To better understand the links between mycorrhizal fungi and eucalypts, CSIRO, in partnership with the Australian National University, Greening Australia and Upper Snowy Landcare Network, are testing whether inoculating ribbon gum seedlings with mycorrhiza improves their growth when planted into areas that have suffered severe forest dieback.
This means germinating the eucalypt seedlings with soil rich in mycorrhizal fungi to begin forming the mutualistic bonds between the fungi and young plants before planting in the environment.
“There is growing evidence that landscape disturbances from overgrazing, land clearing and soil erosion can seriously disrupt the connections plants make with soil fungi and other microbes,” says CSIRO Senior Research Scientist Dr Ben Gooden.
“Depletion of these microbes can in turn limit how well native vegetation can recover from those disturbances.
“But if we can reconnect the seedlings with mycorrhizae prior to planting at dieback affected areas, we hope that those plants have higher rates of survivorship and long-term growth”.
Leading the rollout of this large-scale project, Dr Gooden together with fellow researchers began the experiment by collecting soil from healthy patches of ribbon gum woodland across the Snowy Monaro region with no (or only very little) signs of canopy dieback.
“This field soil was integrated with potting mix and placed in black forestry grow tubes over which ribbon gum seeds were sprinkled,” Dr Gooden says.
“After six months we found that many of the tubes containing the healthy live soil developed fungal fruiting bodies that look like mushroom caps of known mycorrhizal species.
“This was a really exciting moment for us – as it was a sure sign that the eucalypt seedlings have begun forming the connections with desirable fungi prior to planting at the degraded dieback sites.”
Monitoring survival and growth
With the seedlings now planted at Gegedzerick Travelling Stock Reserve in the heart of the Snowy Monaro dieback zone, Dr Gooden says the team will monitor their survival and growth over the next two years.
“This will help us understand the long-term benefits of microbial inoculation for revegetation in dieback areas,” he says.
“We will also periodically harvest some eucalypt seedlings and undertake detailed DNA analysis to ascertain the presence and identity of the different fungal species connecting with their root systems.
“If we do find that areas suffering severe dieback have reduced diversity of mycorrhizae, this may explain why those dieback-affected areas are slow to recover – or can’t recover at all, even under ideal climate conditions for eucalypt growth.
But planting seedlings is time and labour intensive, says Dr Gooden.
“It requires long-term investment in seed collecting, propagation in greenhouse facilities, preparing a site for planting, fencing and ongoing plant maintenance, such as watering seedlings under severe drought conditions,” he says.
“Developing effective ways of reconnecting seedlings with these beneficial microbes prior to planting in the field will be a game changer for revegetation of degraded landscapes.”
This project is part of a broader collaboration between CSIRO, Upper Snowy Landcare Network, Greening Australia and the Australian National University supported through funding from the NSW Environmental Trust. The experiment was enabled through valuable time contributions from volunteers from Greening Australia and Landcare who assisted with preparing the inoculation treatments and planting the seedlings at Gegedzerick in October 2022.