Emily Simpson, University of Sydney; Aaron Greenville, University of Sydney; Chris Dickman, University of Sydney, and Glenda Wardle, University of Sydney
For three years before the Black Summer megafires of 2019–20, cool season rainfall in southeastern Australia was only half that of normal. When fires broke out, intensely dry conditions helped them spread through areas typically resistant to burning, such as wetlands and rainforests.
After the fires came three years of above-average rainfall, and scientific reports charted rapid vegetation regrowth.
It’s easy to think the rains let nature repair itself. But is that the full story? In our new research, we looked at what happened to seven severely burnt plant communities in the Greater Blue Mountains, west of Sydney.
We found a mixed picture. Wet sclerophyll forests supporting tall eucalypts skirted by ferns recovered within five years. But Blue Mountains rainforests did not recover from burns of any severity. Nor did grassy woodlands.
How can we track recovery?
To find out how well forests and other communities recovered from the Black Summer fires, we tracked recovery across the million-hectare Greater Blue Mountains World Heritage Area. Seven severe fires hit this region over the 2019–20 summer. The biggest was the Gospers Mountain fire, which burned an area seven times the size of Singapore – the largest fire from a single ignition point in Australia’s recorded history.
Five years on, if you had looked out across the Blue Mountains landscape you might have thought recovery was well under way. Thick scrub quickly replaced charred undergrowth in many areas. Across our study sites, many burned regions had denser understoreys – the layer of vegetation between forest floor and canopy – than unburnt areas.
But recovery isn’t just about how much vegetation is there. It’s about whether the layers of shrubs and canopy trees that make up a functioning plant community have returned to their previous state.
After fire moves through, some plant species rapidly regrow, outcompeting their rivals for light and water. But these species are often short-lived, failing to survive beyond a few years. On the other hand, species such as eucalypts and banksia may take years to regrow but then live much longer.
This means depending on when we take our measurements, we might mistake a temporary pulse of growth for real recovery.
To address this issue, our study tracked plant growth using a time-series of satellite images. This way, we could see the full trajectory of recovery, not just a snapshot.
A plant community that has fully recovered from burns will look stable year to year and mirror patterns of growth at unburnt sites. Poorly recovered areas, in contrast, will show distinct fluctuations.
We used this method to track recovery from low- to extreme-severity fire at 760 forest, heath and wetland sites in the Greater Blue Mountains region.
What did we find?
An earlier large-scale synthesis two years after the Black Summer fires found wet forests had suffered greater declines than drier plant communities.
Our findings five years after the fires tell a more complex story.
Some wet forests, such as wet sclerophyll, were able to take advantage of the heavy rains to recover fully within five years.
Others were less fortunate. The Blue Mountains are home to rare remnant rainforest, featuring stands of trees such as coachwood, lilly-pilly and sassafras, whose ancestors date back to Gondwana. We found these had not recovered after fires of any severity.
The region’s grassy woodlands also showed no sign of recovery when burned, despite being dry, open areas.
Why? We suggest the intense rains following Black Summer made it difficult for the community’s characteristic grasses and tussocks – adapted to dry conditions – to successfully re-establish.
What if the big dry hadn’t ended?
The years of heavy rain after the Black Summer fires were unusual. If drought had continued, would we have seen different recovery outcomes?
To tease out this question, we modelled what recovery would have looked like for plant communities under drought stress.
Simply put, all would be worse off. In particular, wet sclerophyll forests and freshwater wetlands showed declines five times greater than other plant communities.
This is likely due to the fact that the plant species in these ecosystems need heavy rainfall to re-establish. For instance, freshwater wetlands often need to be completely flooded for species to return.
These communities may be pushed to the point of collapse if drought persists after the next megafire.
What can we do?
Ferocious fires have swept through Europe in recent months and the southern summer is likely to coincide with a strong El Niño event, bringing a higher risk of intense heatwaves, drought and fire.
As climate change accelerates, these extreme conditions will become more common. To protect our plant communities, we must tailor our fire-management strategies. No two communities respond to fire the same way, and recovery outcomes differ greatly depending on whether post-fire conditions are hot and dry or cool and wet. A one-size-fits-all approach won’t work.
During the Black Summer fires, a team of firefighters protected the last wild stand of the ancient, fire-averse Wollemi pine in the Blue Mountains.
To help rainforests and other fire-averse plant communities survive, we will need technologies that help us rapidly detect and suppress new fires – and long-term monitoring programs to let us know when species are at risk.
In other landscapes, we must rethink our reliance on fuel-reduction burns. This is because plants are at higher risk of damage from fire if they have been frequently burnt in the past. When we repeatedly burn an area, we inflate this risk and condemn many plants and animals to staggering losses in future fires.
Of course, all these efforts won’t matter if we fail to tackle the underlying reason why fires are getting more frequent and severe – climate change.
Emily Simpson, Doctoral Candidate in Ecology, University of Sydney; Aaron Greenville, Senior Lecturer in Spatial Agricultural and Environmental Sciences, University of Sydney; Chris Dickman, Professor Emeritus in Terrestrial Ecology, University of Sydney, and Glenda Wardle, Professor of Ecology and Evolution, University of Sydney
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Aaron Greenville 