It's the defining characteristic of what we call the bush, a source of habitat and sustenance, a warm embrace that greets a traveller returning from abroad.

It's the dominant flora in much of the continent, and invariably the most handsome.

And it seems perfectly adapted: written into both science and folklore of the sunburnt country, where bushfires are a regular part of life, is the eucalypt's ability to survive, and thrive, after heavy burning.

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But a disturbing reality is emerging in some areas two years after the Black Summer bushfires that devastated the south-east, burned more than 24 million hectares of bush and caused dozens of human deaths.

Some of the gum trees aren't coming back.

In pockets of bushland from the South Coast to the back of the Blue Mountains, to the New England Tablelands, residents and researchers have been noticing that the rich and joyous regrowth that we have come to expect after fires has stalled, or never begun.

Where there should be epicormic sprouting - a riot of fresh, bright green shoots bursting from an array of blackened trunks - there stand just the dead trees, at once both the ornamental gravestones and the ghosts of those they represent.

What was different about these fires?

Did they burn hotter, as some thought? Did they move slower, with fire consuming an entire tree instead of being blown by fierce winds to race across a forest?

Or was it something in the conditions prior, which may happen again in the near future?

Rachael Nolan is one of the researchers who wants to understand the seeming increase in tree mortality in the wake of Black Summer.

And Dr Nolan, a postdoctoral research fellow at Western Sydney University, is hoping for help from the public identifying areas of eucalypt mortality, through the Dead Tree Detective citizen science project.

While eucalypts clearly benefit from fires, Dr Nolan explains that this is not because the seeds need extreme heat to be released from their pods, as popular belief may have it.

"They're generally thought of as being really resilient to fire," she said.

"But in 2019 we'd been hearing these reports that even before the fires the forests were really struggling.

"People were sending us photos of entire hill slopes where everything had turned brown almost overnight.

"It really raised the question of when the forests are hit by this huge drought and then the fires, are they still as resilient as they usually are?

"This is important because we rely on our forests for so many things - we're increasingly looking to them to offset our anthropogenic carbon emissions, they provide vital habitat ... and we just want forests in the landscape.

"Now's a good time to study them because we've had all this rain now, for about two years, so they should be looking good. If they're not, I want to try and understand where these areas are and why they're not recovering. We've had some reports of some areas already."

Rather than eucalypts needing fire for seeds to be released, Dr Nolan said the post-fire environment was beneficial.

"It's not so much the heat - it's more that after a fire there's a great environment for seedlings," she said. "There's high light on the forest floor, you've got a flush of nutrients from that ash bed, there's less competition. It's really the perfect environment to put your seedlings out and for them to get a bit of height, some growth."

Dr Nolan said with sustained drought in the preceding years, followed by a "flash drought" of no rain and high temperatures, it's likely gum trees had depleted their reserves - stores of carbohydrates underground - and couldn't call on them for a boost once the bushfires had all but destroyed the tree. Hence, increased eucalypt mortality.

Dr Nolan works with the NSW Bushfire Management Research Hub, a multi-university partnership led by the University of Wollongong. She encouraged people to use the Dead Tree Detective reporting system, easily located with an internet search engine.

Previous work by Dr Nolan and colleagues at the hub found that while the Black Summer fires burned a greater extent of bush than seen recently, the fires were not necessarily more severe in intensity across the dominant "dry-forest communities". But they reached other "wetter" areas they may not have in previous fires.

Shoalhaven Mayor Amanda Finley has seen it all too clearly, having driven through vast areas burned in 2019-20.

"There's patches on the Kings Highway that are still sticks," she said.

"The road between Turpentine Rd and Sassafras (west of Tomerong), that's still sticks, and that was just lunar after the fires. The road going out to Manyana and Bendalong, there's sections there that are still sticks. You can see the bush is trying on the understorey, but the understorey that's coming back is not the assemblage of plants that were there. It's the same on the road into Conjola.

"There's a lookout on the way up to Mt Bushwalk [inland from Conjola], when you look down towards the coast you can still see, two years later, a line in the bush where there's dead forest that was burnt, and live forest on the other side."

Cr Finley said logging regimes in the region needed to change to take the dead forests into account.

"We should be focusing on how we can support those forests coming back," she said. "I think some of that will not come back for 100 years. If you're looking at mature forest that had 100-year-old trees in it, they're not coming back in a hurry.

"I do a lot of bushwalking; those forests were silent for 12 months. Some of the spots are still nowhere near what they were, and it's heartbreaking."

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