The mighty mountain ash can grow as tall as a 30-storey building. But if these giants disappear, it's not just the forest that will suffer.

A couple of hours' drive from Melbourne, you'll find giants.

On a winter's day, they fade in and out of the gentle swirling rain, seemingly melting into the mist before abruptly bursting into view once more.

These ghostly mountain ash forests, in an area known as Victoria's Central Highlands, were home to Gunaikurnai, Taungurung and Wurundjeri peoples for tens of thousands of years.

Some towering individuals extend their statuesque trunks almost 100 metres towards the sky.

Mountain ash can live up to 500 years. They are Earth's tallest flowering plants, and one of the tallest tree species on the planet.

It's no wonder the second part of their botanical name Eucalyptus regnans means "ruling" or "reigning".

The tallest living mountain ash we know of is in southern Tasmania — a behemoth nicknamed Centurion that stands a bit over 100 metres from forest floor to foliage crown.

In the forest near Marysville in Victoria is a mountain ash known as the elephant tree.

It's not as tall as Centurion, but is still impressive, with a trunk that measures 13.6 metres around its base.

And it's old — about 400 years.

That means when European colonisers first sailed from Bass Strait into Port Phillip around the turn of the 19th century, the elephant tree was already nudging 200 laps around the Sun.

Such enormous evergreens appear rock solid; capable of withstanding whatever the years throw at them.

But their future, even in the near term, looks shaky.

These forests are one Australian ecosystem that might collapse in the next decade under a warming world, according to an IPCC report released in March this year.

And there's more to lose than a few old trees.

Forest-dwellers at risk

David Lindenmayer knows the area well.

Today, he's a forest ecologist and conservation biologist at the Australian National University.

But back in 1983, he was working in the mountain ash forests near the town of Marysville as a fresh-faced science graduate.

His job was to take a census of sorts, but instead of people, he surveyed Victoria's state animal, Leadbeater's possum (Gymnobelideus leadbeateri).

He wanted to know how many there were, where they lived and what aspects of their habitat were crucial to their survival. This was no small task, as the pocket-sized marsupials are most active at night and prefer to hang out high in the treetops.

Leadbeater's possums had already come back from the dead once, in a way. In the 1930s, they were thought extinct — driven to their demise by farming and fire.

But in 1961, they were rediscovered in the Central Highlands' mountain ash.

Professor Lindenmayer set to counting in 1983, and eventually broadened his census to other resident mammals.

"There's a beautiful assemblage of possums and gliders in that forest, including greater gliders, yellowbelly gliders, mountain brushtail possums," he says.

"When I first started working, about one in every three large old trees had a possum or a glider in it.

"Now the numbers are about one in every eight or every nine trees."

When he'd go out spotlighting in the early days of his career, the most common species he'd catch in the light was the greater glider.

"Now that species has declined by about 80 per cent in the past 20 years."

In July this year, the greater glider was reclassified from "vulnerable" to "endangered" at a national level.

And the little Leadbeater's possum?

"They've declined by about 50 per cent," Professor Lindenmayer says.

Crucial to the survival of these and other forest-dwellers are stands of old-growth forest that contain trees more than 120 years old.

Older trees are full of hidey holes where all manner of animals — not just possums but a whole bunch of birds too — make their nests and feed on leaves and buds.

Fewer big old trees mean fewer places to live and less food.

And big old trees are already few and far between.

Back in the 1980s, about 25,000 to 30,000 hectares of old-growth mountain ash forest stood in the area, Professor Lindenmayer says.

"Now it's 1,886 ha. The abundance of big trees has really dropped through the floor."

Guardians of the water supply

Animals further afield — Melburnians — rely on old-growth forest too.

That's because most of Melbourne's water supply comes from the mountains to the east of the city where the mountain ash live.

As snow and rain fall on the ranges, it trickles through the soil to the water table underground, and burbles out again on the surface as springs.

These springs feed creeks, which feed rivers, which feed reservoirs, which feed into our homes.

So what do old-growth mountain ash trees do for our water supply?

It's more about what they don't do.

While mountain ash keep growing throughout their whole life, their twilight years are spent growing far slower than when they were saplings.

They shoot up around a metre every year during their first 70 years or so. This needs a lot of water, so some of that reservoir-bound rainfall is slurped up by thirsty roots instead.

Keep as much old-growth forest intact as possible, and Melbourne's water supply is protected too.

Why are these old trees vulnerable?

You don't have to look far to see where threats to these forests come from.

The narrow shoulder of the winding road from Marysville to the elephant tree is peppered with signs warning of logging trucks.

Black scars of fires from years gone by peek from beneath new growth.

Either frequent burning or logging on their own is bad enough, Professor Lindenmayer says — but together, they spell disaster.

Let's start with fire. Mountain ash and its slightly smaller cousin alpine ash (Eucalyptus delegatensis), also found in this area, are "seeders" and need fire to reproduce.

After a bushfire, the trees sprinkle thousands of tiny seeds on the ground.

Mountain ash need to be around 20 years old before they make seeds — and that's normally fine.

This is a cool, wet part of the country, and fires tend to be spaced out enough to let young trees mature to reproduction age.

But climate change has thrown a spanner in that balance between death and rebirth, and is driving fires more frequent than the trees have evolved to handle.

Young, recently logged forest is more vulnerable to fire for a few reasons.

First up, the stuff that's left on the forest floor after the big trees are taken away — branches, leaves and the like — can be fuel for fire, Professor Lindenmayer says.

"Plus wind speeds are higher, which is a really important part of fire dynamics."

And then there's the baby trees themselves, which are more flammable than their older siblings.

If fires wipe the mountain ash slate clean more than two or three times a decade, and the forest doesn't have time to recover, they disappear, and other trees take over.

It's not just young trees that are vulnerable to burning. The biggest and oldest mountain ash trees can survive fires, but they can only take so much.

Repeated fires — and not even particularly intense ones — can kill off the big old trees, says Lachie McBurney, a research officer with ANU who lives near and monitors the ash forests.

If a fire whooshes through a patch of forest, it might not kill big trees, but can leave a blackened scar on their base.

"When you get a fire scar in the bottom of a big tree, that's like a chimney door for when the next time a fire comes through," Mr McBurney says.

"So even if you get a really light burn [next time], like a fuel reduction burn … that'll actually turn that into a chimney and kill that tree.

"Even a massive giant we think, because it's big, it should stand for longer, but that's not actually the case."

Throw in a commercial logging regime that harvests trees under 50 years old, and the forest as a whole remains younger, Professor Lindenmayer says.

The forest's age also affects the amount of carbon it locks up.

Mountain ash forest can hold 1,900 tonnes of carbon per hectare — up to 10 times more than tropical rainforest — but it needs to be old growth.

So along with using more water, young forests also store less carbon than their older counterparts.

Respect for arboreal elders

As the world warms, the outlook isn't great for mountain ash.

To give the forests the best chance of survival, we need to stop logging them immediately, Professor Lindenmayer says — not by 2030, which is when the Victorian government said it would pull the pin on native forest logging.

The timber industry has a long history in the Central Highlands. By the 1920s, these ash forests supported around 120 sawmills.

In its heyday, thriving communities, complete with schools, were scattered throughout the forest.

These days, all but a couple of houses are gone, torn down or destroyed by fire. Abandoned machinery sits rusting among the trees.

What also remains is an enduring effect on Melbourne's water supply.

Professor Lindenmayer and his colleagues calculate the city loses 15 billion litres of water each year thanks to logging in the city's most important drinking water catchment, the Thomson Catchment.

That's enough water for a quarter of a million people.

And if logging in the catchment continues at its current rate, the equivalent of 600,000 people's water will be lost from Melbourne's water supply every year by 2050.

"As time has gone on, it's become clearer and clearer that the level of pressure on the mountain ash system is just unbearable," Professor Lindenmayer says.

"It's not possible to keep logging the forest the way it's being logged, and to have so much fire in the system, and to expect any of the forest species to be able to survive — including the mountain ash forest itself."

It's easy to feel insignificant when gazing up at these centuries-old giants.

But their fate, for better or worse, lies with us.

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