Every year at this time, thousands flock to national parks in Tasmania to see the stunning autumn display of Australia's only native winter deciduous tree — the fagus.

But this year the state's parks are closed until further notice thanks to coronavirus, which means few will get to see the "turn of the fagus" with their own eyes.

Kate Burton is one exception. She's been working as a ranger at Cradle Mountain National Park for six years and recently caught the first flush of the turn.

"I'm witnessing something that will occur regardless of what's happening in our crazy world," Ms Burton said.

Kate Burton has worked at Cradle Mountain National Park for six years. ( Tasmanian Parks and Wildlife )

She was doing track work earlier this month at an altitude of 900 metres above sea level when she noticed some of the leaves starting to turn colour. After some fresh snow falls, the turn really kicked off.

"It's spectacular and I feel very fortunate to be able to still see it," she said.

"There's nowhere else in the Australian landscape where you can see huge tracts of native woodland just changing colour in autumn.

"I guess that's why it brings people from all round the world specifically to Tassie to see it."

A cold snap can help bring on the turn. ( Tasmania Parks and Wildlife Service )

Especially disappointed will be nature photographers and "leaf peepers" — yes, that's a thing — travelling far and wide to catch the turn of autumn leaves in different parts of the world.

The fagus — official name Nothofagus gunnii — is a compact alpine deciduous beech tree, believed to have been in Tasmania for 40 million years.

Strangely, Australia has relatively few native deciduous trees and the fagus is the only one that loses its leaves in preparation for the cold season.

It can look rather shrubby and have twisted ground-hugging branches and roots — which has earned it the nickname "tanglefoot" by bushwalkers who go off track and get caught up in it.

As this shot from last season shows, swathes of turning fagus can create stunning landscapes. ( Rob Blakers )

Its distinctive crinkly leaves are very photogenic, especially when they change colour for a few weeks over April and May.

"You've got your beautiful yellows, oranges, pinks and reds and it's absolutely stunning when it's carpeting whole mountain slopes or around waterfalls," said Ms Burton.

While the leaves usually turn from yellow to red, she's recently come across a unusual stripe of red leaves in among a bunch of green.

Fagus leaves starting to turn. ( Tasmania Parks and Wildlife Service )

What makes a plant change colour?

While there are limited studies of the fagus, what we know from research into other winter deciduous trees reveals the physiological processes behind autumn displays.

It all comes down to trees detecting falling temperatures and decreasing daylengths, said Jim Weller, a plant scientist at the University of Tasmania.

Like us, plants are rhythmic beings and have internal biological clocks that are affected by light, which enables them to tell when days are getting shorter.

As soon as they detect this they stop growing and start building up their energy stores instead.

At this time of year the leaves of the fagus turn yellow, orange and red. ( Rob Blakers )

Once a plant detects it's getting cooler as well, this signals it's no longer worth the leaves photosynthesising — using sunlight to make energy.

This is when it starts to break down pigments and other chemicals in its leaves to salvage nutrients like nitrogen.

The array of autumn colours seen in winter deciduous trees is a reflection of how pigments are broken down.

"The leaf is a soup of different pigments — green ones, yellow-orange and reddy-purple, depending on the species — and these get lost late in the season at different rates," Dr Weller said.

Green chlorophyll is the first pigment to disappear, leaving the leaf yellow, orange or red. ( Rob Blakers )

The first to disappear is the green chlorophyll, the key pigment involved in photosynthesis.

Once the green goes, the leaf colour is determined by the remaining pigments including the yellowy-orange "carotenoid" pigments that play a background role in photosynthesis, and the reddy-purple "anthocyanin" pigments, which serve as sunscreens and have other protective functions.

"These do get removed from the leaf too, but just not as quickly as chlorophyll," Dr Weller said.

A forest of red-leaved deciduous beech and pencil pines covered in snow. ( Rob Blakers )

Colours can also vary from autumn to autumn, depending on environmental factors such as how quickly the temperature falls heading into winter, Dr Weller said.

Eventually, the plant decides to cut its connection to the leaves altogether — and this is when the autumn leaves fall.

"Most deciduous plants have a mechanism where they actively degrade a little cell layer at the base of the leaf stalk," Dr Weller said.

Living fossil under threat from fire and climate change

A living fagus leaf next to a fossil leaf from the same species that is about 40 million years old. ( Greg Jordan )

The fagus is the last living Australian species of a group of plants that once covered the wet forests of the ancient continent of Gondwana, in which land masses including Australia, Antarctica and South America were joined together.

As Australia dried out, the fagus habitat shrunk to just small pockets in western Tasmania, which are mainly in the state's national parks.

This habitat is cool and moist and rarely has fires — in fact, the species is very sensitive to fire.

Although the recent fires in Tasmania did not affect the fagus, a drying climate and increasing fire frequency could eventually affect the range of this "living fossil".