Skip to main content

Lightning, tornadoes and mice: the science of bushfires

A bushfire
What governs the way a bushfire behaves?

Flickr.com: Bert Knottenbeld (CC-BY-SA-2.0)

ADVERTISEMENT

They're a familiar sight on the Australian landscape as the weather warms — cutting a path of destruction through homes, farmland, and the bush.

But bushfires have also played an important role in the cycle of renewal undergone by the Australian landscape for millennia.

How does a bushfire begin and spread, and what happens to the environment once it's moved through?

A fire starts when three elements — oxygen, a fuel source, and an ignition source — come together. But not every blaze will become a bushfire. The more dry, hot, and windy it is, the greater the risk a blaze will take off and spread, pushed forward by the wind.

Peak fire conditions occur when there's a period of significant rainfall that causes plants to grow, followed by a hot spell that dries out this fuel. This means the bushfire seasons vary around Australia.

In the north of the country, bushfires are more likely to appear in winter and spring, but in southern states like Victoria and Tasmania bushfires are more likely in summer and autumn.

How bushfires move

A diagram showing the different ways fire spreads: direct contact, spotting, air currents
Fire spreads in different ways depending on the conditions

Supplied: Australian Academy of Science/Nova

Bushfires typically move in a front — a thin line of burning grass or forest that inches forward as new material catches alight.

Radiant heat from the fire front warms the air ahead, drying out fuel, and causing volatile gases inside wood to escape – thus priming new fuel for the approaching fire.

Grass fires are generally wind driven and spread quickly — moving up to 20 kilometres per hour, according to Dr Jason Sharples from the Bushfire and Natural Hazards Cooperative Research Centre.

Bushfires uphill
A bushfire travels faster uphill than on flat or a downslope

Supplied: Australian Academy of Science/

Forest fires may burn hotter but, because there's more fuel for them, they advance slower than grass fires.

"In the most extreme fire conditions you're still only looking at a rate of spread of a couple of kilometres per hour," said Dr Sharples, adding that 3-4 kilometres per hour would be considered very fast for a forest fire.

And fires move more quickly up a hill than down one.

That's because going uphill, the flames are much closer to new fuel and spread easily, while it's the opposite going down.

'Deep flaming' and fire storms

Strong winds can sometimes blow burning embers ahead of the fire front, setting alight new patches of fuel in a process known as "spotting".

These patches of fire can then quickly grow and join up, forming one giant blaze, hundreds of metres or even kilometres wide. Such an event, known as "deep flaming", is more difficult for firefighters to control.

The heat and smoke given off from deep flaming can even create "pyrocumulonimbus" clouds that form over a bushfire.

A large cloud plume seen from high above the earth
A pyrocumulonimbus cloud over a 4,400 hectare fire in Minnesota in 2011.

Supplied: NASA Goddard Space Flight Center

Sometimes these clouds create thunderstorms, fuelling lightning strikes that can in turn start new fires.

"If you're not getting any rain out of your thunderstorm, and you're just getting lightning hitting dry fuel, it's just another source of ignition," Dr Sharples said.

On rare occasions, "fire tornadoes" can even result from pyrocumulonimbus clouds. These tornadoes form in the same way as regular tornadoes — a combination of wind shear, hot moist air, and cold dry air — only in this case, those conditions arise from the bushfire itself.

Dr Sharples said a fire tornado appeared during Canberra's 2003 bushfires, in which four people were killed and more than 400 buildings destroyed. The tornado was responsible for destroying a number of houses on Canberra's edge.

How are animals affected by bushfires?

Animals may survive bushfires if they can escape the area and recolonise it later. Another option is to hide from the fire.

A koala caught in the Victorian bushfires sits by the side of the road near Whittlesea
Koalas are very vulnerable in bushfires.

M Fillinger: IFAW

Even in intense forest fires, the shock of the heat only lasts a matter of minutes, giving animals a chance to make it through, said Professor Ross Bradstock, also from the Bushfire and Natural Hazards CRC.

"If you're a small reptile that uses burrows in the soil or even hollows in trees, you may be able to hide," he said.

"If you're a koala or large possum that's arboreal, you're very vulnerable."

In the aftermath of a fire, some animals starve to death waiting for plants to grow back, while others may become more vulnerable to predators because they have fewer places to hide.

What about plants?

Professor Bradstock said in the days and weeks following a bushfire many plants drop new seeds, while others will quickly re-sprout.

"Even though their foliage may be completely consumed, they haven't died," he said.

"Some plants have the ability to regrow from protected buds."

Others have exceptionally thick bark, which may be scorched but still protects the vulnerable vascular plant material beneath.

A close-up of the tree bark of the swamp paperbark
The thick bark of the swamp paperbark (Melaleuca rhaphiophylla) is an adaptation that helps protect it from fire

Wikimedia Commons: Geoff Derrin (CC-BY-SA-4.0)

Grasses and other plants also have extensive root systems beneath the ground that can help them regrow if they've been burnt off.

These "resilience and persistence mechanisms" are present in most Australian flora, Professor Bradstock said.

"Most of our environments in Australia have been subjected to fire for not just millennia, but timescales well beyond that. So even in rainforests on the mainland, you'll find plants can re-sprout after the occasional fire," he said.

The path to renewal

For many plants, fire is as much a chance at renewal and regrowth as it is a destructive event. In fact, Australian plants have evolved with bushfire to such an extent that some of them can't survive without it.

"Different species have different life cycles, and some of their aspects of reproduction and regeneration may be linked to fire," Professor Bradstock said.

An example of such a plant is the acacia, which requires the heat of a bushfire to crack its seed pods so it can germinate.

Some animals, too, prefer a landscape that's been scorched by fire – like the endangered Hastings river mouse.

A Hastings River mouse
A Hastings River mouse

Flickr.com: Doug Becker (CC-BY-SA.2.0)

"It likes open environments," Professor Bradstock said.

"Once the vegetation has regrown after about five to 10 years it tends to drop out of a system."

In this way, bushfires prompt a boom and bust cycle in animals as vegetation grows and is removed, and various species flourish with the changing ecosystem.

Professor Bradstock said the total time it took for an ecosystem to return to "normality" depended on rainfall and climate.

For example, in the south of Australia, where there is moderate rainfall, there would be substantial regrowth within a year or two.

"If you were looking from a satellite, measuring greenness, you're pretty much back to pre-fire conditions," Professor Bradstock said.

A full recovery, however, usually takes five to 10 years.

Plants that don't have mechanisms to recover from bushfire can be wiped out from particular areas after a big burn. They then rely on seed dispersal from birds or other means to bring them back to the habitat, said Professor Bradstock.

But, he added, most plants without recovery mechanisms are only found in parts of Australia where fire is less common, such as wet and alpine environments in Tasmania.

Want more science from across the ABC?