Wed Mar 22

New research reveals how forests reduce their own bushfire risk, if they're left alone

Written by Philip Zylstra, Adjunct Associate Professor at Curtin University, Research Associate at University of New South Wales, Curtin University

Fire management in Australia is approaching crisis point. Seasons such as the Black Summer three years ago showed how our best efforts in fire-fighting and prescribed burning are insignificant in the face of a changing climate.

But what if forests have their own tools to manage bushfire risk, and we could tap into them?

We know long-unburnt mountain forests in south-east Australia are far less fire-prone ^[1] than more recently burnt areas. And forests in south-west Australia have the lowest fire risk ^[2] when they’ve not been subjected to prescribed burning.

Our study ^[3] just published set out to understand why this occurs, by modelling fire behaviour in iconic red tingle forests of south-west Australia. Our findings offer a clear set of tools for living with fire, even in a warming climate.

The findings offer a tool for living with fire in Australia. Ralph Hurst-Meyers

The ‘fuel load’ dilemma

Most bushfires ^[4] in the south-west forests of Western Australia occur in areas burned specifically to reduce fuel loads.

Current fire management by government agencies focuses on broadscale burning to reduce fuel load. The practice is also known as prescribed, planned, controlled or hazard-reduction burning. It aims to ^[5] reduce the intensity of future fires at a location by burning leaf litter and other fine surface matter.

Fuel-load reduction is premised on the idea that the amount of fuel in a forest determines how flammable it is. The idea comes from early attempts at fire behaviour modelling imported from the United States ^[6] in the 1960s.

But more recent work ^[7] has shown prescribed burning can lead to catastrophic outcomes. This includes the near-destruction ^[8] of critically endangered wildlife populations and the destruction of homes when prescribed burns ^[9] escape.

So what can be done? Our study ^[10] on the iconic red tingle (Eucalyptus jacksonii) deals with this dilemma.

Prescribed burns aim to reduce the intensity of future fires. WA Department of Parks and Wildlife

Getting to know the red tingle

Red tingles are among the tallest eucalypts, growing up to 70 metres. They grow only in a 6,000-hectare stretch along the high-rainfall south coast of Western Australia.

Red tingles can live for more than 400 years. They regenerate well after fire – even an intense fire – and form naturally mixed aged forests dominated ^[12] by large old trees.

But red tingles are sensitive to frequent fires, no matter how mild. Flames can enter the tree and hollow it out, eventually causing its collapse.

a large tree, left, and a collapsed tree after fire, right

Left: A red tingle tree. Right, a collapsed red tingle after repeated fire. M Howe

The understorey of red tingle forest consists of tall, long-lived shrubs that germinate prolifically after fire. These understoreys thin and open as they age.

The goal of our research was to understand how these changes in red tingle forests affect fire behaviour. To do this, we used an advanced fire modelling tool ^[13] developed by the lead author of this article, Phillip Zylstra, and his colleagues.

The tool doesn’t use a simple number to characterise a forest’s fuel load. Instead, it uses hundreds of thousands of calculations to determine which plants will ignite – and importantly, which plants won’t ignite but will instead calm the flames.

So what did we find? As the understorey of red tingle forest ages and thins, the lower branches of taller plants “self-prune” – in other words, they shed dead leaves and twigs.

When this litter is on the ground, it begins to decay and poses a lower fire risk ^[14] than if it were still suspended.

The lower branches of taller plants, once self-pruned, are then less likely to ignite as fuel. Instead, they act as “overstorey shelter” that reduces wind speed and fire severity. In this way, mature forests control fire rather than drive it.

Our study showed that, due to this calming effect, fires in mature red tingle forests could be extinguished by firefighters most of the time.

By contrast, our study showed that prescribed burning in red tingle forests creates dense regrowth, which burns severely during bushfires. In such cases, our study showed firefighters are often unable to extinguish the flames and must resort to backburning - a risky ^[15] fire suppression technique.

left to right, a burnt tree, a regrowth forest and a mature forest

Left to right, a red tingle forest after a prescribed burn, the resulting regrowth, and mature forest. M Howe; P Zylstra

Making peace with the bush

Contemporary fire management approaches, and their dependence upon broadscale prescribed burns, contrast starkly with the approach of Australia’s First Nations people.

Menang and Goreng Traditional Owners of red tingle and surrounding forests say fire should be used in specific locations, burning only what is needed in small, strategically placed patches ^[17]. Wadandi Pibulmun Yunungjarlu Elder Wayne Webb advised our team that Traditional Owners deliberately excluded fire from tall red tingle forests.

In our contemporary context, we can co-operate with the landscape - balancing the effects of climate change by using specialist fire-fighting skills ^[18] and technological advances ^[19] to reinforce safe forest havens.

The fire risk in WA’s south-western forests, and many other eucalypt forests ^[20], is so much lower if they’re left unburnt and allowed to mature. Our analysis of red tingle forests helps explain why.

One thing is clear: if we still want forests in our flammable country, we must stop burning their defences away.