What is the evidence behind hazard-reduction burning?

This is a common response after destructive fires.

After the Black Saturday fires, and in reference to the lack of hazard-reduction burning, Miranda Devine wrote "it is not arsonists who should be hanging from lamp-posts but greenies."

The same debate erupts after large fires in the United States.

As a scientist who has dedicated considerable time - often at taxpayers expense - assembling evidence on house losses during bushfires, it is frustrating that more hazard-reduction burning is touted as the simple solution to our bushfire crises.

Hazard-reduction burning or prescribed burning is the deliberate burning of forest outside the fire season to reduce fine fuels (dead leaves, loose bark and small branches). Bushfires are more readily controlled when they contain less of this flammable material.

Most Australian forests have evolved with fire and resprout quickly, so hazard reduction burning must be undertaken at intervals ranging from every year to every six years to remain effective.

Hazard-reduction burning is often confused with back-burning. Back-burns are ignited ahead of an active fire front to stop the progress of a bushfire.

Hazard-reduction burning is not always an effective way to protect houses during bushfires for several reasons.

The majority of houses are destroyed during bushfires burning in extremely hot, dry and windy weather. In such conditions hazard reduction burning may not reduce the intensity of a fire sufficiently to permit effective suppression. For example, most houses in Marysville were destroyed during Black Saturday despite a ring of hazard reduction burning.

Hazard reduction burning is more effective if undertaken close to houses, but this is often impractical due to infrastructure such as fences and utilities, livestock, the health hazard from smoke and the risk to property and lives should a hazard-reduction burn escape.

Even where hazard reduction burning has been undertaken, our research indicates that houses with a high cover of trees and shrubs within 30-40 meters remain at a high risk of loss during bushfires.

Hazard reduction burning cannot be undertaken in all forests. Some forests are typically too damp to burn and, when dry enough, cannot be burnt safely.

Many bushfires burn through farmland dominated by pasture or crops as occurred in 2015 near Gawler in South Australia where 91 houses were destroyed. Hazard reduction burning is not feasible in these landscapes.

In every bad fire season, fire management agencies come under attack from media and politicians for failing to meet area targets for hazard reduction burning. This year is no exception.

A focus on area targets for hazard reduction burning is counterproductive for several reasons.

Bad fire seasons are often seasons when the window for safe hazard reduction burning is small, and therefore little hazard reduction burning can be safely undertaken.

Pressuring our fire management agencies to do more hazard reduction in dry years will increase the likelihood of escapes. Thirty two houses were destroyed when a hazard reduction burn escaped near Margaret River in 2011.

A focus on area targets pushes more hazard reduction burning into remote areas where they are easier to execute. A focus on tripling the area of hazard reduction burning in Victoria after Black Saturday had this outcome.

While this might meet an area target, our research indicates that the proximity of hazard reduction burning to houses is much more important than the total amount of hazard reduction burning in the landscape.

Good policy should focus on where we are doing hazard-reduction burning rather than how much we are doing.

Indigenous Australians used fire extensively as a deliberate tool to manage native vegetation and some parts of Australia continue to be managed with fire using traditional knowledge.

However, Indigenous Australians did not have the same constraints on burning as exist in heavily developed and populated landscapes that occur across much of Australia today.

It is also important to acknowledge that Indigenous Australians did not maintain all of Australia's forests as "park-like" with low fuel hazard.

For example, in 1820 Oxley noted near Port Macquarie where some fires have been burning that: it was a continued ascending and descending of the most frightful precipices, so covered with trees and shrubs and creeping vines, that we frequently were obliged to cut our way through.

Myself and other scientists have recorded even-aged stands of eucalypts that predate European settlement, indicating that some forests experienced intense, stand-replacing bushfires prior to the arrival of Europeans. Thus, intense bushfires are likely to have been a feature of Australia's environment even with Indigenous burning.

Given hazard reduction burning loses effectiveness during severe weather conditions when most houses are lost, is impractical to undertake near all houses, and is not by itself the most effective way to protect houses during bushfires, there are other strategies that must be adopted in addition to hazard-reduction burning.

Our research indicates that houses are more likely to be saved during a bushfire where trees and shrubs have been thinned or removed to a distance of 30-40 meters, the grass is mown and the garden kept green.

This creates a zone of defensible space that increases the likelihood that houses can be successfully defended by adequately prepared persons during a bushfire.

Appropriate house design, construction materials and maintenance also increase the likelihood that houses survive bushfires.

However, all of these strategies become less effective as weather conditions become more extreme, so adequate insurance and early evacuation should remain part of the bushfire plan for everyone living in a bushfire-prone landscape.