Prescribed burns could make forests more flammable

A new review challenges the way  bushfires are controlled in Australia, finding that disturbing natural forests with activities such as logging and prescribed burning, can make them more flammable, not less.

The research, published in the journal Biological Reviews, challenges the notion that broadscale interventions are necessary to manage fire risk in forests. The researchers argue that these interventions are grounded in models that don’t account for the long-term dynamics of the forests.

“We need to be thinking about forestry and fire management in a more holistic way and look to limit actions that could be increasing flammability,” says lead researcher David Lindenmayer, an expert in forest ecology from the Australian National University, ANU.  

“We’ve understood for a long time now that logging can make bushfires worse, but it’s only in the last few years that evidence is showing that prescribed burning could be doing the same thing.”

Prescribed burns, also known as hazard reduction burns, are a mitigation tool that uses controlled fire to reduce fuel loads, to minimise the intensity and severity of bushfires. Their science is explored in first episode of the “Debunks” podcast season on bushfires: do hazard reduction burns work?

The review summarises existing evidence that a “pulse of flammability” may follow disturbances – such as fire, logging, and clearing. Flammability here refers to both the likelihood of fire and different aspects of fire severity.

They found that while disturbances, such as prescribed burns, decrease flammability in the short term by removing biomass and creating bare ground, they can drive extended periods of additional flammability as vegetation regrows from the ground level, where it can act as fuel.

Then as new biomass transitions from fuel to overstorey shelter in the long term, they found that older forests may enter a long-term period of greatly reduced flammability.

“If they’re too tall to catch fire, plants calm bushfires by slowing the wind beneath them. If disturbance kills those taller plants, replacements regrow from the ground and add to the fuel,” says co-author Philip Zylstra, a fire behaviour researcher from Curtin University in Australia.

Old forests tend to be cooler, more moist and more sheltered. Whereas young, disturbed forests are characterised by warmer, drier, and more variable microclimatic conditions.

“Fire sensitive species thrived for millions of years because so many forests naturally create these less flammable environments,” says Zylstra.

“By limiting disturbance, forests can reach an appropriate age where they can be better protected from the increased frequency and severity of Australian bushfires.”

Overhead photograph of the impacts of logging on a mountain ash forest in victoria
Logging in a mountain ash forest in Victoria, Australia. Credit: ANU

In their review Lindenmayer and Zylstra developed and tested a conceptual model that can predict areas where disturbance, such as logging or prescribed burns, is likely to cause this surge in flammability and bushfire risk.

“Very frequent burning close to homes or control lines could create defendable spaces, but large, remote and infrequent burns maintain the landscape at maximum fire risk because they undermine the natural controls that forests place on fire,” says Zylstra.

In forested areas where disturbance is found to stimulate this flammability, they recommend that management actions should consider the long-term benefits of “limiting disturbance-based management like logging or burning that creates young forests and triggers understorey development” and “protecting young forests from disturbances and assisting them to transition to an older, less-flammable state.”

They also stress the need to reinforce the fire-inhibitory properties of older, less-flammable forests through rapid fire detection and suppression.

“We need to invest heavily in rapid response remote area firefighting specialists and embrace new technologies that allow us to detect fires and suppress them faster,” says Lindenmayer.

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