Extensive logging can increase bushfire severity in Victoria’s mountain ash forests, according to researchers.

The risk of crown fires – which spread rapidly through the tree canopy – is greatest in mountain ash forests that have been regrowing for about 15 years, the University of Melbourne and Australian National University study shows.

Clearfell logging allowed such young trees to become established before the devastating Black Saturday fires of 2009, according to the researchers, who examined 10,000 sites.

“We tested for several environmental factors that could influence [fire severity], including slope and aspect, but found the tree age had by far the greatest influence.”

Crown fires were most prominent in mountain ash trees aged 7-36, with the peak at about 15 years. In older trees the risk of crown fire drops because the canopy is much higher.

Clearfell logging continues in unburnt areas of mountain ash, and the researchers have encouraged the Victorian Government to consider their findings.

“Changes are urgently needed to logging policies in these forests. Any future logging must be negligible in its cumulative impact.

“Given the large area already logged and the huge impact of Black Saturday, an expanded formal reserve system would serve an important role in protecting remaining unburnt areas of mountain ash forest.”