Commercial logging of moist native forests creates conditions that increase the severity and frequency of bushfires, an international study claims.

The finding by Australian, Canadian and U.S. researchers is based on a review of previous studies and is published in the latest issue of the journal Conservation Letters.

Prof. David Lindenmayer of the Australian National University says the team focused on how industrial logging practices in native forests might change fire loads, fire frequency and susceptibility to ignition.

"The evidence from rainforests is unequivocal, the evidence from the wet forests in North America is unequivocal and the evidence is starting to build in Australia as well. When you mess with [native wet] forests, they become more flammable," he said.

The researchers found the removal of trees by logging creates canopy openings and this in turn alters microclimatic conditions, especially increased drying of understory vegetation and the forest floor, Lindenmayer said.

"Work in tropical rainforests suggests that when microclimatic conditions are altered by selective logging, the number of dry days needed to make a forest combustible is reduced," he said.

Vegetation composition shifts

In one study, uncut native forest would generally not burn after less than 30 rainless days, but selectively logged forest would burn after just six to eight days without rain.

Lindenmayer said logging also influences the fire regime by changing the density and pattern of trees, altering the spacing between tree crowns and the composition of plants.

Philip Burton of the Canadian Forest Service and University of Northern British Columbia in Prince George also contributed to the study.

According to Lindenmayer, logging in the moist eucalyptus forests of East Gippsland in southeastern Australia "has shifted the vegetation composition toward one more characteristic of drier forests that tend to be more fire prone."

He said clear-felling of moist forests in southern Australia can create more fire fuel because it leads to the development of dense stands of regrowth saplings.

Lindenmayer said these young forests are more flammable, which often lead to older, less flammable forests abutting them to burn.

Rethink buffer zones: researcher

Logging slash — the debris left by logging — can also sustain fires for longer than fuels in unlogged forests, he said, and can harbour fires when wind conditions are not suitable to spread the blaze.

Lindenmayer said the industry needs to think strategically about where they log and rethink buffer zones and how big they might need to be.

For example, he points to the Ponderosa pine forests in the U.S. that are now being thinned to restore tree distribution to its more natural state as a way of reducing fire risk.

He said these forests became denser and had a higher fire risk after the native bison, which used to roam through the forest creating large gaps between trees, were decimated.