The review found climate change was the key contributor to increased bushfires in Australia, with rising temperatures and more intense dry spells priming the forests for burning. However, disturbance of forests, such as from logging, exacerbated the impacts.
Extraction of timber even by thinning typically leaves behind so-called slash residues of branches and other debris, accounting for as much as 80 per cent of the tree’s biomass. That dead debris serves as a “flash fuel” that increases the forest’s flammability, said Chris Taylor, a research fellow at the ANU’s Fenner School, and another of the review’s authors.
The fragmentation of the canopy also results in warmer and drier conditions for the remaining vegetation as litter and the soil desiccate, contributing increased flammability, the paper noted.
“In Australian forests, for the same ambient values of temperature and humidity, fuel moisture contents may be 2 to 3 per cent lower in full sun than in shade,” it said.
After a disturbance whether from logging or bushfires, the forests in much of south-eastern Australia typically experience “an explosion of growth”, Dr Taylor said. Thousands of young trees can cluster per hectare of logged forest.
The research also indicates that from about eight years after logging until about 30 years later – when the forest begins to mature and larger gaps appear between tall trees – the forests are be more susceptible to severe bushfires.
Over time, the so-called ladder fuels of growing trees tend to break up, making it harder for flames to reach the canopy. Moss and other moisture trapping vegetation also build up at ground level, inhibiting fire movement.
“We should be creating and allowing forests to grow … as an important fire risk management strategy,” Dr Taylor said.
Another threat to forests is the increased frequency of bushfires. Species such as mountain ash that need fires to propagate their seed only reach maturity after about 20 years and such stands will be wiped out if the return period of fires is shorter than that, Dr Taylor said.
Comment was sought from NSW’s Forestry Minister John Barilaro, and also NSW’s state-owned Forestry Corp. A spokesman for the Institute of Foresters forwarded an opinion piece written by Kevin Tolhurst from Melbourne University and Jerry Vanclay from the Southern Cross University.
The article rejected claims timber harvesting made forests more flammable, saying that a 2016 study had found “no discernible impact of timber harvesting on fire severity at the landscape scale”.
“Scientists suggesting that timber harvesting leads to more severe fires are basing their conclusions on selective, local-scale observations where the only variable being considered is the time since harvesting,” they said.
Griffith’s Dr Norman, though, said their project was based on research “done by hundreds of scientists and peer-reviewed by hundreds more”.
Peter Hannam writes on environment issues for The Sydney Morning Herald and The Age.