Tasmania's logged forests burn significantly more severely in a bushfire than old growth, University of Tasmania scientists have found in a study that provides the strongest evidence yet on a question with real consequences for how the state manages its forests.
The research team, led by Professor David Bowman from the University of Tasmania's Fire Centre , seized on a natural experiment when the 2019 Riveaux Road fire swept through the Huon Valley. The blaze burned across both old growth forest and younger regrowth that had been clearfelled in the 1990s, giving scientists a rare chance to compare the two side by side under identical fire conditions.
The numbers told a clear story. When the fire hit old growth, it burned itself out 32 times. In logged regrowth, more than half the area burned at high or extreme severity.
"This was a natural experiment of a kind we rarely get in fire science. The same fire, the same weather, two different forests side by side. The results were unambiguous," Professor Bowman said.
The reason lies in how the two forests are structured. Logged regrowth is made up of younger, shorter, denser trees whose canopies sit close to the ground, creating what fire scientists call ladder fuels, a continuous path that allows fire to climb from the forest floor straight into the treetops.
Old growth forests have tall canopies well above a relatively open forest floor, making it much harder for fire to spread upwards. Regrowth forests also run hotter during fire season because thinner canopies let in more sunlight, drying out the leaves and bark that ignite first.
The study found that under normal fire conditions, old growth forest helped slow and contain fires burning in neighbouring regrowth. The researchers are careful to note, however, that during extreme fire weather all forests can burn severely regardless of their age or structure.
That matters because about one fifth of Tasmania's tall wet forests are currently regrowth younger than 40 years old, much of it the result of forestry activity since the 1980s. As the climate warms, Professor Bowman said finding ways to manage fire risk in these forests was becoming increasingly urgent.
"We now have the evidence base to have an informed conversation about how we manage these forests. The science tells us there is a challenge ahead, and that is exactly why this kind of rigorous, long-term research matters."
The paper is published in the journal Environmental Research Letters .
