An almost forgotten forestry experiment has revealed surprising clues about how mountain ash forests respond to different kinds of disturbance.
Credit: Center for Forest Tree Technology
A decades-old experiment in Victoria’s mountain ash forests is challenging assumptions about the trade-offs between timber production and conservation.
Should a forest remain untouched, or should some trees be harvested to sustain nearby communities and economies? Australians have debated these competing approaches for decades, often treating conservation and timber production as mutually exclusive choices.
Research into a largely forgotten experiment in Victoria’s mountain ash forests suggests the answer is more complicated.
Clearfell logging, which removes every tree from a section of forest, has faced sustained criticism since the 1970s. Conservationists argued that the practice destroys habitat for rare wildlife and diminishes ecological value. Foresters countered that mountain ash forests naturally regenerate after catastrophic bushfires, making clearfelling a comparable disturbance.
The dispute became deeply polarized, but the central scientific questions persisted. Researchers still lacked clear evidence showing how clearfelling affected these forests over time and whether less intensive harvesting methods produced better outcomes.
Aerial views of the Tanjil Bren Silvicultural Systems Project.
L: Small cleared patches varying in size (50-140 metres wide) and shape (square vs. rectangular)
R: Heavy thinning.
Credit: Center for Forest Tree Technology
A landmark experiment
During a major restructuring of Victoria’s timber industry in the mid 1980s, the state government invested the equivalent of A$26 million today to investigate alternatives to clearfelling. The project focused on silviculture, the science of establishing, growing, and managing forests.
Between 1987 and 1989, researchers established the Silvicultural Systems Project at Tanjil Bren in Victoria’s Central Highlands and Cabbage Tree Creek in East Gippsland.
The landmark experiment compared forest regeneration under a broad range of management approaches. Researchers tested conventional clearfells (250–350 meters wide), smaller patch clearfells (50–140 m wide), and heavy thinning that removed 50–70% of the trees. Large areas of unharvested forest remained untouched to provide benchmarks for comparison.
R: the sign rescued from the forest floor in 2025.
Credit: Center for Forest Tree Technology/Tom Fairman
The forest at Tanjil Bren was dense mountain ash regrowth from the 1939 fires. Hundreds of survey plots were established and thousands of seedlings were monitored. By the early 2000s, the results were clear: mountain ash regenerated best on clearfell sites. Where tall canopy trees were retained or the cleared patches were small, common understory species such as acacia dominated.
The findings were published. The access road grew over. The project sign collapsed. The experiment was forgotten. The debates continued.
But the trees kept growing.
The forest at Tanjil Bren was dense mountain ash regrowth from the 1939 fires. Hundreds of survey plots were established and thousands of seedlings were monitored. By the early 2000s, the results were clear: mountain ash regenerated best on clearfell sites. Where tall canopy trees were retained or the cleared patches were small, common understory species such as acacia dominated.
The findings were published. The access road grew over. The project sign collapsed. The experiment was forgotten. The debates continued.
But the trees kept growing.
Thinning produced an unexpected outcome
In 2014, we visited the Tanjil Bren site as part of a University of Melbourne subject on silviculture in native forests. A colleague, Simon Murphy, had been involved in its establishment and suggested our students see the long-term outcomes of different silvicultural systems side-by-side. What we found was astounding.
The original results still held: 30 years later, the mountain ash in the large cleared patches and clearfelled areas was vigorous and abundant. But it was the heavily thinned parts of the forest that really caught our attention.
The trees left behind after thinning were enormous. While they were only 80 years old, the largest averaged well over a meter in diameter at chest height, and some were more than 1.5 m. Thinning had given these trees more space and resources to grow, and they had responded dramatically. The largest trees in the thinned stands were 20% bigger in diameter than those in the unharvested control areas.
Credit: Jeremy Johnson
More remarkable still: the thinned stands of trees had stored as much or more carbon in their wood than the unharvested control areas. In just 30 years, the heavily thinned forest had not only recovered all the carbon removed during harvesting, but had also gained enough additional carbon to equal the increase seen in the unharvested controls.
These findings have profound implications. Bigger trees are more resistant to fires, so when the next fire occurs, they are more likely to survive. Bigger trees are also more likely to form hollows – a critical and declining habitat for rare species such as the greater glider and Leadbeater’s possum.
The acacia trees that grew in most of the silvicultural treatments are also vital for Leadbeater’s possums, which use it to forage and move around the forest. Camera traps across the sites confirmed this: Leadbeater’s possums were found in every silvicultural treatment plot we surveyed – but none of the unharvested areas.
More remarkable still: the thinned stands of trees had stored as much or more carbon in their wood than the unharvested control areas. In just 30 years, the heavily thinned forest had not only recovered all the carbon removed during harvesting, but had also gained enough additional carbon to equal the increase seen in the unharvested controls.
These findings have profound implications. Bigger trees are more resistant to fires, so when the next fire occurs, they are more likely to survive. Bigger trees are also more likely to form hollows – a critical and declining habitat for rare species such as the greater glider and Leadbeater’s possum.
The acacia trees that grew in most of the silvicultural treatments are also vital for Leadbeater’s possums, which use it to forage and move around the forest. Camera traps across the sites confirmed this: Leadbeater’s possums were found in every silvicultural treatment plot we surveyed – but none of the unharvested areas.
Forest management needs more than one answer
The Tanjil Bren SSP offers three important lessons.
First, long-term silvicultural experiments are irreplaceable. Australia’s forests contain many forgotten experiments that could transform our understanding of how forests respond to human intervention. But all too often, they end up neglected, unfunded, and forgotten. We should find them before it is too late.
Second, forest thinning is not a trade-off. Our findings show this for mountain ash, and other studies in North America have shown this for conifer forests. It can deliver wood for local industry, absorb carbon and improve habitat for wildlife. It is a win-win-win outcome that challenges the false dichotomy of conservation versus production that has dominated public debate for decades.
While thinning cannot (and should not) be done everywhere, it may be a useful tool to create more variety in tree ages and sizes, particularly in forests impacted by clearfelling or high-severity fire.
Third, and perhaps most importantly: there is no silvicultural silver bullet for managing forests. The long-forgotten experiment at Tanjil Bren has taught us that there are options available for managing our forests beyond clearfelling that can contribute to the societal demand for wood, maintain carbon stocks, and conserve biodiversity.
The Life of Earth
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