Tree species
What’s the best way to manage a forest? Australians have argued about this for decades. The debate is often framed as binary: do we protect forests entirely, or harvest trees to support local economies and communities?
The reality is more interesting, as our new research on a long-forgotten experiment in Victoria’s mountain ash forests reveals.
Since the 1970s, scientists and the public have questioned the use of clearfell logging – where all trees in a block of forest are cut down. Conservationists argued it destroys habitat for rare species and reduces the forests’ ecological value. Foresters said mountain ash is adapted to catastrophic bushfires, and clearfelling is the forestry equivalent.
The argument was heated and the polarization unhelpful. But the underlying questions about the impacts of clearfelling, and how they compared to less intensive forestry options, remained unresolved.
A landmark experiment
In the mid-1980s, the Victorian government launched a major restructure of the timber industry, and invested today’s equivalent of A$26 million to explore alternatives to clearfelling. This would use silviculture, the branch of forestry that deals with establishing, growing and tending forests.
The Silvicultural Systems Project was established at two sites between 1987 and 1989: Tanjil Bren in Victoria’s Central Highlands, and Cabbage Tree Creek in East Gippsland.
It was a landmark experiment, comparing forest regeneration across the full range of silvicultural approaches. These included traditional clearfells (250–350 meters wide), small patch clearfells (50–140 meters wide), and heavy thinnings, where 50–70% of trees were cut. Extensive areas of unharvested forest were kept as benchmarks for comparison.
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.
Astounding results
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 1.5m. 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.
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.
Lessons for the future
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.