A forgotten 30-year forest experiment just delivered a huge surprise

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.

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.

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.