Australian scientists started a forest experiment but forgot about it. The trees kept growing; 30 years later, researchers were shocked by the results

For years, the debate around Australia's forests was presented as a simple choice. Should forests be left untouched, or should trees be harvested for timber and local industries? But an old experiment in Victoria's mountain ash forests showed that the answer may not have been that straightforward. The experiment began in the late 1980s, after years of arguments over clear-cut logging. Scientists wanted to see what would happen if forests were managed in different ways rather than simply cutting down entire sections. The results were recorded, but the project eventually lost funding and was largely forgotten. The trees, however, carried on growing.

The project was known as the Silvicultural Systems Project, or SSP. According to Victoria's Forests Bush Fire Heritage, it followed the Victorian government's Timber Industry Strategy released in 1986.

Trials were established at Tanjil Bren in Victoria's Central Highlands and Cabbage Tree Creek in East Gippsland. The Tanjil Bren site was made up of mountain ash forest, while the other location involved mixed-species coastal forest.


Researchers tested several methods of harvesting and regeneration. These included clearfelling, where most or all trees in an area are removed, along with different-sized gaps, seed-tree systems and areas where a portion of the existing canopy was retained.

The studies were unusually broad. They looked at tree growth, soils, water quality, biodiversity, economics and operational issues, rather than concentrating on just timber production.

The original trials were intended to continue for more than 50 years. Instead, the main monitoring lasted about 12 years before departmental restructuring and funding cuts brought the project to an end.

Then the experiment was left behind

By the early 2000s, the initial results had already answered some of the questions researchers had been asking.

Mountain ash regenerated particularly well in large cleared areas. Smaller openings and areas where more of the original canopy remained often developed more acacia and other understorey vegetation.

Then something unusual happened. The experiment itself faded from attention.

The access road became overgrown and the sign marking the project eventually collapsed. The forest was no longer being treated like an active scientific experiment, even though the trees were still developing.

That changed in 2014 when researchers and students from the University of Melbourne visited Tanjil Bren to examine the long-term effects of the different treatments. Simon Murphy, who had been involved in establishing the experiment, helped draw attention to what had happened at the site.

What they saw gave them a much longer-term picture than the original researchers had been able to obtain.

The trees left after thinning had become enormous

The original findings about regeneration still largely held up three decades later. But the heavily thinned areas produced the most unexpected result.

Trees that had been left standing after much of the surrounding forest was removed had grown to an extraordinary size.

The largest trees in these areas averaged more than a metre in diameter at chest height, with some exceeding 1.5 metres. Researchers found that the largest trees in the thinned stands were about 20% wider than those in the unharvested control areas.

The explanation was fairly straightforward. With fewer competing trees around them, the remaining mountain ash had access to more space and resources and were able to put on substantial growth.

But tree size was not the only surprise.

The forest recovered its lost carbon

Researchers found that the heavily thinned stands had stored as much, or even more, carbon in their wood than the unharvested comparison areas.

Within about 30 years, the trees in the thinned forest had replaced the carbon removed during harvesting and had accumulated additional carbon comparable to the increase recorded in the unharvested areas.

The researchers also found potential benefits for wildlife.

Large trees are more likely to survive fire than smaller trees, while older and larger trees are more likely to develop hollows. Those hollows are important habitat for species including the greater glider and Leadbeater's possum.

Acacia growth also mattered because Leadbeater's possums use these trees for foraging and moving through the forest.

Camera traps used by researchers found Leadbeater's possums across all of the silvicultural treatment plots they surveyed. None were detected in the unharvested areas they examined.

Why the forgotten experiment matters

The long-term results do not provide one simple answer for how every forest should be managed. Instead, they show how different approaches can produce very different forests over time.

A 2026 study by Kaitlyn L. Hammond, Craig R. Nitschke, Raphaël Trouvé and Patrick J. Baker examined the Tanjil Bren experiment after roughly 30 years. The researchers found that the treatments produced distinct forest structures, with overstorey-retention areas developing the largest trees and greatest variation in tree sizes.

The study also suggests that alternative silvicultural systems can help create more complex forest structures than traditional clearfelling approaches.

For the scientists involved, perhaps the biggest lesson is that long-running forest experiments can become valuable in ways that are impossible to see after only a few years.

The Tanjil Bren site had effectively been left to continue its experiment on its own. Decades later, the trees provided a result that could not have been captured by the original short-term measurements.