A long-running Victorian forest experiment has produced a surprising result: about 30 years after heavy thinning, the largest trees in treated stands were roughly 20% greater in diameter than those in unharvested controls, and the stands’ wood-carbon stocks had caught up with or exceeded the controls. Those are findings reported by the study’s authors for one mountain ash site—not a general verdict that thinning is best for forests.
What was the forest experiment?
The Silvicultural Systems Project (SSP) was established between 1987 and 1989 at two Victorian sites: Tanjil Bren in the Central Highlands and Cabbage Tree Creek in East Gippsland. The recent findings at the center of the story concern the Tanjil Bren site, a mountain ash forest.
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The project compared different ways of harvesting or retaining forest, alongside areas left unharvested as benchmarks. Its treatments included:
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Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →- Traditional clear-cuts: 250–350 metres wide.
- Small patch clear-cuts: 50–140 metres wide.
- Heavy thinning: removal of 50–70% of trees.
- Unharvested areas: benchmark plots with no harvest treatment.
These treatments do not represent interchangeable versions of the same intervention. Clear-cutting removes trees across an area, while thinning retains part of the stand; the unharvested areas provide a comparison rather than a treatment.
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What did researchers report when they returned?
In their account of a 2014 visit to Tanjil Bren, the authors describe a long-term result in the heavily thinned stands: the largest trees were about 20% greater in diameter than the largest trees in unharvested controls. This is a comparison of the largest trees, not a claim that every tree in a thinned stand grew larger.
The authors also report that, after roughly 30 years, the wood-carbon stocks in thinned stands had recovered the carbon removed during harvesting and accumulated additional carbon comparable to the increase in the controls. They say those stands stored as much or more carbon in their wood as the unharvested areas. The reported carbon result concerns carbon in wood; it should not be read as a complete accounting of all forest carbon or as proof that thinning is always carbon-neutral.
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The results are described by the paper’s authors—Kaitlyn L. Hammond, Craig Nitschke, Patrick Baker and Raphael Trouve—in their explanatory account, and in their 2026 Forest Science paper, “Alternative Silvicultural Systems Maintain Multiple Values in Tall Eucalypt Forest” (DOI: 10.1007/s44391-026-00076-6). The available description does not establish the plot counts, measurement methods, statistical uncertainty or exact carbon-accounting boundaries, so the findings are best treated as reported outcomes at this site.
Why this is not a simple case for thinning
The project’s early results, as summarized by the authors, showed mountain ash regenerated best on clear-cut sites, while smaller openings or retained tall canopy favored common understory plants such as acacia. The later findings about large trees and wood carbon in thinned stands measure different outcomes over a longer timescale. One treatment therefore cannot be called “best” without specifying the goal.
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Camera traps detected Leadbeater’s possums in every surveyed silvicultural treatment plot at Tanjil Bren, but not in surveyed unharvested areas. That is a site-specific survey result, not evidence that thinning generally benefits the possum or that unharvested forest is unsuitable habitat. The authors also discuss acacia understory as habitat for foraging and movement.
The authors caution that thinning cannot, and should not, be done everywhere. They suggest it may help create greater variety in tree ages and sizes, particularly in forests affected by clear-cutting or high-severity fire, while stressing that there is no single silvicultural solution for every forest.
Quick Recap
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What the findings do—and do not—show
- They show: the authors report larger maximum tree diameters and wood-carbon stocks at least comparable to unharvested controls in heavily thinned Tanjil Bren stands decades after treatment.
- They do not show: that all thinned forests outperform unharvested forests, that thinning is suitable everywhere, or that the possum camera-trap observations establish a general wildlife benefit.
- They do not settle: how these results apply to other forest types, places, thinning intensities or management goals.
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