Tanjil Bren, in Victoria’s Central Highlands, holds a stand of mountain ash where the trunks of the biggest trees now measure more than a metre across. They stand in plots that foresters cut hard in the late 1980s, removing roughly half to seven-tenths of the trees, then left alone for three decades while the experiment itself dropped out of sight.
When University of Melbourne researchers re-measured the site, the heavily thinned plots had produced the largest trees of any treatment, and had stored as much carbon in wood as the untouched forest, or close to it, depending on which write-up of the result is read. The finding was published in the journal Forest Science on July 1, 2026.
A Silvicultural Systems Project that lost its signpost
The experiment was set up between 1987 and 1989 as the Silvicultural Systems Project. It compared several ways of harvesting tall eucalypt forest: traditional clearfells 250 to 350 metres wide, small patch clearfells 50 to 140 metres wide, heavy thinnings that took out 50 to 70 percent of trees, and unharvested control areas, according to the authors’ account republished by Mirage News.
It then went quiet. Early results showed that clearfells encouraged mountain ash to regenerate, and by the early 2000s the project attracted no further attention or funding. Timber and Forestry E-News relays the authors’ description of the details: the project sign collapsed and the access road became overgrown. Monitoring resumed in 2014, with field measurements restarted in 2019, Nautilus reports, which is what made a 30-year comparison possible.
Trees more than a metre across
The size result is the centrepiece of the study. Kaitlyn Hammond, Craig Nitschke, Patrick Baker and Raphael Trouvé write that trees in the heavily thinned stands averaged well over a metre in diameter at chest height, and that some topped 1.5 metres. The largest trees in those thinned stands were 20 percent bigger in diameter than the largest in the unharvested controls, a result WoodCentral also carries.
The formal paper uses different language. Its abstract in Forest Science, titled “Alternative Silvicultural Systems Maintain Multiple Values in Tall Eucalypt Forest”, compares seven approaches: clearfelling, seed tree removal, three gap sizes between a quarter and two hectares, and two retention levels, with 30 percent and 50 percent of the overstorey left standing, plus unharvested controls. Leaving 30 to 50 percent of the overstorey standing corresponds to the 50 to 70 percent removal described in the authors’ own summary, so the hardest-cut plots are the ones the paper calls its low-retention treatments. The paper says those retention treatments produced the largest trees of any treatment, along with the greatest basal area and size inequality.
The comparison with clearfelling sharpens the point. In the authors’ description, clearfells and patch clearfells restarted the forest, and the experiment’s first published results had shown that they promoted mountain ash regeneration. Three decades later, the Forest Science results put clearfell and seed tree treatments at moderate basal areas with uniform size distributions, while the gap treatments favoured smaller-diameter trees and Acacia. The report does not isolate why the thinned plots grew the biggest trees, and none of the sources read measures light, water or soil, so the mechanism stays an open question.
Retention treatments also gave the greatest size inequality, the paper reports, meaning a wider spread between the biggest and smallest stems than in any other treatment, the structural complexity the authors say alternative systems can accelerate in tall eucalypt forest.
Carbon, and where the result stops
On carbon the two accounts differ in tone, and both are worth stating. The authors’ summary for general readers says the thinned stands had stored as much or more carbon in their wood than the unharvested control areas, having recovered all the carbon removed during harvest. The journal abstract is more cautious: carbon recovered rapidly in retention systems, regaining all harvested biomass within 30 years and approaching control levels. Tree fern biomass stayed substantially higher in the undisturbed controls.
The authors also report that Leadbeater’s possum, a threatened species, turned up in every harvested treatment plot surveyed and in none of the unharvested areas, which they link to acacia regenerating in the disturbed plots.
They are explicit about limits. The results come from mountain ash in Victoria’s Central Highlands, and the authors write that thinning cannot and should not be done everywhere. They describe the outcome as a win-win-win for timber, carbon storage and wildlife, not a case for cutting forests generally.
One comparison stays open. The 20 percent figure describes the largest trees, not the average tree, and no source gives a stand-level carbon total per hectare for each treatment, so how closely thinned plots truly matched the controls rests on the paper’s own wording, which is that carbon approached control levels.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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