In the late 1980s, the government of Victoria, Australia, spent heavily on an ambitious forestry experiment. Then the project was largely forgotten. Three decades later, researchers went back — and the results surprised them.
Scientists from the University of Melbourne report that, after 30 years, some heavily thinned patches of mountain ash forest had stored as much carbon in their wood as forest that was never logged, or more. They described the findings in The Conversation, based on a study published this summer in the journal Forest Science.
What the experiment was
The Silvicultural Systems Project was set up between 1987 and 1989 at two sites in Victoria’s Central Highlands, Tanjil Bren and Cabbage Tree Creek. According to the researchers, the investment was equivalent to about A$26 million in today’s Australian dollars.
The forests were mountain ash, one of the tallest tree species on Earth, regrowing after the major bushfires of 1939. The trial compared several harvesting approaches side by side, including:
- Clearfelling: cutting nearly everything in patches about 250 to 350 meters (roughly 800 to 1,150 feet) wide.
- Seed tree and gap systems: leaving a few trees, or cutting small openings.
- Thinning and retention: removing 50% to 70% of the trees and leaving the rest standing.
- Unharvested controls for comparison.
After the early years, the site was, in the researchers’ words, neglected and unfunded. They began revisiting it in 2014, measuring trees and surveying wildlife across the old treatment plots.
Why thinning can grow bigger trees
The idea behind thinning is simple. In a crowded young forest, trees compete for the same sunlight, water and nutrients. Many stay skinny. Removing some of them gives the survivors more room and resources, so they can put on girth faster.
Foresters have long used thinning to grow larger, more valuable timber. The question this experiment could finally answer, decades later, was what that trade-off looks like over a long time span: does removing trees now cost a forest carbon and habitat for good, or can the forest make it back?
What they found
Trees in the heavily thinned areas, now about 80 years old, averaged more than 1 meter (about 3.3 feet) in diameter at chest height. Some topped 1.5 meters (nearly 5 feet). The biggest thinned trees were about 20% larger in diameter than the biggest trees in the unharvested areas.
That growth matters for carbon. With fewer neighbors competing for light and water, the remaining trees grew faster. The researchers say the heavily thinned stands made back the carbon removed at harvest and then matched or exceeded the carbon stored in the wood of untouched forest.
The study itself reports that the two retention treatments regained all their lost basal area (a measure of how much ground is covered by tree trunks) within 30 years.
A surprise for wildlife
Leadbeater’s possum, a small, rare marsupial, turned up in every type of harvested plot the team surveyed — but in none of the unharvested areas.
Big trees also matter for wildlife because hollows, which many animals depend on for nesting, form almost exclusively in large trees. The study found the retention treatments and controls had substantially more predicted hollow-bearing trees than other harvesting methods.
The study did not find significant differences in overall plant species composition or diversity across the treatments.
Why US readers might care
Thinning is a hot topic in American forests, too, where it is often discussed as a tool for wildfire risk and forest health. The researchers note that similar outcomes have been seen in North American conifer forests. The Victorian trial adds rare long-term evidence to that debate, because few forestry experiments are tracked for three decades.
The catch
The authors are clear that there is “no silvicultural silver bullet.” The results apply to mountain ash forests and may not carry over to other forest types. Thinning “cannot (and should not) be done everywhere,” they write. And the carbon findings are about what is stored above ground in trees, which is only part of a forest’s total carbon picture.
The bigger lesson may be about patience. A long-term experiment that nearly vanished turned out to hold some of the most useful answers.
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