More than half a century after bulldozers rolled through a Far North Queensland rainforest, researchers from the University of the Sunshine Coast’s Forest Research Institute (FRI) have returned to discover the impact, and what never grew back. While rainforest soils and much of the vegetation have recovered remarkably well, the largest trees are still missing 56 years later, evidence that the way forests are harvested can leave ecological legacies lasting generations. 

By revisiting logging tracks first created in 1968, the team has uncovered evidence that is reshaping how scientists think about sustainable tropical forestry.

Semi-annually, researchers from the FRI’s Tropical Forests and People Research Centre head into the Wet Tropics of Far North Queensland to revisit permanent research plots first established decades ago.

The 16-ha study area was selectively logged from July 1968 to May 1969, approximately 56 years prior to this study, though some of the long-term rainforest plots were established 80 years ago by the Queensland Department of Forestry to study the effects of different forestry and silviculture practices on tree growth. 

With records that stretch back to the 1930s, this research is among Australia's longest-running tropical forest studies.

This year's expedition to Far North Queensland continued that legacy, combining decades of field observations with new technology to answer questions about forest recovery, carbon storage and biodiversity that couldn't be answered anywhere else.

"It's one of the most exceptional forest ecosystems on the planet," says Research Fellow Dr Alexandra Catling.

"We can go to a particular site, find an individual tree that was measured 80 years ago, and see how all of the different dynamics that have taken place in that forest since then have ultimately affected its growth."

Read the published article in Elsevier: 

Logging legacies in the Australian tropics: 56-years of skid trail recovery

This year's fieldwork builds on a landmark study just published in Elsevier by the team, who revisited the remains of logging operations carried out in the late 1960s, where bulldozers harvested a heavy volume of 86 cubic metres across 18 trees per hectare. 

Using original Queensland Department of Forestry maps and harvest records, the team relocated skid trails – the narrow paths bulldozers used to drag logs from the forest – 56 years after they were last used. 

What they found challenges common assumptions about tropical logging.

The rainforest had recovered remarkably well. Soils had regained their structure, dense understorey vegetation had returned and the canopy had largely closed. 

But one important legacy of logging remained. Large trees were still missing from the old skid trails more than half a century later. 

"The most prominent logging legacy was the absence of large trees," says Dr Catling.

The findings suggest that while tropical forests can recover from carefully managed selective logging, the infrastructure used to harvest timber leaves a footprint that can endure for generations.

Importantly, those skid trails occupied only about 3.9 per cent of the harvested forest, highlighting how thoughtful planning can dramatically reduce long-term impacts. 

This year's field season expanded that work by examining another piece of the puzzle: how damaged trees influence forest carbon storage.

Current methods for estimating forest carbon generally assume trees are solid. But many older trees develop hollows after being damaged by storms, insects or logging, meaning they may contain less wood, and therefore less carbon, than standard calculations assume.

Researchers revisited the unique damage experiment established in 1967 to track the fate of individual trees damaged during logging to ask, did they survive? Did they develop hollows? And how should those hollows be accounted for in carbon estimates?

The answers matter not only for improving carbon accounting but also for understanding wildlife habitat, as tree hollows provide homes for countless birds and mammals.

Wood scientist Dr Vilius Gendvilas was on the team that travelled to Far North Queensland, to trial ‘resi’ technology, that can measure wood density inside standing trees without cutting them down. ‘Resi,’ which stands for ‘resistance drilling,’ is a sophisticated power drill instrument which can quickly and non-destructively collect data from plantation grown hard- and softwoods trees.

"It's normally difficult and time-consuming to measure wood density," he says, "but with this machine, I can do it in about one minute."

The rapid, non-destructive technique allows researchers to estimate internal wood density and identify hollows while leaving the forest intact.

For Professor Robin Chazdon, who has worked with the Forest Research Institute since it was established, the long-term nature of the research is what makes it so valuable.

Research in Far North Queensland has continued since the 1930s, creating a unique and extraordinary record of forest change.

"One of our main objectives is to understand whether these forests are still active sinks for carbon, are they still actively storing more carbon than they're emitting, or is that beginning to change?" Robin says. 

Answering questions like that requires patience and time, so each year visiting this site adds another layer to a dataset that spans generations of researchers. 

Integrating different technologies, such as LiDAR (which stands for Light Detection and Ranging), was a unique part of this research. Dr Catling says not many other studies have integrated technologies including camera trapping for vertebrates, pit fall trapping for crickets and beetles, and LiDAR scans of vegetation density combined with traditional hand measures.

"There was a lot of feral pig damage which is another of point of conversation for how forests are managed and how artificial trails influence animal movement," she says. 

As one of Australia's longest-running forest experiments, one which reveals much about how rainforests recover, and how careful, long-term science can guide the future of tropical forest management around the world, the semi-annual expedition to the Wet Tropics of Far North Queensland is also a fantastic classroom, giving research students the opportunity to learn. 

PhD student Lief Erikson Gamalo, who visited the region for the first time, says the trip was both professionally and personally transformative, demonstrating "a lot of techniques" that can be applied in his own work.

This research in the Wet Tropics of Far North Queensland shows that rainforests are incredibly resilient, but recovery is not the same as returning to how they were before. More than 50 years after logging stopped, the forest looked healthy again, with vegetation and soils largely restored, but the biggest trees, which store large amounts of carbon and provide important habitat for wildlife, were still missing.

The study shows that even small areas disturbed by logging, such as the tracks used to remove timber, can leave a lasting impact for generations. The good news is that these impacts can be reduced through careful planning, proving that sustainable forestry is possible when we understand how forests respond over the long term.

We acknowledge the Traditional Owners of the areas we worked in, Mbabaram Country and Yidinji Country, and thank our visiting colleagues who were a significant part of this trip:

• Prof Jerry Vanclay - Southern Cross University

• Dr Georgina Falster - Adelaide University

• Prof Stewart Fallon - The Australian National University (ANU)

• Dr Matthew Brookhouse - ANU

• Sabrina Dao Roth (PhD student) - ANU