A roar, a deluge of snow, and then silence—avalanches can be deadly, but they often leave behind little lasting evidence of their passage. Now researchers have turned to tree rings to reconstruct records of avalanches in the past.
Analyzing the wood of hundreds of living pine trees near a ski resort in Bulgaria, a team of scientists has found evidence of dozens of large avalanches that have hit the area over two centuries. Understanding the frequency of potentially destructive avalanches can inform risk management efforts and land-use planning, the researchers suggest. They published their results in the journal Dendrochronologia in June.
The Pirin Mountains in southwestern Bulgaria are home to centuries-old pine and fir forests. Many of the trees have stood for centuries, but some have steeply sloping trunks, while others bear visible scars such as broken branches.
“They have signs that they were damaged in the past,” said Momchil Panayotov, one of the study’s authors and a dendrochronologist at the Forestry University in Sofia, Bulgaria.
According to many researchers, this damage was caused by avalanches. The mechanical impact of the snow coming down the slope can seriously damage a large tree and even uproot it completely. But visible damage does not reveal when exactly an avalanche occurred, which is important for reconstructing records of hazardous events. To determine when and where avalanches fell in the Pirin Mountains, Panayotov and Nikolay Tsvetanov, also an author of the study and a dendrochronologist at the Forestry University, turn to tree rings.
Trees that have experienced an avalanche develop telltale signs in their rings, Panayotov said: “Survivors keep the record.”
In 2020 and 2021, Panayotov, Tsvetanov and several students collected wood samples from hundreds of pine trees in the Pirin Mountains that showed signs of damage. The team focused on three known avalanche corridors in the Banderitsa Valley, where the ski resort of Bansko is located. The researchers used a tool called an incremental probe to manually extract pencil-width cores from each living tree. Back in the lab, Tsvetanov dried the cores, mounted them in wooden holders, and then sanded them to reveal individual rings. “It’s a very long process,” he said.
The team then compared these tree-ring records to sequences of rings obtained from nearby undamaged trees. This cross-dating allowed the researchers to determine the timing of unexpected features such as stunted growth, scars and missing rings.
“Trees are amazing recorders of past disturbances, including geomorphic hazards like snow avalanches,” said Alison Carroll, a dendrochronologist at California State Polytechnic University, Humboldt, who was not involved in the research.
Panayotov and Tsvetanov found evidence of avalanches that happened as far back as 1600. But definitively concluding that an avalanche occurred that long ago is challenging, Panayotov said, because relatively few trees affected by an avalanche survive for centuries. That’s why the team chose to focus on more recent avalanches.
The researchers concluded that more than 20 large avalanches have occurred since the mid-1800s. Some of these events can be linked to historical records—for example, the 1963 tree-ring avalanche is likely a documented event. happened on February 12 of the same year. But other avalanches seemed to roll down into oblivion. “We have no written histories of these events,” Panayotov said. “We can only rely on tree rings.”
By analyzing the spatial positions of affected trees, the researchers could also estimate the approximate size of each avalanche. For example, they found that the avalanche that occurred in 1969 was unusually large; it reached high up to one of the banks of the corridors. The team also noted that avalanches fell in all three corridors in 1963 and in two corridors in 1931, 1987 and 1996. “There were some specific meteorological situations that favored large avalanches in those years,” Panayotov said, such as strong storms in winter.
But reliably predicting that avalanches are more likely in certain climates will require more data.
“You need time series that go further back in time to get good correlations between climate conditions and avalanches,” said Markus Stoffel, an environmental scientist at the University of Geneva who was not involved in the study.
Panayotov and Tsvetanov hope their results will contribute to the safety of winter sports enthusiasts. Both researchers have personal interests: Tsvetanov is a snowboarder and Panayotov is a skier who participates in mountain rescue operations and helps monitor avalanche training and safety at the Bansko ski resort. “I’m a regular there,” he said.
