A rumble, a deluge of snow, and then silence — avalanches can be deadly, but they often leave little lasting evidence of their passage. Now, researchers have turned to tree rings to reconstruct records of past avalanches.
By analyzing the wood of hundreds of living pine trees near a ski resort in Bulgaria, a team of scientists found evidence of dozens of large avalanches that struck the area over two centuries. Understanding the frequency of potentially destructive avalanches could inform risk management and land use planning, the researchers suggest. They published their results in June in the journal Dendrochronologia.
The Pirin Mountains in southwestern Bulgaria are home to primeval forests of pine and spruce. Many of the trees have been standing for centuries, but some have steeply sloping trunks, while others bear striking scars such as broken branches.
“They show certain signs of having been damaged in the past,” said Momchil Panayotov, one of the study’s authors and a dendrochronologist at the University of Forestry in Sofia, Bulgaria.
Many researchers believe that the damage was caused by avalanches. The mechanical impact of snow rushing down the slope can severely damage a large tree, even uprooting it. But visible damage does not reveal exactly when an avalanche occurred, which is important for reconstructing data about the dangerous events. To determine when and where avalanches occurred in the Pirin Mountains, Panayotov and Nickolay Tsvetanov, also an author of the study and a dendrochronologist at the University of Forestry, focused on tree rings.
Trees that experience an avalanche develop clear signs in their rings, Panayotov said: “The 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 Bunderitsa Valley, home to the Bansko ski resort. The researchers used a tool called an increment borer 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 down to reveal individual rings. “It’s a very long process,” he said.
The team then compared that tree-ring data with sets of rings obtained from nearby undamaged trees. That cross-dating allowed the researchers to determine the timing of unexpected features such as suppressed growth, scarring and missing rings.
“Trees are great recorders of past disturbances, including geomorphic hazards like snow avalanches,” said Allyson Carroll, a dendrochronologist at California State Polytechnic University in Humboldt, who was not involved in the study.
Panayotov and Tsvetanov found evidence of avalanches as far back as the 17th century. But it’s hard to definitively conclude that an avalanche occurred that long ago, Panayotov said, because relatively few avalanche-stricken trees survive for centuries. So the team chose to focus on more recent avalanches.
The researchers deduced that more than 20 large avalanches had occurred since the mid-1800s. Some of those events could be linked to historical records; for example, the avalanche that appears in tree rings dated to 1963 is likely a documented event that occurred on Feb. 12 of that year. But other avalanches seemingly rolled downhill into oblivion. “We don’t have a written history of these events,” Panayotov said. “We can only rely on tree rings.”
By analyzing the spatial positions of the affected trees, the researchers were also able to 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 on one of the corridors’ banks. The team also noted that avalanches occurred 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 severe winter storms.
However, to reliably predict that avalanches are more likely under certain climatic conditions, more data are needed.
“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 a vested interest: Tsvetanov is a snowboarder, and Panayotov is a skier who participates in mountain rescues and helps monitor avalanche education and safety at the Bansko ski resort. “I go there regularly,” he said.