A rumble, a flood of snow, and then silence—avalanches can be deadly, but they often leave behind little lasting evidence of their passage. Now, researchers are turning to tree rings to reconstruct records of past avalanches.
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 hit the area over two centuries. Understanding the frequency of potentially devastating avalanches can inform risk management and land-use planning efforts, the researchers suggest. They published their results in the journal Dendrochronologia in June.
The Pirin Mountains in southwestern Bulgaria are home to old-growth pine and fir forests. Many of the trees have stood for centuries, but some have gnarled trunks, while others bear visible scars such as broken branches.
“They have definite signs of being damaged in the past,” said Momchil Panayotov, one of the study’s authors and a dendrochronologist at the University of Forestry in Sofia, Bulgaria.
That damage, many researchers believe, was caused by avalanches. The mechanical impact of snow rushing 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 the hazard event data. 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, turned to tree rings.
Trees that experience 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 valley of Bunderitsa, home to the ski resort of Bansko. The researchers used an instrument called a seed drill to manually extract pencil-width cores from each living tree. Back in the lab, Tsvetanov dried the nuclei, mounted them in wooden holders, and then sprayed them to reveal individual rings. “It’s a very long process,” he said.
The team then compared those tree-ring recordings to sequences of rings taken from nearby undamaged trees. This cross-dating allowed researchers to determine the timing of unexpected features such as stunted growth, scars and lost rings.
“Trees are wonderful recorders of past disturbances, including geomorphic hazards like snow avalanches,” said Allyson Carroll, a dendrochronologist at California Polytechnic State University, Humboldt, who was not involved in the research.
Panayotov and Tsvetanov found evidence of avalanches that occurred as far back as the 1600s. But concluding that an avalanche happened that long ago is challenging, Panayotov said, since relatively few trees affected by an avalanche survive for centuries. Therefore, the team decided to focus on the most recent avalanches.
The researchers found that more than 20 large avalanches had occurred since the mid-1800s. Some of these events can be linked to historical records—for example, the avalanche that appears in tree rings dating back to 1963 is probably an event documented to have occurred on February 12 of that year. But other avalanches seemingly rolled downhill into oblivion. “We have no written history of these events,” Panayotov said. “We can only rely on tree rings.”
By analyzing the spatial positions of affected trees, researchers can also estimate the approximate size of each avalanche. For example, they found that the avalanche that occurred in 1969 was extremely large; reached up the banks of one of the corridors. The team also noted that avalanches hit all three corridors in 1963 and two corridors in 1931, 1987 and 1996. “There were some specific meteorological situations that favored large avalanches in those years,” Panayotov said, as strong storms in winter.
But reliably predicting that avalanches are more likely under certain climate conditions 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 research.
Panayotov and Tsvetanov hope their results will contribute to keeping winter sports enthusiasts safe. Both researchers have a special interest: Tsvetanov is a skier and Panayotov is a skier who participates in mountain rescue efforts and helps oversee avalanche education and safety at the Bansko ski resort. “I’m a regular there,” he said.
