Ancient tree rings hold clues to volcanic cooling events

In a frigid region of northern Norway, a strange phenomenon in tree rings is helping scientists reveal the climate impact of a distant volcanic eruption more than a century ago. Researchers found that unusually cold summers left distinctive blue rings in trees and shrubs, providing a natural archive of historical temperature drops.

The study, published in Frontiers in Plant Science, reveals how these blue rings form when summer temperatures are too low for normal plant growth, preventing cell walls from developing properly. This unique color becomes visible when wood samples are treated with special dyes in the laboratory.

In the Iskolas Mountains in northern Norway, scientists examined core samples of 25 Scots pine trees and 54 common juniper shrubs and found evidence of two particularly severe cold snaps in 1902 and 1877. The timing coincides with the timing of major volcanic eruptions—Mont Pelli in Martinique and Cotopaxi in Ecuador—suggesting a possible link between volcanic activity and regional climate effects.

“The blue rings look like unfinished growth rings and are related to cold conditions during the growing season,” explained Dr. Agata Buchwal of Mickiewicz University in Poland, who led the study. The findings show that trees and shrubs respond differently to these cold events, with pines showing greater sensitivity.

Careful analysis by the research team found that blue growth rings are relatively rare, appearing in only 2.1% of pine tree rings and 1.3% of juniper shrub growth rings. However, during the extremely cold summer of 1902, blue rings were extremely common, appearing in 96% of pine trees and 68% of juniper shrubs studied.

These cold-induced formations can have lasting effects on affected trees. Dr. Pawel Matulewski, the study’s second author, said, “For pine trees in northern regions, blue rings have the potential to weaken the trees, making them more susceptible to mechanical damage or disease.”

The 1902 event coincided with the coldest June on record in the region, significantly shortening the growing season. While the trees successfully formed normal early-season growth rings, the late-season wood showed clear signs of disruption in development, resulting in a distinct blue color when stained.

The research process itself proved challenging, requiring expertise beyond traditional tree-ring analysis. “Blue ring studies require longer protocols than classic dendrochronological studies,” noted Dr. Buchwal, who emphasized the importance of skilled laboratory technicians in the research.

The timing of the cold event appears to affect the intensity of blue ring formation. Research suggests that late-summer cooling events, such as the one in August 1877, may result in less pronounced blue rings than early-season cooling. This change could help scientists figure out not only the occurrence of cold events, but also their seasonal timing.

While a link between the 1902 cold wave and the Peli eruption seems plausible, the relationship between the 1877 Cotopaxi eruption and Norway’s August cooling remains less certain. Researchers acknowledge that other unknown factors may have contributed to the drop in temperatures.

Looking to the future, Dr. Buchvar envisions expanding this research through collaboration: “We hope to inspire other research groups to look for blue rings in their materials. If a network of blue rings based on trees and shrubs could be created to reconstruct the northern forest Long term cooling event, that would be great.

The study highlights the value of natural climate records preserved in living organisms. As scientists work to understand historical climate patterns and their drivers, these blue rings in northern trees and shrubs could be invaluable in reconstructing past environmental conditions and their connection to global volcanic activity.