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Ancient Tree Rings Hold Clues to Volcanic Cooling Events

In the frigid reaches of northern Norway, a curious phenomenon in tree rings is helping scientists uncover the climatic impact of distant volcanic eruptions from over a century ago. Researchers have discovered that unusually cold summers leave behind distinctive blue-tinted 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 drop too low for proper plant growth, preventing cell walls from developing normally. This distinctive coloring becomes visible when wood samples are treated with special dyes in the laboratory.

At Mount Iškoras in northern Norway, scientists examined core samples from 25 Scots pine trees and 54 common juniper shrubs, uncovering evidence of two particularly severe cold spells in 1902 and 1877. The timing of these events coincides with major volcanic eruptions – Mount Pelée in Martinique and Cotopaxi in Ecuador – suggesting a possible connection between volcanic activity and regional climate effects.

“Blue rings look like unfinished growth rings, and are associated with cold conditions during the growing season,” explains Dr. Agata Buchwal of Adam Mickiewicz University, Poland, who led the research. The findings show that trees and shrubs respond differently to these cold events, with pine trees showing greater sensitivity.

The research team’s meticulous analysis revealed that blue rings were relatively rare, appearing in only 2.1% of pine tree rings and 1.3% of juniper shrub rings. However, during the extreme cold summer of 1902, the blue rings were remarkably widespread, appearing in 96% of pine trees and 68% of juniper shrubs studied.

These cold-induced formations may have lasting consequences for the affected trees. According to Dr. Pawel Matulewski, the study’s second author, “In the case of pine trees in boreal regions, blue rings have the potential to weaken the tree, making it 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 trees managed to form normal early-season growth rings, the late-season wood showed clear signs of disrupted development, resulting in the telltale blue coloring when stained.

The research process itself proved challenging, requiring specialized techniques beyond traditional tree ring analysis. “Blue ring studies require a much longer protocol than classical dendrochronological studies,” notes Dr. Buchwal, emphasizing the importance of skilled laboratory technicians in the research.

The timing of cold events appears to influence the intensity of blue ring formation. The study suggests that late-summer cooling events, like the one in August 1877, may result in less pronounced blue rings compared to early-season cooling. This variation could potentially help scientists pinpoint not just the occurrence of cold events, but also their seasonal timing.

While the connection between the 1902 cold spell and Mount Pelée’s eruption appears plausible, the relationship between Cotopaxi’s 1877 eruption and the August cooling in Norway remains less certain. The researchers acknowledge that other unidentified factors might have contributed to the temperature drop.

Looking ahead, Dr. Buchwal envisions expanding this research through collaboration: “We hope to inspire other research groups to look for the blue rings in their material. It would be great to establish a blue ring network based on trees and shrubs to reconstruct cooling events at the northern treeline over long timescales.”

The study underscores the value of natural climate records preserved in living organisms. As scientists work to understand historical climate patterns and their driving forces, these blue rings in northern trees and shrubs may prove invaluable in reconstructing past environmental conditions and their connections to volcanic activity worldwide.


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