Summarize this content to 2000 words in 6 paragraphs In the frigid landscapes of northern Norway, an unusual phenomenon etched into the rings of trees and shrubs has offered a glimpse into the Earth’s climatic history.Scientists have discovered that peculiar “blue rings” in wood samples reveal the aftermath of chilling volcanic eruptions that disrupted growing seasons over a century ago.When summers turn abnormally cold, trees and shrubs struggle to grow properly. The cell walls of their new growth fail to lignify—the process of hardening and strengthening—leaving behind blue-stained rings when wood samples are dyed.By studying these blue rings, researchers can pinpoint periods of extreme cold, with the latest findings highlighting significant cooling events in 1902 and 1877.
A close-up of a young green pine tree (main) and an image of a volcanic eruption (inset).
A close-up of a young green pine tree (main) and an image of a volcanic eruption (inset).
Esa Hiltula/Daniel Jara/Getty
The cooling may be linked to major volcanic eruptions: Mount Pelée on Martinique in 1902 and Cotopaxi in Ecuador in 1877. These eruptions likely cast a volcanic winter over the northern hemisphere, impacting the trees at Mount Iškoras in Norway, where the study was conducted.”Blue rings look like unfinished growth rings, and are associated with cold conditions during the growing season,” Agata Buchwal, lead author of the study, said in a statement.Buchwal’s team analyzed samples from 25 Scots pine trees and 54 juniper shrubs, staining and photographing their growth rings under a microscope.The study revealed that blue rings were more common in pine trees than in junipers, appearing in 2.1 percent of the trees’ rings compared to 1.3 percent in shrubs.Notably, 1902 and 1877 stood out: 96 percent of the pines and 68 percent of the shrubs displayed blue rings in 1902, while 84 percent of pines and 36 percent of shrubs exhibited them in 1877.Junipers, found farther north than trees, appeared better adapted to cold conditions, showing fewer blue rings.”That is why I love to study shrubs; they seem to be the true heroes of the north,” Buchwal said.
A blue ring formed in 1902 in a tree in northern Norway.
A blue ring formed in 1902 in a tree in northern Norway.
Pawel Matulewski and Liliana Siekacz
The blue rings in trees formed during the latewood stage, at the end of the growth season, when cold conditions abruptly halted lignification.Pawel Matulewski, a co-author of the study, noted that these rings could weaken trees, leaving them more vulnerable to mechanical damage and disease.”If this phenomenon persists over several years, it can impede the tree’s recovery in following years,” Matulewski said.Temperature records confirmed the extreme conditions in 1902, when the coldest June on record delayed the start of the growing season. In contrast, 1877’s cooling occurred in August, suggesting that blue rings could help differentiate between early- and late-season cold events.The study adds to evidence that volcanic eruptions leave a long-lasting mark on global climates. Mount Pelée’s eruption in May 1902 may explain the cold June that year, while Cotopaxi’s eruption in late June 1877 aligns with the cold August.The team identified other blue rings in the samples too, but less cold weather and missing temperature data for the region hampered more detailed analysis.The authors cautioned that future work will need in-situ weather data since there may have been some temperature variation between the trees and the nearest weather station.”We hope to inspire other research groups to look for the blue rings in their material,” said Buchwal. “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.”Do you have a tip on a science story that Newsweek should be covering? Do you have a question about blue rings? Let us know via [email protected], A., Matulewski, P., Sjöberg, Y., Piermattei, A., Crivellaro, A., Balzano, A., Merela, M., Krže, L., Čufar, K., Kirdyanov, A. V., Bebchuk, T., Arosio, T., & Büntgen, U. (2025). Blue rings in trees and shrubs as indicators of early and late summer cooling events at the northern treeline. Frontiers in Plant Science, 15. https://doi.org/10.3389/fpls.2024.1487099
