Enigmatic Blue Tree Rings Uncover History’s Harshest Summers

A simple blue stain may unlock trees’ memories of cold summers and volcanic winters, offering scientists a precise new tool to reconstruct ancient climates through timber anatomy.

By combining the biological stains safranin and astra blue, researchers can highlight areas with low lignin, a compound that strengthens plant cell walls. When trees and shrubs experience colder conditions, lignification slows down, creating distinctive blue-stained rings.

An international research team found these blue rings serve as reliable climate markers, reflecting centuries of environmental changes. To investigate further, they analyzed wood samples from 25 Scots pine trees (Pinus sylvestris) and 54 common juniper shrubs (Juniperus communis) from Norway’s Mount Iškoras, near the northern treeline.

Applying safranin turned cell walls pink, but when followed by astra blue, lignin-deficient areas darkened, forming clear blue rings. “Blue rings look like unfinished growth rings and indicate cold conditions during the growing season,” explains dendroecologist Agata Buchwal from Adam Mickiewicz University in Poland.

Pine Trees as Climate Indicators: More Sensitive to Cold Than Shrubs

Trees exhibited more blue rings than shrubs, suggesting shrubs adapt better to cooling events and thrive further north. About 2 percent of pine rings and 1 percent of juniper rings turned blue, highlighting pine trees as more cold-sensitive and stronger climate indicators.

Major drops in lignin production occurred in 1902 and 1877, with 96 percent of pine rings and 68 percent of juniper rings turning blue in 1902, and 85 percent of pine rings and 36 percent of juniper rings in 1877. These patterns likely link to cold summers, possibly triggered by distant volcanic eruptions.

“Summer temperatures in 1902 and 1877 were particularly unfavorable for woody plant growth,” the researchers note. However, they suggest future studies should explore whether drought or other factors also contribute to blue ring formation.

Blue rings remain a relatively new area of study, with ongoing questions about their consistency across species and regions. This research brings clarity, reinforcing that multi-colored rings can signal shifts in seasonal temperatures, growing periods, and global climate events.

“We hope to inspire other researchers to look for blue rings in their material,” says Buchwal. “Building a blue ring network using trees and shrubs could help reconstruct cooling events along the northern treeline over long timescales.”

Read Original Article: Science Alert

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