Here was a ring to rule them all. It came not from the fires of Mordor, but from a tree in New Zealand. And its secrets were written not in Elvish script, but in the annual pattern of its growth.
Gretel Boswijk, a dendrochronologist from New Zealand’s South Island, has been deciphering that pattern for a decade. By matching up the lines in older and older trees, she and her team at the Tree-Ring Lab of the University of Auckland have been building a precisely dated annual calendar. Today, they’ve completed a tree-ring chronology going back 4,500 years—the longest in the Southern Hemisphere.
Only a handful of tree-ring calendars of that length exist, including the Huon pines of Tasmania and the venerable bristlecone pines of the American Southwest.
"This is the ‘Holy Cow’ range," said Jim Speer, a dendrochronologist who uses tree rings to reconstruct fire history at Indiana State University.
Boswijk has begun using her record to date 19th-century wooden buildings from the time Europeans reached the North Island. In the U.S., researchers used similar calendars to determine the age of the Pueblo Bonito ruins of New Mexico and the Kentucky log cabin Abraham Lincoln was born in.
But the rings also tell another story. Like pages in the Earth’s diary, they chronicle the weather patterns of each year. Some trees provide clues to El Niño, the global pattern of storms that occurs every five to seven years, said Anthony Fowler, a dendroclimatologist and founder of the Tree-Ring Lab.
El Niño had been little studied from the Southern Hemisphere. By interpreting tree rings, Fowler found that El Niños have been getting more intense over the past 500 years. With Boswijk’s new chronology, he can go back in time to see how unusual this trend is in the context of thousands of years. Earlier this month, Fowler presented the new chronology to the European Geosciences Union at its annual conference in Vienna.
Though the chronology is not yet complete—”these things never are,” said Boswijk—the journey to get here was long. To extend the record back in time, researchers chased down preserved logs in the swamps of northern New Zealand. Boswijk found herself crouched in the dark rafters of a cottage built for Māori King Tawhiao, wielding a power drill. Eventually, the puzzle came together.
"It was as if the gods of science were smiling on us," Fowler said.