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.

We caught up with Boswijk on what makes tree rings powerful and precious today.

What kind of trees do you work with?

Here in New Zealand we work on kauri, which is an iconic species. It's a grand tree, and it was used so much in the colonial period for all sorts of structures and buildings. So you find yourself not just looking at tree rings but asking all those other questions around a piece of wood.

What makes its rings so special?

Kauri tree rings are ideal because they're clearly defined. So we can determine what is a ring and what isn't. Plus it's a long-lived species. We have trees standing in the forest today such as Tane Mahuta—the largest living kauri tree, whose name means "Lord of the Forest"—that may be on the order of 2,000 years old.

How did this record come to be?

First we used the living trees, which gave us a record back to the 13th century. Then we had the archaeological resource, which is the material that's in buildings. Then we have the swamp kauri, trees that were buried in bogs and protected from decomposition. That resource extends back across the Holocene.

And there are deposits of ancient kauri, trees that were alive anywhere from 20,000 to 60,000 years ago. So they give us these windows in time that we can use to look at climate or environmental conditions. Not many countries in the world have such deposits.

What's unique about the kauri region of New Zealand?

We have these amazing big trees, forests that you do not find anywhere else in the world. Starting with Polynesians coming in, the Māori, you have the interaction of people with that forest both for an economic resource and on a spiritual level. Then with the European contact you have this absolutely incredible history of engagement again with this forest and this tree species.

We can look at parallels in other settler societies, but this is unique because of the small geographic region and the response of people to working with kauri. So there are multiple layers of stories in this.

How can tree rings help us understand archaeology?

They can help you tell when something was built, particularly if your sample goes right out to the last growth ring put on before the tree was cut down. That's called the "waney edge."

It gives you a fixed date before which the event could not occur. And if you understand your structure and you understand about wood production and aspects like seasoning and stockpiling, it will help you date when the structure was put up.

How do you sample wood from an archaeological site?

If the building is staying in situ then it's possible to take out very small samples from the timbers using a borer or corer. That's a power drill with an attachment. So you take out a "plug"—which is a bit like a long, thin cigarette—and that's what you use for your analysis.

It's tiring. You can take maybe eight to ten samples in a day, but you have to go slowly. What you get is a thin sliver that gives you a particular view of the ring widths.

What would you like to find out about the history behind the kauri tree, which was heavily milled by Europeans when they arrived?

I would like to get inside the mill towns and learn more about them. They were noisy, dirty, and dangerous. People died in the mills and drowned in the rivers. There were actions taken, unions formed, there are all these stories that we barely scrape the surface of when we go into a museum.

The Kauri Museum in Matakohe is fabulous—but it's quiet. It's clean. It doesn't smell. And it's got lots and lots of fantastic objects on display. But we also need to try and get at the life that existed behind those objects.

What's the importance of chronology?

It's that potential that rests in this record. I've expended a lot of energy and effort into building a record, and it has to be accurate and it has to be precise. My part is a bit like baking a cake. I'm not so interested in eating it—but I'm quite happy if other people eat it.

How did you get into tree rings?

My PhD [dissertation] was a tree-ring analysis of a buried forest. It's a lowland raised mire in eastern England that sort of sits like an upturned jelly on the landscape. The question was, Were all those pine trees contemporary? Was it one big phase of pine and then the water rose and the trees died?

I did tree-ring analysis of a few hundred samples from across the bog, and it showed that you actually had episodic growth. Then I returned to New Zealand to take up a post-doc [project] here that extended into looking into swamp kauri and the archaeological wood.

What's the coolest thing about working with tree rings?