Trees are timekeepers. Count the concentric growth rings circling the heartwood of a chopped log and you’ll know a tree’s age.
It’s a fun fact, for sure, but tree-ring dating (technically known as dendrochronology) goes far beyond determining how old a tree is. Trees are also meticulous record keepers of climatic conditions. By unraveling the rich data stored in tree rings, scientists can do everything from dating archaeological sites and preventing forest fires to documenting planetary history and offering a crystal ball into our environmental future.
"Trees are natural archives of information," says Ronald Towner, an associate professor of dendrochronology and anthropology at the Laboratory of Tree-Ring Research at the University of Arizona in Tucson. "They stand in one place for a long time, sort of recording in their rings the environment around them. Anything that affects a tree — precipitation, temperature, nutrients in the soil, fires, injuries — can show up in the rings."
Lords of the rings
Wood typically grows season by season, adding a new layer a year. In this way, trees progressively build trunks strong enough to support their many branches and hold them upward toward the sun so leaves can undergo photosynthesis. Look at a cross section of log and you’ll see these growth rings fanning out from the older inner rings to the newer outer rings.
In general, rings can be used to determine a tree’s age, particularly in species like oaks that reliably produce an annual ring. There are exceptions to the one-ring-a-year rule. Pines, for instance, may occasionally miss a year or even double up for two yearly rings, and trees living in unique microclimates (such as being located near a stream with an abundance of water) can experience either enhanced or stunted ring growth. For the most part, though, if you count 65 rings in 2018, you know a tree’s first shoot pushed through the soil in 1953.
Likewise, a singular ring’s width — whether thick or thin — offers clues about the growing conditions a tree encountered that year. "Generally, in a good year trees put on a fat ring, and in a bad year they put on a narrow ring," Towner says.
You can learn more about how tree rings form and what they reveal about climate conditions in this video.
Trunks full of treasure
That’s only the beginning of what dendrochronologists can surmise from tree rings.
For one, they can use them to determine when and where a tree was cut down — in other words, what time period and location it came from. To do this, they first create a master chronology, essentially a database of tree ring patterns going back in time for a given geographic region.
Because all trees growing near one another experience the same conditions, their rings will look the same in any given year. That is, they’ll be equally wide or narrow with no two years exactly the same.
Dendrochronologists start by drilling out a pencil-sized core sample from a living tree using an increment borer. Rest assured, no trees are harmed (though rare mistakes have occurred, as in the time the world’s oldest tree was accidentally killed in 1964).
Dendrochronologists extract a rod of wood tissue that contains the tree's full ring structure. (Photo: Hannes Grobe/AWI/Wikimedia Commons)
Next, ring patterns are plotted out year by year, providing a precise picture of growing conditions over time. Many chronologies stretch back thousands of years, long before written records, using samples from very old trees and ancient wood found on the ground. (And that's just the tip of the iceberg. You can learn more from Towner and others at this PBS page about dendrochronology.)
"We’ve got bristlecone pines in California that are 5,000 years old and oak chronologies in Germany that go back 9,000 years," says Towner.
The tales trees tell
Say you want to know when a fallen tree toppled in the forest. Simply cross-date (match) its ring patterns to the master chronology for your area. If its rings line up for years 1790 through 1902, you know that’s exactly when it lived and died. No fancy technology needed.
Dendrochronologists have used this method to do a host of fascinating things, including:
Dating the cliff dwellings of Mesa Verde using wood charcoal found at the site. "Because charcoal doesn’t burn to ash, it preserves the ring structure, which we can see under a microscope," says Towner. Charcoal samples suggest the Colorado cliff dwellings, once occupied by Ancestral Pueblo Indians, were built around 1250 and abandoned by about 1280 due to a severe drought.
Tree rings found in wood charcoal that was discovered at the cliff dwellings in Mesa Verde National Park allowed researchers to date the site to the 13th century. (Photo: Niagara66/Wikimedia Commons)
Preventing massive forest fires. Tree-ring chronologies dating to the 1500s show that small forest fires used to occur naturally every three to five years in the southwestern U.S. They scarred but didn’t kill the trees and helped promote new forest growth by burning old pine needles, brush and dead wood. However, chronologies reveal that human interference disrupted these natural patterns, starting in the late 1800s when millions of sheep and cattle arrived and began devouring brush and other fire fuel. As a result, the fires stopped. Later, as ranching declined and fires started up again, the Forest Service implemented a policy of always putting them out. By the 1990s, excessive buildup of brush and pine needles began causing mega fires, often wiping out millions of acres of trees at a time. Forest ecologists are now working to restore the natural historical fire patterns revealed in tree rings.
Charting climate change. Dendrochronologists have amassed a long historical record of global temperature variations, revealing recent stunning changes. "Since about 1950, particularly since the '70s, we’re seeing things we’ve never seen before," says Towner. "Rising temperatures means a longer growing season, so we’re seeing some trees grow faster and their rings getting bigger. It’s beyond the range of natural variation." Translation: Temperatures are spiking more than previously seen in thousands of years, and the increase coincides with surging carbon emissions from human activity.
Uncovering environmental mysteries that may help us navigate the future. According to tree-ring chronologies across the globe, 540 was a catastrophic year. "Trees in completely different environments all around the world grew smaller rings," says Towner. One theory is that a comet broke up in the Earth’s atmosphere. Although it didn’t hit the Earth, it may have created dust clouds and massive forest fires from fragments raining down and shortened the growing season that year. Such knowledge could help us prepare for future cosmic cataclysms.