Every tree has a story to tell
From a secret location 10,000 feet high in the White Mountains of California, its thick, gnarled limbs stretch skyward. "Methuselah" is the oldest living tree found on earth. It's a 55-foot-tall bristlecone pine that's nearly 5,000 years old. It's as old as the great pyramids of Egypt. Older, by a thousand years, than Hammurabi's reign in Babylon.
Far from being a silent witness to history, Methuselah and other old trees have stories to tell. They are stories that scientists are now able to read more clearly - stories about huge volcanic eruptions, forest fires, and droughts. But trees can also tell gentler stories about log cabins, precious violins, and even three-part paintings that may have been split up long ago.
Have you ever counted a tree's rings on a stump to see how old it was? In temperate climates like that of the United States, each ring represents one year of growth. People have known this for a long time. But in the 1920s, Andrew Ellicott Douglass discovered something else about the rings: Environmental conditions (temperature, rainfall, sunlight) could help determine the width of the rings. Not only that, but patterns of wide and narrow rings could also be compared from tree to tree if the trees were the same species. Comparing younger trees to older ones, one could build a timeline of growth-ring patterns reaching back hundreds, even thousands of years. (See illustration.)
This new science of tree-ring dating was named dendrochronology (den-droh-cruh-NOL-uh-jee). The word comes from two Greek words: "dendron" (tree) and "chronos" (time).
Examining Methuselah by taking core samples using a special drill, scientists found a curious pattern. There was a series of very narrow rings. Scientists think that the rings are evidence of volcanic eruptions 3,600 years ago. The eruptions filled the atmosphere with ash and soot, enough to block the amount of sunlight reaching Earth. Temperatures fell, and the bristlecone pine grew slowly.
Many of the earth's major natural events - forest fires, century-long droughts, insect plagues, and glacial freezes - have left their marks on the world's oldest trees.
Dendrochronologists can use tree-ring patterns not only to learn about the earth's past, but also to uncover the secrets of historic objects made of wood. Abraham Lincoln's first cabin, for example, or Colonial-era houses, or the world's most famous violin.
Dendrochronologist Henri Grissino-Mayer tells how he was asked to solve a historical mystery involving Lincoln's birthplace cabin: "I was called in for a documentary being filmed called 'Lincoln: Man or Myth,' showing aspects of Lincoln's life that were debatable," Professor Grissino-Mayer says. He works in the department of geography at the University of Tennessee in Knoxville. "One was his 'birthplace cabin.' " The National Park Service had already conceded that it wasn't Lincoln's birthplace, but that's what the documentarymakers wanted to prove.
Grissino-Mayer conducted dendrochronological tests on logs from the cabin in Hodgenville, Ken., matching their tree-ring pattern to a well-researched database of ring patterns for those trees in that region.
According to the pattern of the rings, the logs had been cut down in the 1840s or 1850s. But Lincoln was born in 1809. He was a grown man when this cabin was built. Digging deeper, Park Service historians found that a promoter had built the cabin in the 1890s from logs taken from dilapidated cabins in the area. It may have been the site of Lincoln's birthplace, but it wasn't Lincoln's cabin.
The most famous violin in the world is known as the "Messiah." It is believed to have been made by the Italian violinmaker Antonio Stradivari in 1716. But the music world was rocked in 1998 when its authenticity was challenged by Stewart Pollens of New York's Metropolitan Museum of Art.
Grissino-Mayer and his colleagues carted 200 pounds of equipment - measuring tools, a special microscope, a digital camera and imaging system, and a laptop - to the Ashmolean Museum in Oxford, England, where the violin was kept.
"It was scary," Grissino-Mayer says. "The 'Messiah' violin was worth $12 million to $20 million, and they just took it out of the case and took the bridge and strings off and said, 'Here you go.' It took three of us six hours of hands-on work to study it."
The growth rings in the wood used to make the violin were beautifully visible, all 109 of them. Grissino-Mayer and his colleagues measured each one several times. Their conclusion? The wood is Norway spruce from the Italian Alps. It grew between 1577 and 1687. Stradivari lived from 1644 to 1737.
The violin, very possibly, is worth the money - but not necessarily. Knowing how old the wood is doesn't tell you when a wooden object was made. Tree-ring dating does not date the object, only the wood. All that Grissino-Mayer will say for sure is that the violin's wood dates from Stradivari's time.
In fact, "Very often [the] best violin copies were made with very old wood," says Roman Barnas, head of violinmaking and restoration at North Bennet Street School in Boston. Even today, the kind of wood the Italian masters used - wood of the right age and region - can be obtained from old buildings.
Ron Spronk of Harvard University in Cambridge, Mass., had a similar quest. A research curator at Harvard's Straus Center for Conservation, Mr. Spronk is investigating paintings on wood at three different museums in Belgium and the Netherlands. Spronk suspected that they were once a triptych - a three-paneled painting - done by a Renaissance Flemish painter. Dendrochronological tests told him that the panels were cut from the same tree. Spronk still needs more information to prove that the three paintings were once one, but the tree-ring evidence has strengthened his case.
Dendrochronology may not completely answer questions about the past, but it can make the past clearer. With historic houses, for example, tree-ring dating can stand tradition on its head.
Dendrochronology, used with documentary research (looking at old papers) and archaeology, can shed light on the lives of the people who lived in a particular building, according to Anne Grady. She's an architectural historian and preservation consultant. Over the years, she has done research and planning for more than 60 dendrochronological studies of old buildings in New England, including old homes.
In only one-quarter of the cases, the building studied was as old or older than its supposed date. The other 75 percent of the time, the building studied was more recent than it was thought to be, sometimes by 30 or 40 years.
"Some building owners were clearly disappointed," Ms. Grady says. "I understand how they feel. There is a cachet attached to having a very old building."
Take the Robert Pierce house in Dorchester, Mass., near Boston: "It had been recognized for generations as being built in the 1640s or 1650s," says Historic New England's Peter Gittleman, who organizes public tours of Colonial-era houses. When dendrochronology changed the date [to 1683], we lost Robert. He never set foot in the house; he'd died before construction. It's amazing how many signs we had to change."
When it comes to forest fires, there are good ones and bad ones.
Conventional wisdom once held that all forest fires were bad. Every forest fire - whether started by lightning or by human carelessness - was put out quickly. But it gradually dawned on forest managers that some fires were helpful. "Burns" are a natural part of a forest's life. Without periodic small fires, leaf litter and twigs build up on forest floors. All that fuel leads to hotter, more dangerous fires. Henri Grissino-Mayer calls them "forest killers."
Professor Grissino-Mayer specializes in forest fires: "Before the 19th century," he says, "fires cleaned the forest. Now they don't - there are more intense wildfires. We have changed the balance of nature."
The National Park Service and the USDA Forest Service have asked Grissino-Mayer and other dendrochronologists to research forest history. Fires leave scars in tree rings that can be dated and "read" to see how often naturally occurring fires happened. Using this information, scientists can figure out how frequently a forest should burn to keep it healthy.
"They want to condition the forests to burn the way they used to," explains Grissino-Mayer. "How often? You look at tree rings. When? Look at tree rings. How big should the fire be? You look at tree rings."
