The Trees Tell Tales With a Ring of Truth

The trail up Mt. Baden-Powell zigzags four miles through canyon oak, Douglas fir and finally limber pines before reaching a barren summit 9,400 feet above sea level.

Huffing and puffing, researcher Franco Biondi and assistant Shirley Imsand unload gear and break for water near the top. To the south, a shimmering haze blankets San Gabriel Valley communities; a look north reveals the seemingly endless floor of the Mojave Desert.

Biondi is checking for clues to the past climate that are hidden within the rugged sentinels that guard the wind-swept peak. By painstakingly counting the rings of the gnarled trunks of limber pines, Biondi has found that some of the living trees are 1,250 years old or older, perhaps the oldest in Southern California.

Dead snags that are still upright go back as far as AD 497.

The world’s oldest living trees are the 4,765-year-old bristlecone pines that survive at 14,000 feet in the White Mountains along the Nevada state line several hundred miles north of Los Angeles.

“What surprised me was to find [ancient trees] so close to downtown L.A.,” said Biondi, a tree-ring researcher, or dendrochronologist, from the Scripps Institution of Oceanography.

The trees also record climate changes. They grow more during wet years than dry ones, and by correlating annual growth spurts shown by the rings, Biondi is assembling a 1,000-year chronology of drought and rainfall, including El Nino events, for the Los Angeles region.

He has found that the limber pines, named for their flexible branches, have grown more in the last two decades than at any time during the previous 1,000 years. This does not appear to be the result of an increase in moisture. He suspects that the growth may reflect the deposition of nitrogen in the soil from air pollution, acting as fertilizer, or perhaps increasing levels of atmospheric carbon dioxide.

“These last 20 years are unusual,” Biondi said. “Something is happening there, but I don’t know what it is.”

Biondi is one of a growing number of climate researchers who are peering back in time by looking at nature’s own records instead of those kept by humans.

Tree ring dating isn’t exactly a new science, but it is getting a new spotlight as research time and money are increasingly focused on global warming. In the 1920s, geologist A.E. Douglass of the University of Arizona first used tree rings from the timbers in adobe houses to date a Native American settlement at Pueblo Bonita, N.M., to AD 800, double the age that researchers had thought.

Since then, scientists have used tree rings--as well as coral reefs, pollen trapped in muddy lake sediment and even old pack rat burrows--to estimate temperature and rainfall patterns thousands of years ago. By understanding the past climate, scientists say, they will have a better understanding of possible human influences on our present one.

Climate Records Explain Conditions

“If scientists want to understand changes in climate in the 20th century, we need to look at the climate before the Industrial Revolution and understand the natural rhythms of climate variability,” said Jonathan Overpeck, director of the National Oceanic and Atmospheric Administration’s paleoclimatology center in Boulder, Colo.

Overpeck and Susan Woodhouse of the University of Colorado at Boulder used natural climate records to determine that droughts in the past 2,000 years in the central United States were much longer, more severe and more widespread than the great Dust Bowl drought of the 1930s. In research published last year, they found that extreme droughts lasting 20 to 30 years were a regular occurrence up to 400 years ago, and could occur again.

Other researchers are using tree rings to explain climate’s effects on human civilization. In Virginia, researchers at the College of William and Mary used tree rings from a 900-year-old bald cypress to find that the Jamestown Colony of Virginia and the “Lost Colony” of Roanoke Island were plagued by the driest period in 770 years.

And in Lake Tahoe, rings from 500-year-old trees cut down a century ago revealed that the region is overdue for a major wildfire.

Biondi has been studying tree rings from the Sawtooth Mountains of Idaho to the Sierra San Pedro Martir of northern Baja California, Mexico, to get a large-scale view of climate change on the Pacific coast of North America. It’s not a perfect science, however, and the vagaries of environment can throw off a climate chronology. “These are living organisms, not instruments,” Biondi said of the trees.

The limber pines on Mt. Baden-Powell respond to winter rainfall, growing more during wet years. Other trees, like the Douglas fir and white bark pines in Idaho, grow more during warm summers. Rain doesn’t affect them as much.

Non-Climatological Factors Considered

Forest fires, lightning strikes and insect pests also can affect annual rings. Other factors are the amount of sunlight and competition from other trees. These things have nothing to do with climate, so researchers have to take lots of samples in lots of places.

And while tree rings have proven valuable for climate studies in the Northern Hemisphere, they are nearly useless in the tropics because tropical trees don’t live long and don’t have annual rings.

Some of the best trees for tree ring study are at high elevations. Last summer, Biondi hiked to 14,000 feet among a range of active volcanoes in the central Mexican states of Mexico and Colima. Trees that grow at the tree line are especially sensitive to changes in climate, and can serve as a sort of early warning system.

Biondi is one of 14 international researchers examining the ecology and climate signals from high-altitude trees from Alaska to the tip of Chile in a study sponsored by the Interamerican Institute for Global Change Research in Brazil.

On the steep, rocky north slope of Baden-Powell, the afternoon’s work is about to end. Biondi and Imsand, a Pasadena naturalist who helped collect tree ring samples last summer, have found some of the trees to be uncooperative.

The hard, compact wood fibers twist like a candy cane, all the better to resist strong winds and pests, as well as the coring tools used by researchers. Biondi drills the tool into the wood like a corkscrew, probing as far as he can.

He has already broken one tool by hitting a rock concealed by the tree’s growth; another probe turned up rotten inner wood, useless for study. But Biondi hits pay dirt with a sample from a tree dating to AD 1241. “This is a beautiful core; it’s a dream come true,” says the Italian researcher. “It’s in perfect condition.”

After recording the tree’s location, Biondi wraps the fragile, chopstick-sized cores in paper, and heads down the mountain.