Old Trees’ Growth Spurt Has Some Researchers Worried

High on the craggy, wind-swept bluffs of the Southwest, ancient trees that have seen more than a millennium of history are telling a new story.

Scientists who have been studying the old trees for several years now say they are growing far faster than at any time in the last 1,000 years.

“There’s just an unprecedented increase in growth,” says Thomas Swetnam, a forest ecologist with the University of Arizona’s Laboratory of Tree Ring Research.

“Something very unusual is happening,” he adds. “Something on the scale of 1,000 years unusual. And that’s a little worrisome.”

Swetnam and other experts at the lab are not sure exactly what has caused the escalated growth, but the smart money says the old trees may be the best evidence yet of dramatic changes that are beginning to develop in global weather patterns.

“My hunch is it’s a climate shift,” Swetnam says. “It’s some combination of precipitation and temperature.”

The sudden growth surge is happening only at the higher elevations, Swetnam says, suggesting that these old trees are enjoying a little longer spring than normal.

Scientists at the lab are amazed and baffled by the discovery. But the way this century has gone, nothing should really surprise them.

“During this century we’ve had one of the wettest periods and one of the driest periods of the last 1,000 years,” he says, making the 20th century extreme, to say the least.

The 60-year-old laboratory has pioneered in the study of tree rings, which record each year’s growth in bands of light and dark wood. The width of each band corresponds to the amount of growth for that season, and research by Henri Grissino-Mayor of the lab has shown that variability in growth depends primarily upon the amount of water available to the tree during its growth season.

That has enabled scientists to reconstruct rainfall patterns over the last 2,000 years by drilling cores from old, living trees and from trees long dead, including some used in prehistoric archeological settlements.

In general, the patterns revealed by the growth rings correspond to meteorological data from weather stations throughout the Southwest. Grissino-Mayor, who has reconstructed Southwestern weather back to 136 BC, found that 70% of the variation in growth patterns was caused by changes in annual rainfall. Temperature changes also played a part.

Although the Southwest has been wetter than normal over the past two decades, some increase in the growth rate would be expected, but nothing on the scale that Swetnam and his fellow researchers found when they began drilling into some of the oldest trees in the region. Oddly enough, the trees have lived so long because they have adapted to the harshest of environments.

All six sites studied by Swetnam are around 9,000 feet high. Most are on bluffs far above the desert floor.

“The trees grow very slowly from the very beginning of their lives because they are growing in very stressful sites,” Swetnam says. “They grow in small pockets of soil in crevasses in rocks. That’s partly why they have lived so long. There’s no contiguous canopy of trees so fires can’t go through there, and usually insects are not so much of a problem. So they get to be very old.”

The oldest Douglas fir trees in western North America were used in the study. These 1,200-year-old trees found a home in the most unlikely place, old lava beds that offer little of nutritional value. There is no moist subsurface for the trees to sink their roots into for nourishment through dry years, so their growth rings reveal the annual rainfall with extreme accuracy.

Despite their age, the gnarled, twisted trees are less than 3 feet in diameter, and reach a height of only 15 to 20 feet.

They grow so slowly that drilling cores show a century of growth in only about 1 inch of thickness.

But all that began changing in the mid-1970s, according to the story told by the trees themselves. Many of the annual growth rings more than doubled in size. The last 20 years now take up half an inch of the drilling core.

“It really jumps,” Swetnam says.

There are no meteorological stations at those remote sites, so the researchers have no base for comparing data. So, the cause of the growth spurt is open to debate.

Swetnam believes two forces are probably at work: more rain, and higher temperatures, which translate into a longer growing season. The long-postulated global warming scenario calls for higher temperatures and wetter seasons in the Southwest, so that clearly is one option.

But so does the El Nino effect, and that condition, caused by a huge pool of warmer-than-normal water in the Pacific Ocean, is occurring with increasing frequency.

But Swetnam says he cannot rule out other causes as well. Air pollution from nearby cities increases the amount of nitrates in the atmosphere, and it could be that the trees are finding smog a source of higher nutrition.

But nearly all scenarios have one thing in common, Swetnam says. They all point to “people as the indirect cause.”

Lee Dye can be reached via e-mail at leedye@compuserve.com