TUCSON — 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.
