Prickly tip-off to ozone decline?

Like Stetson hats and chili peppers, the saguaro cactus symbolizes the U.S. Southwest. A large stand of saguaros near Tucson, Ariz., has even achieved the status of a national monument. But ecologists are now investigating whether these tall, branching cacti, which often live for hundreds of years, might also provide another symbol: a preview of the plant damage to occur during the chlorofluorocarbon-induced degradation of Earth's ozone layer.

An unexplained, accelerating die-off of the Arizona saguaros, preceded by a browning and thickening of the outer flesh (cuticle) and the loss of spines, may result from an increased exposure to ultraviolet-B (UVB) rays as the stratospheric ozone layer thins, says ecologist Kate Lajtha at Boston University. If so, she asserts, "the saguaro might be the first plant species to show damage from increased UVB activity."

Lajtha noticed that the saguaros initially develop the so-called "barking" appearance only on their southern sides, which receive the most sun. She ruled out the possibility that the thickened cuticle represents an attempt to conserve fluids, because healthy and damaged saguaros contain the same proportion of water in relation to the amount of carbon dioxide drawn in. She also ruled out frost damage from winter cold snaps, since frozen cacti get mushy and die quickly. Further, searches for disease-causing organisms and comparisons of environmental lead levels absorbed by each side of the plant turned up no differences, she says.

"It's got to be something in the sunlight," concludes Lajtha, although she concedes, "I'm still skeptical [about the ozone link]."

Lajtha, in collaboration with the U.S. Park Service, now plans to place UVB-shielding glass sleeves around a group of healthy cacti in the Saguaro National Monument. "If UVB is causing the barking, we ought to see a difference [between shielded and unshielded saguaros] within five years," she explains.