Some saguaro cacti on a south facing slope of the Tucson Mountains on private land of Tucson looked a sickly orange when photographed this week.
The shooter, Bill Peachey, a longtime Tucson-area geologist who studies saguaros professionally, says he believes the strange coloration on these 17 saguaros stemmed from the hard freeze that struck Tucson in early February. In an email to colleagues and friends written Wednesday, the day he shot the pictures, he said this sight was a strange occurrence because thousands of saguaros surrounding this area weren’t orange..
“Quite interesting because of the color. I’ve never seen that color before,” said Peachey, who has studied a 300-saguaro plot on two acres of private ranchland in the Rincon Valley for 14 years.
But in a return email to Peachey and others written Thursday morning, Organ Pipe National Monument plant ecologist Sue Rutman opined that she’s not convinced that the freeze lies behind the cacti’s coloration.
“Look at the callous tissue around the edges of the necrotic tissue. That takes longer to develop than a few days,” Rutman said.
Peachey fired back that as recently as January 25, 2010, he had walked into this area as he was dissecting another batch of saguaros in an adjacent area and saw absolutely no discoloration at that time. One of the emails he sent out showed three shots of the most orange saguaro taken on March 2 -- on Jan. 25, it was green, he said. Also, the owner of this property told him that in mid-February, the saguaros looked brownish, he said.
“The "telephoto" image was taken from several thousand feet away,” Peachey wrote in an email, speaking of a sweeping shot of the entire area containing the orange saguaros. “I immediately zeroed in on these without a single prompt beforehand as to their location in the terrain. Sooo, I have extreme doubt that these would have gone undetected for very many days.”
Another naturalist and cactus researcher, Bill Thornton, wrote back in a followup email that the discolored cacti appeared to be suffering from epidermal browning, a common skin disease among saguaros that has been studied but is still not well understood. Thornton wrote that the browning may be a natural consequence of aging.
Thornton added, “In these saguaros, the additional stress of several hard reezes may have aggravated the condition.”
A 10-year study of epidermal browning at Saguaro National Park reported in 2000 that past studies had failed to prove any links between the browning and air pollution or to ultraviolet radiation exposure. It said that a possible cause might be heat loading, in which high surface temperatures on an affected saguaro cause localized tissue damage. The research was financed by the U.S. Geological Survey and the University of Arizona.
The study also found that saguaros that died during the 10-year study period had higher levels of browning than those that survived, suggesting a link between browning and mortality.
Dale Turner, a co-author of that 2000 report, got a look at Peachey’s photos this week, and prounounced them “pretty weird.”
“I don’t understand it. There’s a good chance it was the freeze, but I haven’t been to the site,” said Turner, a biologist who works as a conservation planner for the Nature Conservancy.