Betsy Pierson, a University of Arizona associate professor of plant pathology, said wildfires are particularly heinous because they wipe out mature saguaros as well as future generations that would have replaced the 200-year-old giants.
"The loss of these precious reproductive and newly established plants is critical," Pierson said.
A study of a 1,200-acre fire in Saguaro National Park in May 1994 revealed that 6 percent of 436 saguaros in the burn area died within six weeks. All were less than 3 feet tall.
Researchers found that a quarter of the saguaros died within six years of the fire, most of them in the first year.
Other fires are even more destructive.
Mark Holden, a Saguaro National Park biologist, said another 2-acre fire in the park in 1994 killed 90 percent of the saguaros.
Still, the estimated number of saguaros throughout Saguaro National Park increased during the 1990s by 35 percent to 1.3 million, according to Dale Turner and Carianne Funicelli in a study for the U.S. Geological Survey.
Researchers are unaware of any hard estimates of the entire population of saguaros, which are found only in Arizona and Sonora.
When wildfire strikes, the number of saguaros that die depends on the height of the flames. Saguaros growing on a slope tend to have more of their stems scorched than those on flatter land.
The plants at ground level die first, including young saguaros that often had to struggle five years under a nurse plant to reach 1 inch.
Kim Stone, horticultural specialist at the Boyce Thompson Arboretum near Superior, said nurse plants, which protect young saguaros from sun, frost and predators, contribute to saguaro deaths during fires by adding fuel to the blazes.