Congratulations on the Sid Goodloe story (HCN, 4/15/96), which stuck a cattle prod into conventional narratives.
I need to explain my sure-to-be-maligned comments about saguaros. I lump them, properly I think, with "woody" plants. But I do not mean to imply that they, like piûon-juniper, have exploded over the landscape. They have always been around.
To my knowledge, no one knows the fire history of the Sonora Desert. Certainly, the desert can burn (driven by winter rains, not as in the mountains by summer drought). I would guess that an average site would experience three fires a century, some places more, some less. Saguaros survive light burns readily and their fire adaptations have been recorded by a British (!) ecologist. But they succumb to the fire-girdling that can result when woody plants and debris build up around them and hold flame.
Last summer a lightning-caused fire blasted the McDowell Mountains Park in north Scottsdale. Was this event within historic ranges or not? Yes and no. I believe its occurrence was perfectly normal but its effects probably abnormal due to an excessively "woody" landscape composed largely of shrubs, low-growing trees, and (probably) an increased density of succulents.
In the absence of any evidence to the contrary, it seems to me that the most plausible narrative is that the Sonora desert restates the larger fire narrative of the Southwest: overgrazing and drought pummeled the prior biota, woody plants have replaced grasses, that fires burn less frequently but with greater intensity. Worse, the niche vacated by indigenous grasses and ephemerals has filled with exotic pyrophytes like red brome, which support flash fires. Wildfire from roads and housing developments is now pushing into the desert like a bow wave.
Pyne is professor of history at Arizona State University.