Saguaro mortality 10 years after the River Fire, Arizona

Giant saguaro (Carnegiea gigantea) cacti lure tourists to the Tonto National Forest (TNF), Arizona. By the early1990s, over 30 percent of TNF mature saguaro habitat had burned. Recent fires reveal saguaro loss from fire continues (e.g. the 2005 Cave Creek Complex fire, the second largest fire recorded in Arizona, burned 248,310 acres of mostly desert habitat). Persistent management concerns for these burned areas include: saguaro retention and regeneration, type conversion, and non-native species invasion. Few studies have analyzed fire effects on this long-lived cactus making prediction of post-fire recovery difficult. We previously reported a 10 year post-fire saguaro mortality of 32 percent and reduced height growth (0.9 m) on the low severity 1993 Vista View Fire near Four Peaks, Mesa District, TNF. Here we report on growth and mortality of saguaro 10 years after the nearby 1995 high severity River Fire. Saguaro were randomly selected using the point quarter method along three 350 m transects. Two transects were measured in 1994 before the wildfire. One of these transects burned near the fires edge; a third transect, added in 1997, suffered the full impact of the 1995 wildfire. Saguaro injury and mortality were less near the fire perimeter. In 1997, overall saguaro mortality was 28 percent. By 2005, saguaro mortality increased to 50 percent. During the first post-fire decade, fire exposed saguaro grew an average of 0.9 m in height while unburned saguaro increased 1.3 m. Ten year results from both fires show saguaro mortality increased and growth was stunted over time. Additionally, higher fire severity increased burned saguaro mortality, but did not decrease apical growth. Several decades may be necessary to determine the cumulative effects of the River Fire on saguaro.