Predicting long-term Saguaro (Carnegiea gigantea) survival following wildfire

Background/Question/Methods Giant saguaro (Carnegiea gigantea) cacti are a tourism favorite on the Tonto National Forest (TNF), Arizona. Thousands of acres of this desert shrub community have been scorched by wildfires during each of the last 3 decades.  Both the saguaro habitat and scenic vistas will take a century or more to return to their previous grandeur.  Presently, this ecosystem and its resource managers face challenges from recurrent fire, uncertain saguaro retention and regeneration, plus invasion of non-native species. Our analyses of long-term fire effects on saguaro populations may help managers predict and address post-fire recovery.  Here, we compare saguaro mortality 10 years after two very different wildfires.  Eight plots were established with six  point quarter transects, running north to south, in burned and unburned areas of The Rolls, near Four Peaks Road, Mesa District, TNF. The closest saguaro in each of four quarters of circular plots was measured at eight points, set 50 m apart, along 350 m transects.  After 10 years, the following mortality and growth was observed for 164 saguaro.

Results/Conclusions

Unburned areas had 4 percent saguaro mortality and an average apical growth of 1.13 m. Mortality was 51 percent with an average height growth of 0.9 m on the May 1993 Vista View Fire.  The adjacent July 1995 River Fire had 77 percent mortality with an average growth height of 0.7 m.  High saguaro mortality and stunted growth were observed in both burns. Areas of high compared to low fire severity almost doubled saguaro mortality.  Apical growth rate of surviving burned saguaro was about 22 percent less in the high vs. low severity fire areas.  Two years Pre- and Post-fire precipitation events were high to normal for the low severity Vista View fire but were normal to low for the high severity River fire.  Six to seven low-precipitation years the decade following both fires likely affected saguaro post-fire survival, growth and regeneration success. High fire mortality and difficulties in predicting future fire and climatic events suggest that resource managers may want to employ pro-active restoration efforts for burned saguaro ecosystems particularly after high severity or recurrent wildfire.