Monday, 17 November 2003
Delayed mortality: saguaro cacti are still dying 10 years after wildfire!
Sonoran desert vistas supporting Giant Saguaro (Carnegiea gigantea) cacti attract millions of visitors to the Tonto National Forest (TNF), Arizona each year. Unfortunately fires occurring during the last few decades burned large portions of mature saguaro habitat--landscapes now devoid of centuries old behemoth cacti. Based on a developing understanding of disturbance effects on saguaro, TNF is modifying grazing and fire policy on its desert rangeland. They have placed a greater emphasis on the protection, maintenance and recovery of this valuable resource, yet little is known about long-term consequences of historical range practices. The arson-set ignitions known as the Vista View Fire burned saguaro habitat near Four Peaks, Mesa District, TNF during mild fire-weather conditions in May 1993. A 1994 post-fire study evaluating fire effects on saguaro and associated vegetation compared unburned and burned habitat. Survival and mortality of saguaro and associated vegetation was measured using point quarter techniques along four 350 m transects. Initial saguaro mortality was recorded at 19 percent in the burned areas yet long-term mortality was expected to increase based on the 90 percent injury recorded. Delayed saguaro mortality from other fires had been documented for up to 5 years after injury. Preliminary analysis now shows that after 10 years saguaro mortality attributable to the Vista View fire has increased to almost 30 percent. Similarly, some of the associated vegetation that had sprouted after the fire eventually died. Although many obviously fire injured saguaro continued to grow in height and produce additional arms evident deterioration and partial decay suggest more will eventually die from injury suffered during the 1993 fire. Higher numbers of juvenile saguaro were observed in unburned when compared to burned areas. Further comparisons between burned and unburned vegetation will elucidate how the extended drought during the last decade influenced mortality of saguaro and associated vegetation. Another decade or two may be necessary to determine the full impact of the Vista View fire on this saguaro population.
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