Changes in scale and bark stem surface injuries and mortality rates of a saguaro cacti (Carnegiea gigantea, Cactaceae) population in Tucson Mountain Park

Rates that stem surfaces of saguaro cacti (Carnegiea gigantea (Engelm.) Britt & Rose) accumulate scale and bark injuries and the mortality rates of cacti were determined on a population of 1149 saguaro cacti in 50 field plots over the 9-year period of study (from 1993-1994 until 2002). Twenty-three percent of the saguaro population had few surface injuries throughout the 9-year period while 27% showed a marked increase in stem area with scale and bark injuries. Thirty percent of all cacti had more than 80% stem areas with combined scale and bark injuries on south-facing stem surfaces throughout the study period. Finally, 20.3% of the saguaro population died over the 9-year period, a rate of 2.3% per year. Thirty-three percent of all cacti that died by 2002 exhibited few surface injuries in 1993-1994 while 54% of the cacti that died over the period had more than 98% stem areas with combined scale and bark on south-facing stem surfaces in 1993-1994. In this manner, stem scale and bark injuries on south-facing surfaces were usually associated with the death of saguaros. The annual mortality rate of 2.3% appears high considering that this species may live for more than 200 years.