Epidermal browning of saguaro cacti (Carnegiea gigantea): Relative health and rates of surficial injuries of a population

Epidermal browning appears to be occurring on virtually all medium and older-aged saguaros [Carnegiea gigantea (Engelm.) Britt. and Rose] throughout the Sonoran Desert. The purpose of his research was to determine if a population of 253 saguaros could be accurately subdivided with regard to health based upon surficial injuries and to determine rates of injuries (morbidity) and death (mortality) by repeated 2-3 year observations of the sampled population. Surficial injuries such as mean percentage of spines present and of crest and trough areas without visible surface blemishes were determined. Data show that the cactus population can be subdivided accurately into (1) cacti that are healthy, (2) cacti that exhibit a health decline, (3) cacti with poor health (many surficial injuries), and (dr) cacti that died during the observation period. The changes in percentages of surficial injuries for cacti that exhibited a health decline (group 2 above) were used to determine rates of morbidity; this group comprised 27-49% of the entire cactus population among the three parameters observed. For one group of saguaros (between 11 and 36% of the population), there was up to a 31% decline in health annually. Mortality rates were only between 3 and 4.5% annually, below some other published estimates. From available evidence it appears that the health of a saguaro population can be determined accurately with a relatively small number (253) of cacti examined over a short (2- or 3-year) period because (a) surficial injuries of individual saguaros were determined, (b) three parameters of injury occurred at relatively similar rates, (c) the percentages of cacti that underwent these changes were relatively large, and (d) the negative changes in health were relatively large.