Epidermal browning of saguaro cacti ( Carnegiea gigantea): is it new or related to direction?

Abstract

Browning of epidermal surfaces of saguaro cacti ( Carnegiea gigantea) is associated with premature death of the species in the northern Sonoran Desert. Analyses of authentic historical photographs of mature saguaros from 1903 to 1987 demonstrated a significant (1) loss of spines, (2) decrease in apparently healthy crests, and (3) increased trough barking over the period. All three of these changes occurred to saguaros of over 115 years of age at a projected rate of 4–5% per decade. Analyses of epidermal characteristics of saguaros at the Saguaro National Monument were performed to determine if epidermal browning was related to compass direction. Results show that both scaling and barking of crests and troughs and loss of spines, all symptoms of epidermal browning, were 10 times higher on southern exposures (between 140 and 220°) compared with northern exposures (between 320 and 40°). The two troughs of a crest were evaluated separately. The results demonstrated that injury to one trough of a crest could be as much as four times greater than that of the second trough.