Twenty-one species of tall, long-lived columnar cactus species show extensive bark coverage (epidermal browning) in the Americas. Each species shows more bark coverage on equatorial-facing surfaces than on polar-facing surfaces. In addition, controlled experiments with supplemental UV-B irradiation show the initial stages of bark coverage. These two facts suggest that UV-B irradiation is the cause of sunlight-induced bark coverage. This sunlight-induced bark leads to premature morbidity and mortality. Saguaro cactus (Carnegiea gigantea (Engelm.) Britton & Rose) of southern Arizona is the most researched cactus species with regard to this bark coverage. The current effort focused on bark coverage rates on 12 cactus surfaces at 1.7 m height above ground by comparing logistic curves of relative bark coverage among stem surfaces. Logistic curves have been used to document progress of many diseases in humans. In this study, logistic curves were best fit to data with least squares analysis in all cases to build a model of bark formation on cactus surfaces. Percentages of bark formation were estimated on south-, east-, west-, and north-facing crests and the two associated troughs of each crest on each cactus plant, data from the 12 surfaces were obtained for analysis. Because south-facing crest surfaces exhibited bark formation before other surfaces, bark coverage rate on the 11 other surfaces was compared with bark coverage on south crests. Bark formation occurred on east and west crests about 3 yr after bark formation on south crests, and bark on north crests occurred about 8 yr after bark formation on south crests. On all surfaces, bark formation on troughs occurred after bark formation on crests. On average, the delay in bark coverage on south-, east-, west-, and north-facing troughs was 6−7, 3−5, 5–8, and 4−6 yr, respectively, compared with their respective crests. Young saguaros do not exhibit bark coverage. However, once bark coverage began on south-facing crests at 1.7 m above the ground, on average all stem surfaces at 1.7 m had complete bark covering within 45 yr. This rate of bark coverage is consistent with mortality rates of more than 2% per year for the past several decades among adult saguaros.
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