When did bark coverages begin on saguaro cacti in southern Arizona?1

Adult plants of saguaro cacti (Carnegiea gigantea) have bark on their surfaces that lead to death. Archived photographs of saguaros taken before 1945 show no bark on surfaces. In contrast, data after the 1980s show that bark is present. Thus, there is a gap between the times when no bark is present and when bark is present. To close this time gap, films produced in the Tucson area were viewed to determine if bark was present during this gap. Concurrently, backward projections of data from 1994 to 2017 indicate that bark should not have occurred before 1960. The only motion picture with adequate saguaros for analysis is Backlash, filmed in Tucson Mountain Park in Tucson, Arizona in 1956. The film contained hundreds of saguaros in panoramic view. Only 67 saguaros from the film could be evaluated. Bark on saguaros did not occur in Tucson before 1956. The following have been established: ultraviolet B radiation (UV-B) causes bark formation; bark occurs on 23 species in the Americas with a wide variety of habitats; once bark coverages begin on south-facing surfaces, all surfaces at 1.7 m above ground level are covered within 45 yr; once north-facing troughs of saguaros have more than 80% bark coverage, death occurs within 8 yr; UV-B levels have increased over past decades. Therefore, UV-B is the primary cause of mortality for adult saguaros in the Tucson, Arizona area.