Areole Changes during Bark Formation on Saguaro Cacti

Abstract: Spines are an important component of cactus species. Besides providing protection from predation, they reduce heat loading, reduce transpiration, and aid dispersal of stem segments for vegetative propagation among others. Bark formation occurs on surfaces of more than twenty species of columnar cacti in the Americas. For all species tested, bark formation is related to direct sunlight exposure. Extensive bark coverage leads to premature death for saguaro cacti (Carnegiea gigantea). Preliminary evidence indicates that changes in areoles with spines occurred during the bark formation on C. gigantea. Four hundred, sixty-one paired photographic images of areoles of north- and south-facing surfaces of shoots with bark percentages on adjacent troughs in 2017 were analyzed to determine differences between north- and south-facing surfaces. As in past studies, south-facing surfaces had higher bark percentages (mean bark coverage 62%) than north-facing surfaces (mean bark coverage 19%). For north and south areoles, mean numbers of central spines were 2.70/1.55; radial spines 3.20/1.50; and apex spines 1.81/1.13, respectively. When data were pooled to compare numbers of central, radial and apex spines as well as spine color and spine thickness, all were significantly related with bark percentages. Decreases in numbers and characteristics of spines precede bark formation on adjacent troughs. Overall, the results show that crests deteriorate faster than adjacent troughs. To our knowledge, this is the first report to document changes in spine numbers and characteristics with bark coverage on a cactus species.