Periderm coverages on five species of long-lived columnar cactus species of central Mexico

Abstract

Sunlight exposures have resulted in periderm coverages on more than 40 species of tall, long-lived columnar cactus species in the Americas. Periderm coverages start on equatorial surfaces and eventually cover all surfaces. Periderm coverages lead to tissue morbidity and eventual cactus death. After more than 80% of periderm coverage on saguaro (Carnegiea gigantea) cacti, the probability of cactus death was greater than 95% within eight years. Periderm coverage characteristics were not similar for five species of tall, long-lived columnar cactus species endemic to the Tehuacán-Cuicatlán Valley (18° N), Puebla, Mexico. For four species, trough coverages were higher for south-facing surfaces than north-facing surfaces. For these four species periderm ratios were near 2:1, S:N. This 2:1 ratio is coincident with the ratio of direct sunlight exposures on S: N surfaces. For one species, the amounts of periderm were low so no differences in periderm coverages occur of south to north were present. Unlike cacti at other locations, crests of species of this study did not show a directional effect. The results indicate that periderm formation occurred first on south-facing surface and eventually occurred on other surfaces. Results with Random Forest, an algorithm, reinforce results of periderm percentages. Random forest was used to determine surfaces coverages that predict periderm coverages on other surfaces. As expected, neighboring surfaces were the best predictors of periderm coverages. Other surfaces were not good predictors of periderm coverages indicating that other surfaces had periderm coverage before or after the predicted surfaces. Random Forest data were the same for all five species. Although, the patterns of the five species showed some variability, they all support that hypothesis that periderm coverages start on south-facing surfaces and eventually occur on north-facing surfaces and they are caused by sunlight.