“Unexpected epiphytes” - Cacti growing on trees in the xerophytic scrubland of the Barranca de Metztitlán, México

Epiphytic growth in Cactaceae has been typically linked to moist habitats. However, there are scattered reports of terrestrial cacti growing on other plants in arid environments. This phenomenon was first mentioned over a century ago but largely ignored by the scientific community. The current study assessed cacti substrate preference and analysed epiphyte-host tree relationships in a xerophilous scrubland in Mexico. It also compared the size structure of epiphytic and terrestrial individuals of common cacti species. Epiphytic growth was found in at least 9 of the 13 locally occurring cacti species. We observed, i.e., globose cacti (Mammillaria polythele, M. crinita) that mainly grew on trees and even produced flowers when growing epiphytically, arborescent cacti (Isolatocereus dumortieri, Myrtillocactus geometrizans) with a high proportion of small individuals growing on trees and larger ones only found as terrestrials, and cacti such as M. longimamma and Opuntia spp., with a clear preference for terrestrial growth. Although we identified seven possible host species at the study sites, 99% of epiphytic cacti were found just on just one of them (Neltuma laevigata). The only other host was I. dumortieri. The study of this phenomenon broadens our understanding of functional diversity in arid ecosystems and provides an area for exploring evolutionary processes and mechanisms of coexistence in one of Mexico's most emblematic plant families. Furthermore, it contributes to the current discussion of the evolution of epiphytism in general.