A reduced role for water transport during the Cenozoic evolution of epiphytic Eupolypod ferns

Abstract

Summary The Cretaceous–Cenozoic expansion of tropical forests created canopy space that was subsequently occupied by diverse epiphytic communities including Eupolypod ferns. Eupolypods proliferated in this more stressful niche, where lower competition enabled the adaptive radiation of thousands of species. Here, we examine whether xylem traits helped shape the Cenozoic radiation of Eupolypod ferns. We characterized the petiole xylem anatomy of 39 species belonging to the Eupolypod I and Eupolypod II clades occupying the epiphytic, hemiepiphytic, and terrestrial niche, and we assessed vulnerability to embolism in a subset of species. The transition to the canopy was associated with reduced xylem content and smaller tracheid diameters, but no differences were found in species vulnerability to embolism and pit membrane thickness. Phylogenetic analyses support selection for traits associated with reduced water transport in Eupolypod 1 species. We posit that in Eupolypod epiphytes, selection favored water retention via thicker leaves and lower stomatal density over higher rates of water transport. Consequently, lower leaf water loss was coupled with smaller quantities of xylem and narrower tracheid diameters. Traits associated with water conservation were evident in terrestrial Eupolypod 1 ferns and may have predisposed this clade toward radiation in the canopy.

Functional‐trait contrasts between terrestrial and epiphytic ferns in Taiwanese subtropical cloud forests

“…For example, recent work suggested that the evolution of photosynthesis‐related trait co‐regulations and trade‐offs, such as the LES, has been more strongly constrained by leaf water transport capacity in ferns (Sessa & Givnish, 2014; Zhang et al., 2014). The reason behind this is probably a more limited potential for controlling evaporation, low water use efficiency in terrestrial ferns (Brodribb & Holbrook, 2004; Zhang et al., 2014), and constrained hydraulic structure in epiphytic ferns (Pittermann et al., 2023). Because of these limitations, distinct life forms may exhibit different degrees of vulnerability to drought stress, which may account for their relatively small niche width at the more resource‐conservative part of the global LES trait space (Díaz et al., 2016).…”