Hydraulic properties of fern sporophytes: Consequences for ecological and evolutionary diversification

Premise of the study: Ferns are an important component of both tropical and temperate forests; yet, our understanding of the water relations of their sporophyte generation is limited. Indeed, to date there has been no large scale survey that attempts to clarify how ferns fit into current ideas of plant water relations. This study examines several tropical ferns with the goal of understanding how these characters vary between species from various habitats and across life forms METHODS: We measured stipe hydraulic conductivity, water potential, and vulnerability to cavitation along with photosynthetic variables and leaf allometry of 21 species from 14 genera to identify physiological trait assemblages across taxa. Key results: Epiphytic ferns have significantly lower hydraulic conductivity and a vascular system more resistant to cavitation (i.e., higher P₅₀ values). They reached lower mid-day water potentials and produced leaves with reduced stipe lengths and reduced laminar area relative to terrestrial species. Xylem specific hydraulic conductivity (KS) was correlated with the mean hydraulic diameter of tracheids in terrestrial species, but not in epiphytes. There was no evidence of safety-efficiency trade-offs in any group. CONCLUSIONS: When compared across life forms, our data shed light on physiological mechanisms that may have allowed for terrestrial ferns to move into the epiphytic habit. When compared across a diverse assemblage of terrestrial plants, we find that resistance to water flow in fern stipes is significantly higher than that recorded from the stems of seed plants.