Trait plasticity enables trees and shrubs to live as epiphytes throughout the coast redwood canopy

Abstract The radiation of plants into epiphytic niches has largely been studied by comparing closely related taxa that contain both epiphytic and terrestrial species. While these studies have led to important insights, they leave unanswered an important question—what traits allow for the initial colonization of the epiphytic niche? Our study addressed this knowledge gap in an old‐growth coast redwood ( Sequoia sempervirens ) forest, which has few obligate epiphytes but an abundance of accidental epiphytes. We measured structural and water relations traits in four woody species that are largely terrestrial but also commonly exist as epiphytes in the canopy of coast redwood forests: Vaccinium ovatum , V. parvifolium , Gaultheria shallon , and Tsuga heterophylla . We also measured these traits on one obligate epiphytic fern Polypodium scouleri , and one obligate terrestrial fern, Polystichum munitum . Traits were measured across a height gradient from ground level to the upper canopy (68.9–92.4 m). We found that accidental epiphytes exhibited many trait shifts as they occupied higher niches. Height above the ground explained significant variation in stomatal density, specific leaf area (SLA), leaf thickness, and stable isotope composition, which likely are responses to increases in light and decreases in water availability. Despite consistent variation of some traits with height, our study also highlights unique trait combinations for different taxa living as epiphytes. It is extremely rare to study within‐species trait shifts that enable normally terrestrial species to occur as epiphytes, and our study uniquely leveraged the lengthy ground‐to‐canopy height gradient offered by this coast redwood forest to resolve how and why some species are able to make the transition to epiphytism. We found consistent variation in the expression of SLA, stomatal density, turgor loss point, and minimum leaf conductance between terrestrial and epiphytic individuals, despite our focal taxa including a wide range of growth forms (i.e., ferns, shrubs, and trees). These results highlight the potential importance of trait variation in supporting an evolutionary niche shift towards epiphytism. In contrast, some traits we measured exhibited inconsistent variation between epiphytic and terrestrial individuals, indicating that despite some coordinated trait shifts, unique trait combinations facilitate the colonization of the epiphytic niche.