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

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 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.