DOI: 10.5061/dryad.70rxwdc8j
title: Data from: Trait plasticity enables trees and shrubs to live as epiphytes throughout the coast redwood canopy

description: 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.
description: <strong>Site Description</strong>
 Our study took place in Jedediah Smith Redwoods State Park in
 Northern California. This property contains ~10,000 acres of old-growth
 forest at the northern edge of the range of <em>Sequoia
 sempervirens</em>. The region experiences cool, wet winters followed
 by warm, dry summers. The average high temperature, in August, is 18.9º C
 and the average low temperature, in January, is 4 C (weather.gov).  Annual
 precipitation averages 1803 mm per year (weather.gov). In addition, forest
 vegetation intercepts coastal fog that frequently moves through this
 habitat; these inputs comprise approximately 30+% of the overall water
 budget (Dawson 1998). Since epiphyte colonization is often associated with
 the time that supporting architectural features have been in place, these
 groves support the greatest density of vascular epiphytes (Sillett 1999;
 Williams & Sillett 2007). Nine old trees were
 chosen for this study. Diameter, which was measured a few meters above the
 ground at a height above the basal trunk flare (~2-3 m), ranged from
 3.5-5.1 m. These individuals ranged in height from 68.9 to 92.4 m. Prior
 to climbing, each tree underwent a risk assessment by a team of
 experienced research climbers and certified arborists. Lines were set
 using a crossbow with modified arrows connected to a thin monofilament
 line. Once the line was in place, first a 2mm cord, and then a static
 climbing rope, was connected to the monofilament line, which was then
 pulled through the crown (Williams and Sillett 2007). Thin nylon cords
 were kept in the trees during the study to secure the climbing path, but
 at the end of the study, this cordage and all research supplies were
 removed. The approximate coordinates of the study location are 41.76,
 -124.11. Details of the study trees are not provided to protect the
 identity of the trees.
 <strong>Microclimate</strong> In
 three of the study trees, 5-6 iButtons (DS1923-F5#) that measure
 temperature and relative humidity (RH) were placed along a vertical
 transect from the ground (~1m) to the upper crown of each tree (77-87m).
 Sensors were placed on a shrub near the ground, at the height of the
 lowest part of the tree’s crown,n and then at additional points along the
 vertical transect where epiphytes were abundant. Data were logged
 automatically every 20 minutes for the month of October in 2019. Though
 the trees were within 1 km of one another, they were grouped in three
 microsites. One tree in each microsite contained the iButtons. The vapor
 pressure deficit, which is an important metric of evaporative demand on
 plant tissues, was calculated using the following equation:
 VPD =
 (1-(RH/100))*SVP                                                     Eq.
 1 Where SVP is the saturated vapor pressure, which is
 calculated using the following equation (Monteith and Unsworth,
 2008):          SVP = 0.61078 e ^ (17.27T / T + 237.3).
                                   Eq. 2
 <strong>Study Species</strong>
 Three of the five focal species were ericaceous shrubs and lived
 primarily as terrestrial species: <em>Vaccinium ovatum</em>,
 <em>V. parvifolium</em>, and <em>Gaultheria
 shallon</em>. Another focal species,s <em>Tsuga
 heterophylla</em>, western hemlock, grows primarily as a terrestrial
 tree that can achieve heights of 50-70 m.  <em>Polypodium
 scoulei</em> is the only one of our focal species that is an
 obligate epiphyte. Since it rarely occurs on the ground, we also included
 a common terrestrial fern, <em>Polystichum munitum</em>, for
 comparison. We recognize that these two fern species represent more
 genetic variation than would be expected within a species. While an
 examination of traits between two ferns and within the other taxa does not
 represent identical processes, we include this complementary pair of
 closely related species because it provides an opportunity to compare
 intra and interspecific trait variation across epiphytic and terrestrial
 populations. <strong>Sample
 Collection</strong> Within the crown of each
 study tree, we selected an approximately even spatial distribution of up
 to 15 individual epiphytes of each species for detailed study. Since the
 climbing and collection were labor-intensive, samples were collected from
 1-2 trees per day. Samples were collected in the morning, and were then
 brought back to the field lab, where stems were cut under water and
 samples were placed in the dark to rehydrate overnight. Terrestrial
 individuals of the study species were also marked close (&lt;50m) to
 each of the study trees. We attempted to match the size of the terrestrial
 and epiphyte plants included in the study. For woody species, collected
 samples consisted of branches with several terminal branchlets and many
 leaves,s so that a single sample could be used for all the trait
 measurements. For ferns, many leaves in a clump were collected and bagged
 together. <strong>Size
 Measurements</strong> The length, wid,th and
 depth of the crown, defined as the part of the plant with leaves, were
 measured for each of the study individuals for the four woody species. The
 multiplication of these variables resulted in the crown volume,e which was
 used to evaluate differences in size between terrestrial and epiphytic
 individuals. Sample sizes and crown dimensions can be found in Table
 S1. <strong>Trait
 Measurements</strong> <em>One-time trait
 measurements</em> We made all leaf-based
 measurements on young mature leaves that were free of noticeable insect or
 pathogen damage. Leaf dry matter content (LDMC, g g-1), leaf water content
 (LWC, %) and saturated water content (SWC %) were calculated using leaf
 weights, while specific leaf area (SLA, cm2 g-1) was calculated by
 dividing the fresh leaf area (measured with a leaf area meter-CID
 Biosciences CI-202) by dry mass, which was obtained by oven drying samples
 at 60C for 3 days. We calculated LDMC as the dry mass divided by the fresh
 mass of the sample, LWC as the percentage of the dry mass of a fresh leaf
 sample that is water,er and SWC as the leaf water content calculated after
 the samples were rehydrated overnight. We measured stomatal density (SD, #
 mm-2) by coating the underside of the leaf with nail varnish. Once the
 coating was dry and firm, we peeled it from the leaf surface, placed it
 under a microscope, and counted the stomata using a 10x objective in a
 compound light microscope (Parco model 4008, Westland, MI). We calculated
 stomatal density as the number of stomata divided by the area of the field
 of view. We measured leaf thickness (LT, µm) from a fresh leaf section
 that was viewed under the microscope, and wood density (WD, g
 cm<sup>-3</sup>) was measured with the displacement method on
 sections of the main stem of each sample. Dried and ground samples were
 analyzed for C and N content (% dry weight), and stable isotope ratios
 (δ<sup>13</sup>C and δ<sup>15</sup>N respectively)
 were extracted with an elemental analyzer/continuous flow isotope ratio
 mass spectrometry using a CHNOS Elemental Analyzer (vario ISOTOPE cube,
 Elementar, Hanau, Germany) coupled with an IsoPrime 100 mass spectrometer
 (Isoprime Ltd, Cheadle, UK). The long-term analytical precision is 0.08 ‰
 for C and 0.14 ‰ for N. For C-isotope analyses, the standard used was
 V-PDB, and for N-isotope analyses, we used N<sub>2</sub>-gas.
 All isotope analyses were conducted at the Center for Stable Isotope
 Biogeochemistry at the University of California, Berkeley, CA,
 USA. <em>Traits obtained with dehydration trials
 (minimum leaf conductance and pressure-volume curve
 metrics)</em> Using the methodology described in
 Williams et al. (2017), we generated pressure-volume (PV) curves on a
 subset of individuals from each species and study tree to understand the
 dynamics of water retention and drought tolerance. In analyzing the
 relationship between leaf water potential and the relative water content
 of the dehydrating sample, we estimated several important water relations
 traits including the turgor loss point (TLP, MPa), bulk elastic modulus
 (Ɛ, MPa) and the capacitance before the turgor loss point (CT,
 MPa<sup>-1</sup>) (Tyree and Hammel 1972; Koide et al.  2000).
 In total, we completed 55 PV curves on a minimum of 10 individuals per
 species (or functional group in the case of the ferns). The samples were
 evenly spread across the study trees. Since the sample size was smaller
 for these metrics, we restricted our analyses to compare terrestrial
 versus epiphytic individuals. We also measured minimum
 leaf conductance (<em>g</em><sub>min</sub>, mmol
 m<sup>-2</sup> s<sup>-1</sup>) because this metric
 relates to the ability of a species to regulate water loss once the
 stomata are closed as much as possible (Duursma et al. 2019). To measure
 minimum leaf conductance, we removed leaves from their stems and sealed
 the petiole with parafilm to minimize water loss from the cut surface. We
 weighed the leaves every 30 minutes for 4-6 hours while recording the air
 temperature and relative humidity. We estimated
 <em>g</em><sub>min</sub> as the slope of the mass
 over time divided by the average mole fraction of the vapor pressure
 deficit taken over the measurement period (Scoffoni et al. 2018). To
 account for changes in leaf area during dehydration, we calculated
 <em>g</em><sub>min</sub> using the average leaf
 area from the beginning to the end of the experiment (Scoffoni et al.
 2018). <strong>Data
 Analyses</strong> For functional traits and
 stable isotopes, we collected samples from individuals that occupied a
 range in vertical heights. Since some trait data were not normally
 distributed, we calculated the Spearman correlation coefficient to
 evaluate relationships between vertical height in the host tree where a
 plant was found and functional traits, including leaf thickness, stomatal
 density, leaf water content,t, and specific leaf area. (Table S2).
 Variation in stable carbon and nitrogen isotopes with height was also
 evaluated using Spearman correlation coefficients (Table S3). For water
 relations measures, which were more labor-intensive and yielded smaller
 sample sizes (i.,e. PV-curve parameters and
 <em>g</em><sub>min</sub>), variation between
 terrestrial and epiphytic niches was analyzed, as opposed to evaluating
 variation by height. We also performed additional analyses for functional
 traits, based on the niche occupied. Functional traits exhibited a
 non-normal distribution; therefore, the effect of
 <em>niche</em> was evaluated for each trait using a Wilcoxon
 test.  For <em>g</em><sub>min</sub>, which was
 normally distributed, we analyzed the effect of <em>taxon</em>
 and <em>niche</em> (terrestrial vs. epiphyte) using a two-way
 ANOVA followed by post-hoc one-way ANOVAs on each taxon. For the PV-curve
 parameters, for each species, we either performed an ANOVA or a Wilcoxon
 test depending on the distribution ofthe datat, where the effect of
 <em>niche</em> was analyzed (Table S4). For each group of
 analyses (i.,e. functional traits, PV-curve parameters, and stable
 isotopes), we applied a Benjamini-Hochberg procedure for multiple
 comparisons for each factor to determine the threshold for significance
 (<em>Q</em>=0.05). Lastly, the percent change of functional
 traits between epiphytic and terrestrial individuals of the focal taxawase
 was calculated (Table S5). All data analyses were performed in JMP
 (Version Pro 17, SAS Institute, Cary, NC, USA).
description: # Data from: Trait plasticity enables trees and shrubs to live as
 epiphytes throughout the coast redwood canopy Dataset DOI:
 [10.5061/dryad.70rxwdc8j](https://doi.org/10.5061/dryad.70rxwdc8j) ##
 Description of the data and file structure ### Files and variables ####
 File: Trait_averages_by_individual_2025.xlsx The
 "Trait_averages_by_individual_2025_dryad.xlsx" file contains
 functional trait data (columns J-P) for several species of plants that
 live either as terrestrially or as accidental epiphytes in coast redwood
 trees. Below, find an explanation of each column heading * ID= unique
 identifier for each plant - Life form= category refering to whether the
 study individual was living on the ground or epiphytically * Tree= Host
 tree # - Taxon=Two letter abbreviation refering to the first letter of the
 genus and the first letter of the species. See published paper in
 Ecosphere for details * Taxon 2=The only difference here is that PM and PS
 are grouped together as "Fern"  - Ind=The individual number
 within the species * NRows=the number of leaves that were averaged to
 yield the average values per individual that are presented in the table -
 Abs height (m)= This is the absolute height of the location of the study
 plant * Rel height (%)= this is height relative to the total tree height -
 Mean(SLA (cm2/gm))= specific leaf area * Mean(LDMC (gm dry/gm fresh))=leaf
 dry matter content - Mean(Leaf Leaf Water Content (%))=leaf water content
 * Mean(Wood Density (gm/cm3))= wood density - Mean(Stomatal Density
 #/mm2))=stomatal density * Mean(LeafThickness (um))= mean of the total
 leaf thickness - Mean(Hydrenchymal Thickness(um))= thickness of the layer
 of hydrenchymal cells if they were present.  #### File:
 PV_curve_data_compiled_2025.xlsx The second file, is the PV curve output
 on a smaller dataset. A following is a description of those columns: *
 Seq- this can be ignored, this is the original sequence that the data was
 entered - ID- the unique plant identifier * Taxon-the abbreviation used
 which was the first letter of the genus followed by the first letter of
 the species. Please see the Ecosphere paper for details - Fern- whether it
 is a fern or not * Habit- whether the plant was living as an epiphyte or
 terrestrial plant. The columns headings for the PV curve output parameters
 are below: | Variable | Unit | Description | | :------------- |
 :------------------ |
 :------------------------------------------------------------------------------------ | | SWC | g g\^-1\^ | Saturated water content, normalized by dry mass | | AWF | decimal | Apoplastic water fraction | | Ψs~FT~ | MPa | Osmotic potential at full turgor pressure | | Ψ~TLP~ | MPa | Water potential at turgor loss, "turgor loss point" | | total RWC~TLP~ | decimal | Total relative water content at Ψ~TLP~ | | sym RWC~TLP~ | decimal | Symplastic portion of relative water content at Ψ~TLP~ | | total Є | MPa | Bulk tissue elastic modulus, based on total RWC | | sym Є | MPa | Bulk tissue elastic modulus, based on sym RWC | | C~T~ | MPa\^-1\^ | Hydraulic capacitance before Ψ~TLP~, un-normalized | | C~T,mass~  | g g\^-1\^ MPa\^-1\^ | Hydraulic capacitance before Ψ~TLP~, normalized by dry mass | | C~T,area~ | g m\^-2\^ MPa\^-1\^ | Hydraulic capacitance before Ψ~TLP~, normalized by fresh area | | W~T,mass~ | g g\^-1\^ | Water storage capacity before Ψ~TLP~, normalized by dry mass | | W~T,area~ | g m\^-2\^ | Water storage capacity before Ψ~TLP~, normalized by fresh area | | W~GT,mass~ | g g\^-1\^ | Water storage capacity before Ψ~TLP~, corrected for gravity, normalized by dry mass | | W~GT,area~ | g m\^-2\^ | Water storage capacity before Ψ~TLP~, corrected for gravity, normalized by fresh area | | C~t~  | MPa\^-1\^ | Hydraulic capacitance after Ψ~TLP~, un-normalized | | C~t,mass~  | g g\^-1\^ MPa\^-1\^ | Hydraulic capacitance after Ψ~TLP~, normalized by dry mass | | C~t,area~ | g m\^-2\^ MPa\^-1\^ | Hydraulic capacitance after Ψ~TLP~, normalized by fresh area | #### Access information Other publicly accessible locations of the data: * NA Data was derived from the following sources: * Measured values from field and laboratory