DOI: 10.5061/dryad.51c59zwbk
title: What doesn’t kill you can make you stronger: variation in plasticity in response to early temporally heterogeneous hydrological experience

description: Temporally heterogeneous environments may drive rapid and continuous
 plastic responses, leading to highly variable plasticity in traits.
 However, direct experimental evidence for such meta-plasticity due to
 environmental heterogeneity is rare. Our objective was to investigate the
 effects of early experience with temporally heterogeneous water
 availability on the subsequent plasticity of plant species in response to
 water conditions. We subjected eight plant species from three habitats,
 four exotic and four native to North America, to initial exposure to
 either a first round of alternating drought and inundation treatment
 (Ehet, temporally heterogeneous experience) or a consistently moderate
 water supply (Ehom, homogeneous experience), and to a second round of
 drought, moderate watering or inundation treatments. Afterwards the
 performance in a series of traits of these species, after the first and
 second rounds of treatments, was measured. Compared to Ehom, Ehet
 increased final mean total mass of all species considered together, but
 did not affect mean mortality. Ehet relative to Ehom, decreased the
 initial total mass of native species as a group, but increased the mass of
 exotic species or species from hydric habitats; Ehet also increased the
 late growth of natives, but did not for exotics, and increased the late
 growth of mesic species more than xeric and hydric species. Our results
 suggest that previous exposure to temporal heterogeneity in water supply
 may be not beneficial immediately, but can be beneficial for plant growth
 and response to water stress later in a plant’s lifetime. Heterogeneous
 experiences may not necessarily enhance the degree of plasticity, but may
 improve the expression of plasticity in terms of better performance later,
 effects of which differ for different groups of species, suggesting
 species-specific strategies for dealing with fluctuating abiotic
 environments.  Synthesis. Previous temporally heterogeneous
 experience can benefits plant growth later in life though modulating the
 expression of plasticity, leading to adaptive meta-plasticity. Studies of
 meta-plasticity may not only improve our understanding of the importance
 of variable plasticity in relation how plants cope with environmental
 challenges, but also the costs versus benefits of plastic responses and
 its limits over the long term.
description: The species we used in this study were four
 exotic invasive species - <em>Leucanthemum vulgare</em> Lam.,
 <em>Centaurea stoebe</em> L. ssp.
 <em>micranthos</em> (Gugler) Hayek, <em>Leonurus
 cardiaca</em> L. and <em>Potentilla recta</em> L., and
 four native species - <em>Heterotheca villosa</em> (Pursh)
 Shinners, <em>Gaillardia aristata</em> Pursh,
 <em>Agastache urticifolia </em>(Benth.) Kuntze  and Potentilla
 arguta Pursh (Table S1;
 also see Wang et al.
 2017). Thus, three pairs of exotics and natives
 shared families and one pair were congeners. All seeds were collected from
 natural grasslands in western Montana, USA. These species overlap
 substantially in distributions, but the selected groups of target species
 generally occur in habitats that share positions on a soil moisture
 gradient (for details see Wang et al. 2017). 
 Mortality rates and total mass for all treatment
 combinations were calculated. Overall, 720 individuals survived to the end
 of the experiment, and were used for analyses. Traits of total mass, shoot
 mass and root mass and root to shoot ratio were used to assess the early
 performance of species, with these traits and late growth in shoot mass,
 root mass and total mass used to assess their late performance. Late
 growth (LG) in a trait for each species in all early - late treatment
 combinations was calculated with the formula (1) (Wang et al.
 2017): LG = (<em>Y</em> –
 <em>X</em>) /
 <em>X</em>                                                              (1) where <em>X</em> is the mean trait value in one of the two early treatments (the 1<sup>st</sup> round), and <em>Y</em> is the mean trait value in one of the three late treatments (the 2<sup>nd</sup> round) with the same early treatment. Mean values of shoot mass, root mass and total mass were log-transformed, and mortality rates were square root- and arcsine-transformed (by calculating the arcsine of the square root of mortality rate), to minimize variance heterogeneity. Plasticity in response to late inundation or drought conditions in traits was calculated using the Simplified Relative Distance Plasticity Index (RDPIs, (Valladares, Sanchez-Gomez & Zavala 2006), abbreviated as “PI”. We used the formula (2-1) for mass traits and formula (2-2) for late growth of mass traits to calculate plasticity in a given trait of a species: PI = (<em>Y<sub>2</sub></em>－<em>Y<sub>1</sub></em>) / <em>Y<sub>1</sub> </em>                                                            (2-1) PI = <em>Y<sub>2</sub></em>－<em>Y<sub>1</sub></em><sub> </sub>                                                                     (2-2) where <em>Y<sub>2</sub></em> was the mean trait value in inundation or drought (the 2<sup>nd</sup> round) after an early  treatment (the 1<sup>st</sup> round), and <em>Y<sub>1</sub> </em>was the mean trait values in moderate water conditions after the same early treatment. The PI value was regarded as significant when mean values in two late water treatments differed at the 0.05 level (LSD method in one-way ANCOVA). To compare effects of early heterogeneous vs. homogeneous experience among species or late conditions, we defined “difference variables (Diff-value)” as the difference between individuals with the early heterogeneous and homogeneous experiences in a given trait of a  species (Wang et al. 2017), which was calculated with formula (3) as: Diff-Y = Y<sub>het</sub> － Y<sub>hom   </sub>                                                     (3) where Diff-Y was the difference between individuals with early heterogeneous (E<sub>het</sub>) and homogeneous (E<sub>hom</sub>) experiences in the performance of a trait in a late condition, Y<sub>hom</sub> was the mean trait value in the late condition after E<sub>hom</sub> (control), and Y<sub>het</sub> was the mean trait value in the same late condition after E<sub>het</sub>. For a given trait, Diff-values due to early treatments were regarded as significant when its mean values (in the same late conditions) differed between two early treatments at 0.10 level (LSD method in one-way ANOVA). 
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