# Disentangling the heritable and plastic components of the competitive and facilitative effects of an alpine foundation species

Patrick Al Hayek, Jean Paul Maalouf, Blaise Touzard, Richard Michalet

► To cite this version:

Patrick Al Hayek, Jean Paul Maalouf, Blaise Touzard, Richard Michalet. Disentangling the heritable and plastic components of the competitive and facilitative effects of an alpine foundation species. Journal of Ecology, 2015, 103 (5), pp.1172-1182. 10.1111/1365-2745.12445 . hal-02633542

HAL Id: hal-02633542

https://hal.inrae.fr/hal-02633542v1

Submitted on 7 Aug 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Disentangling the heritable and plastic components of the competitive and facilitative effects of an alpine foundation species

Patrick Al Hayek¹,²*, Jean-Paul Maalouf³, Blaise Touzard¹ and Richard Michalet²

¹UMR INRA 1202 BIOGECO, Université de Bordeaux, Site de Talence, Bâtiment B2, allée Geoffroy St-Hilaire, CS 50023 – 33615, Pessac Cedex, France; ²UMR CNRS 5805 EPOC – OASU, Université de Bordeaux, Site de Talence, Bâtiment B18, allée Geoffroy St-Hilaire, CS 50023 – 33615, Pessac Cedex, France; and ³Addinsoft, 40 rue Damrémont, Paris 75018, France

## Summary

1. Differences in effects between phenotypes of foundation species on subordinate species have been reported, but no study has separated their heritable and plastic components. In a subalpine community of the French Pyrenees, we observed two phenotypes of *Festuca gautieri*: tight cushions in dry convex outcrops with few subordinate species and loose cushions in wet concave slopes with many subordinate species, suggesting differences in effects of the two phenotypes on subordinates.

2. Using two reciprocal transplantation gardens, we studied the responses of the two phenotypes to changes in environmental conditions, thus assessing the contribution of heritability and plasticity to the phenotypic variation. Using a target cross-transplantation experiment, we assessed the plasticity and heritability bases of their contrasting effects. We also quantified the recolonization of resident plants within the reciprocally transplanted cushions.

3. We found that morphological differences between cushions had both heritability and plasticity bases. The two competition experiments showed heritable increase in competitive effects from loose to tight cushions. This was counteracted by plasticity effects, which decreased competition from the benign to the stressful garden.

4. **Synthesis.** We conclude that heritable effects overcome plasticity effects resulting in higher diversity in the loose phenotype from the benign habitats.

**Key-words:** common garden, cushion plants, *Festuca gautieri* (bearskin fescue), phenotypic variation, plant–plant interactions, reciprocal transplantation, stress

## Introduction

Since the call for a new field inquiry bridging community ecology to evolutionary biology (Antonovics 1992; see also Wilson 1976), research in community genetics provided evidence that heritable variation [i.e. variation among individuals that is passed intact from the parents to their offspring] within one species, particularly a foundation species (Ellison *et al.* 2005), may have cascading effects at the community and ecosystem levels through biotic interactions (Antonovics 1992; Whitham *et al.* 2003, 2006, 2008; Pakeman *et al.* 2006; Johnson & Stinchcombe 2007; Gibson *et al.* 2012; Bailey *et al.* 2014). Plants are potentially perfect organisms for assessing the consequences of heritable variation within a focal species on its community. Yet, studies of interactions within plant communities (i.e. plant–plant interactions) have

been minimally explored in an evolutionary context (Lankau & Strauss 2007; Michalet *et al.* 2011; Gibson *et al.* 2012) compared to interactions across trophic levels such as species-specific mutualistic or parasitic interactions (Bailey *et al.* 2009; Bronstein 2009; Crutsinger *et al.* 2014).

In alpine and subalpine systems, plant–plant interactions are known to strongly vary across contrasting local environments induced by elevation and topography. Competition dominates in the subalpine and in concave topographies on deep soils, and facilitation in the alpine and convex topographies on shallow soils (Choler, Michalet & Callaway 2001; Callaway *et al.* 2002; Cavieres & Badano 2009; Michalet *et al.* 2014), consistent with the stress-gradient hypothesis (hereafter SGH, Bertness & Callaway 1994). Facilitation has also been shown to contribute to the diversity of alpine systems (Cavieres *et al.* 2014), increasing diversity at medium to high environmental severity levels by expanding the realized niche of stress-intolerant species into more severe physical

<footnote>*Correspondence author: E-mail: patrick.hayek@gmail.com</footnote>
conditions (Michalet *et al.* 2006). Alpine systems are also
particularly suitable for exploring ecologically differentiated
plant species over a small-scale environmental variation
induced by the relatively rugged topography (Choler *et al.*
2004; Liancourt *et al.* 2013) as topographic heterogeneity
may induce sympatric genetic differentiation within species
(Sambatti & Rice 2006; Lekberg *et al.* 2012).

Phenotypic variation within a plant species may have con-
trasting effects on subordinate species [subordinates hereafter,
that is species found frequently but with low relative cover in
a community including a dominant species (i.e. species found
frequently with high relative cover)] due to their varying traits
(Lankau & Strauss 2007; Crutsinger, Strauss & Rudgers
2010; Michalet et al. 2011; Liancourt et al. 2013). Most cases
of changes in effects of differing phenotypes on subordinates
have been evidenced in studies where the different pheno-
types occur in similar environmental conditions (Callaway,
Nadkarni & Mahall 1991; Crutsinger, Strauss & Rudgers
2010). For example, Callaway, Nadkarni & Mahall (1991)
showed in Californian oak savanna that two phenotypes of
*Quercus douglasii* differing in surface root density induce
contrasting interference effects on their herbaceous under-
storeys. Similarly, Crutsinger, Strauss & Rudgers (2010)
showed contrasting facilitative and competitive effects of erect
and prostrate phenotypes of *Baccharis pilularis*, respectively,
under constant environmental conditions in coastal Californian
dunes. However, most phenotypic variation has been shown
to occur across environmentally contrasting conditions, partic-
ularly across topography and/or soil gradients (Choler &
Michalet 2002; Michalet et al. 2011; Lekberg et al. 2012;
Liancourt et al. 2013). Intraspecific morphological trait varia-
tion in heterogeneous environments can be explained by heri-
table variability and/or phenotypic plasticity [i.e. the capacity
of a genotype to change its expression in response to environ-
mental variation] (Pigliucci 2001; Byars, Papst & Hoffmann
2007; Grassein, Till-Bottraud & Lavorel 2010). Reciprocal
transplant experiments (Joshi et al. 2001; Kawecki & Ebert
2004; Bennington et al. 2012) help revealing local adaptation,
and thus the heritability of trait differences, even over short
range of environmental variation. However, the use of cross-
transplantation competitive experiments is not an easy task in
natural alpine systems. In such hard physical conditions, most
nurse species have too low growth rate to attain a sufficient
size for assessing their effects on other species. For example,
Michalet et al. (2011) found contrasting phenotypic effects of
the alpine foundation species *Geum rossii* on other species in
a field experiment conducted across natural habitats in north-
ern Arizona. They also evidenced the heritable basis of phe-
notypic trait differences in a common garden. However, they
could not straightforwardly conclude on the genetic basis of
changes in effects on other species across habitats because
they could not separate the confounding effects of varying
environmental conditions (i.e. plasticity effects) in a cross-
transplantation competitive experiment with this slow-growing
species.

In a subalpine system of the French Pyrenees, Al Hayek
*et al.* (2014) observed two different phenotypes of *Festuca* L.

gautieri (Hack.) K. Richt. with contrasting patterns of associa-
tion with subordinates in two microtopographic conditions: a
'tight' phenotype with stiff leaves in dry outcrops with few if
any other species within its canopy and a 'loose' phenotype
with soft leaves in wet concave slopes hosting many other
species. In a low-elevation shadehouse experiment, they
showed both the heritability of trait differentiation and the
stronger competitive effect of the tight phenotype as com-
pared to the loose phenotype. However, in a field experiment
in the same subalpine system, they found an opposite result
with a slightly higher competition in the habitat of the loose
phenotype than in the habitat of the tight phenotype. They
concluded that heritable effects counteracted the increase in
competition from the stressful convex habitat to the more pro-
ductive one predicted by ecological theory (Grime 1973; Bert-
ness & Callaway 1994) and evidenced by several authors in
subalpine and alpine systems (Choler, Michalet & Callaway
2001; Callaway et al. 2002; Cavieres & Badano 2009; Micha-
let et al. 2014). They argued that heritable effects contribute
to maintain diversity in this subalpine system through
decreasing competition in the predicted highly competitive
and species-poor concave habitat. However, Michalet et al.
(2014) have shown for alpine and subalpine communities that
an increase in competition from a stressful habitat to a more
benign one might depend on two effects. It could be due to
either an increase in the target performance growing without
neighbours (defined as strict environmental severity effect) or
a decrease in the target performance growing within neigh-
bours (i.e. neighbour trait effect). Thus, the increase in com-
petition with the decrease in stress is eventually due to the
plasticity of competitive effects in absence of interspecific
turnover of nurse species. Al Hayek et al. (2014) could not
separate these two effects in their study because they did not
quantify the plasticity of the competitive effects (changes in
effects of a single genotype on subordinates along an environ-
mental gradient). Moreover, the heritable basis of the differ-
ences in phenotypic traits and facilitative/competitive effects
was not assessed in the natural environment of the cushions
but in a low-elevation shadehouse (with no physical stress),
which is particularly problematic for assessing facilitation
known to occur in stressful conditions (Bertness & Callaway
1994).

Thus, in this study conducted in the same system formerly
explored by Al Hayek *et al.* (2014), we separate the relative
contribution of heritability and phenotypic plasticity to
changes in phenotypic traits and competitive/facilitative
effects across habitats, using two subalpine common gardens
mimicking the natural habitats of the two phenotypes of
*F. gautieri*. Our approach is novel as it allows not only the
isolation of the community level traits of a foundation spe-
cies, but also the examination of their consequences on inter-
actions, which is less well understood. First, we transplanted
adult cushions of the two phenotypes in both gardens and
measured their morphological traits to assess the heritability
and plasticity bases of changes in phenotypic traits between
phenotypes across natural habitats. We then transplanted three
types of targets (*Agrostis capillaris* L., tight *F. gautieri* and
loose *F. gautieri*) within the reciprocally transplanted cushions and outside cushions (open area). This was done in order to assess the heritability and plasticity bases of differences in cushion's competitive and facilitative effects across natural habitats. Additionally, as our ultimate goal is to assess the ecological consequences of phenotypic variation for the diversity of this subalpine system, we counted the number of naturally re-established subordinates (species richness) in all cushions and open areas between cushions in both gardens. This latter survey will allow us to assess the relevance of our target competition experiment for community richness.

## Materials and methods

### STUDY SITE AND TARGET SPECIES

The experimental site is located in the western Pyrenees, 100 km from the Atlantic Ocean at La Pierre Saint-Martin pass (42°58' N, 0°45' W, altitude: 1744 m a.s.l., Pyrénées-Atlantiques, France). Climate is temperate oceanic with a mean winter temperature of -2.2 °C and a mean summer temperature of 11.5 °C; mean annual precipitation is 2850 mm with the highest rainfall occurring in winter. Our target species, *Festuca L. gautieri* (Hack.) K. Richt., is an outbreeder grass (Fletcher & Stace 2000) with a circular to elliptic cushion-like form with thin bright green leaves and yellowish-green inflorescences flowering from July to August. It is a foundation species typical of subalpine and alpine grasslands on rocky calcareous soils in the entire Pyrenees (Saule 1991). Two contrasting phenotypes of *F. gautieri* cushions occur in the system across contrasting topographic and soil conditions (Al Hayek et al. 2014; Le Bagousse-Pinguet et al. 2014). A tight phenotype dominates convex topographic habitats with shallow, stony and relatively dry and unfertile soils, and a loose phenotype dominates concave topographic habitats with deep, less stony and relatively wet and fertile soils (Al Hayek et al. 2014). Tight phenotypes have been shown to be more drought tolerant than loose ones (Le Bagousse-Pinguet et al. 2014). Both phenotypes may occur side by side, as the scale of habitat heterogeneity is <1 m. Moreover, cushions with intermediate traits occur in intermediate ecological conditions, but they are much less frequent than the two extreme phenotypes (Al Hayek et al. 2014). Inflorescence cover is high for tight phenotypes, intermediate for intermediate phenotypes and low for loose phenotypes (Al Hayek et al. 2014). Loose phenotypes have a higher subordinate richness and abundance than intermediate or tight phenotypes [n = 30, one-way analysis of variance (ANOVA): *P* < 0.001, Fig. 1, also see Table S1 in Supporting Information]. Most frequent species of the community are *A. capillaris* L., *Galium pumilum* Murray, *Campanula rotundifolia* L., *Alchemilla conjuncta* Bab., *Lotus corniculatus* L., *Festuca rubra* L. and *Trifolium pratense* L.

### RECIPROCAL CUSHION TRANSPLANT EXPERIMENT

In order to assess the heritability and plasticity of cushion traits in natural conditions, we set up a reciprocal transplant experiment in two experimental gardens mimicking the environmental conditions of the natural habitats of the two phenotypes. On November 2010, we collected eight *F. gautieri* cushions (hereafter genotypes) of each phenotype, tight and loose, at our site. We chose distant, adult and discrete cushion individuals in order to maximize chances of selecting genetically different cushions. Each cushion was separated into 10

tillers of 5-10 leaves, which were grown for 32 months in a glasshouse. Each cushion was planted in a 4-L pot with a soil-less mix of peat moss, perlite and gravels in 1:1:0.5 proportions. Pots were stored during the first winter in an unheated glasshouse. In late April 2011, pots were randomly placed on benches installed in a shadehouse transmitting 50% of light without intercepting precipitation, in order to protect plants from direct sunlight and air desiccation. In 2011 and 2012, between late April and late September, pots were irrigated with 1 L of tap water (watered pots) three times a week [see Al Hayek et al. (2014) for more detailed methodology]. Early July 2013, when the cushions had attained an adult size (approximately 20 cm in diameter), we transplanted them in the two experimental gardens at our site. The first garden (convex site hereafter) was set up in a convex topographic position (Fig. S1) on shallow, stony and relatively dry soil [soil volumetric water content measured with a ML3XTheta-Porbe (Delta-T Devices, Cambridge, UK) 3 days after a rain event, 7.13 ± 1.57, *n* = 25] mimicking the tight cushion's natural microhabitat. The second garden (concave site hereafter) was set up in a concave topographic position (Fig. S1) on deep, less stony and relatively wet soil (soil volumetric water content, 30.43 ± 0.85, *n* = 25) mimicking the loose cushion's natural microhabitat. The two sites (convex and concave) were 50 m apart, with a surface of 46 m² each. In each site, we transplanted five adult cushions of each genotype of each phenotype (phenotype treatment; total cushions per site: *n* = 80). The 80 cushions were randomly transplanted, 50 cm distant from each other and tagged with metal rings. All cushions were watered once after transplantation. Before planting out, resident vegetation was manually eliminated, and a mulching canvas, permeable to water and air, was spread out to inhibit the regeneration of resident vegetation. New recruits or regrowth of resident vegetation was removed again in September 2013 after the growing season. Circular holes of 30 cm in diameter were made in the mulching canvas in order to transplant the cushions, and two paired holes were made in open areas near each cushion. One of the paired open areas was used in the competition experiments; the other one was used to assess the effects of cushions on the natural re-establishment of subordinates. The sites were fenced with metallic nets (height: 1 m - mesh: 10 cm × 10 cm) to prevent herbivory.

In July 2014, we measured five morphological traits that were related to field differences in performance and fitness between phenotypes (Al Hayek et al. 2014). Cushion surface, maximum leaf length, cushion penetration (an index of cushion compactness and interference, Al Hayek et al. 2014), leaf thickness and leaf density were measured on both tight and loose cushions (phenotype treatment) in both sites. All morphological traits were averaged per cushion genotype in each treatment combination (site × phenotype for morphological traits, and site × neighbour for naturally re-established species richness and abundance).

### RECIPROCAL TRANSPLANT COMPETITION EXPERIMENT

In order to assess the concurrent contribution of both the heritable and plastic components to the variation in competitive and facilitative abilities (effect and response) among *F. gautieri* phenotypes, we set up a reciprocal transplant competition experiment in the same sites in July 2013. We manipulated cushion presence and phenotype (neighbour treatment, microhabitat (site treatment) and target identity (target treatment). The neighbour treatment was applied by planting targets either within a tight or a loose cushion, or alone without cushion (open area). For the target treatment, in order to cover a
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Means + SE (n = 30) of species richness (a) and abundance (b) of subordinate plant species within tight, intermediate and loose cushions of *Festuca gautieri* in their natural habitats. Significant results of one-way ANOVAS on the effect of phenotype are indicated: ***, *P* < 0.001. Results of Tukey tests for significant phenotype effect are shown on top of error bars.</figcaption></figure>

representative panel of possible responses to neighbouring cushions (Brooker *et al.* 2008), we used two drought-intolerant types of targets, that is the loose *F. gautieri* phenotype itself and *A. capillaris* (the most frequent subordinates), and a drought-tolerant target, the tight *F. gautieri* phenotype itself. We used both *F. gautieri* phenotypes as targets in order to assess not only their effects but also their responses to neighbours, as both effect and response are important components in determining a species competitive and facilitative abilities (Goldberg 1990; Liancourt, Viard-Cretat & Michalet 2009; Le Bagousse-Pinguet *et al.* 2013). We planted the three target individuals (one tight and one loose *F. gautieri*, and one *A. capillaris*) within each cushion's canopy with an equal distance of 10 cm between them, and in the paired open area near the cushion (in the holes previously made in the mulching canvas). As transplanted targets remained very small during the whole experiment, this distance was sufficient to avoid any interaction between target individuals within a single cushion. We used five different genotypes per target with eight replicates per genotype. A replicate of target genotype was planted in each cushion genotype. This genotypic sampling was applied in order to maximize differences in competitive ability within phenotypes, but was not used as a statistical treatment due to insufficient replication. Transplanted targets were randomly collected at the site in five distant and discrete cushions for each *F. gautieri* phenotype and in five distant and discrete patches for *A. capillaris*. Targets had from 5 to 10 leaves. All 720 targets were tagged with metal rings. Beginning of August 2013, 93 dead targets were replaced.

In September 2013 (after summer season), we recorded survival and total leaf number for each target. We also recorded survival in June 2014 but did not measure growth (total leaf number) since there was very low survival in some treatments. Survival was expressed in percentages calculated for all targets (regardless of their genotype) transplanted within the same cushion genotype, then averaged per treatment combination (site-neighbour-target) before statistical analyses. Total leaf number measurements were also averaged per cushion genotype in each treatment combination.

## SURVEY OF NATURALLY RE-ESTABLISHED PLANT SPECIES

In July 2014, we recorded the number of naturally re-established plant species (richness) and their abundance within all the cushions and in 40 randomly selected paired open areas (neighbour treatment) in both sites (site treatment), using quadrats of 20 × 20 cm, in order to quantify their responses to the effects of both cushion phenotypes. These naturally re-established plant species (resident vegetation) were lastly eliminated at the beginning of autumn 2013 (to limit competition with transplanted cushions). Species richness and abundance were expressed per 400 cm² (mean cushion size) due to differences in cushion surfaces and then averaged per cushion genotype in each treatment combination (site-neighbour) before statistical analyses.

## CALCULATION OF BIOTIC INTERACTION INDEX

We quantified the net effect of cushion phenotypes on transplanted targets and on naturally re-established resident species in each treatment combination using the Relative Interaction Index (RII, Armas, Ordiales & Pugnaire 2004) for target survival (RII<sub>survival</sub>) and total leaf number (RII<sub>total leaf number</sub>) for the transplanted targets, and RII for species richness (RII<sub>Richness</sub>) and abundance (RII<sub>Abundance</sub>) for the naturally re-established species with:

$$RII = \frac{P_{+neighbour} - P_{-neighbour}}{P_{+neighbour} + P_{-neighbour}}$$

where $P_{+neighbour}$ and $P_{-neighbour}$ represent target-type performances (survival and total leaf number) in the presence and absence of a neighbouring cushion, respectively. This index is symmetrical around zero (no significant interaction) and has defined limits between -1 and +1. Negative values indicate competition, whereas positive values reflect facilitation.

## DATA ANALYSIS

Broad-sense heritability of cushion traits (cushion surface, maximum leaf length, cushion penetration, leaf thickness and leaf density) was calculated following Falconer & Mackay (1996) and Connor & Hartl (2004). Each trait was introduced as a dependent variable in the following random-effects model:

$$y = \alpha + \beta_1 \text{Site} + \beta_2 G + \beta_3 (G \times \text{Site}) + \varepsilon$$

where y is the trait vector; α is a fixed intercept; Site and G are the vectors of the random effects associated with site (garden) and genotype, respectively; G × Site denotes the interaction between genotype and site; ε is the residual error; β₁, β₂, β₃ are the random-effect coefficients. For each trait, this model involves four variances: $\sigma_s^2$, $\sigma_g^2$, $\sigma_{g \times s}^2$ and $\sigma_\varepsilon^2$, corresponding to site, genotype, genotype × site and residual variances, respectively. Variances were estimated using restricted maximum likelihood assuming normal distributions for the random effects.

Variance components were used to obtain broad-sense heritability $h^2$:

$$h^2 = \frac{\sigma_g^2}{(\sigma_g^2 + \sigma_{g \times s}^2 + \sigma_\varepsilon^2)}$$

Heritability 95% confidence limits were computed as the 2.5% and 97.5% centiles of heritability estimations arising from 1000 data sets obtained by bootstrapping the original data table at the genotype level. For each trait, the significance of each of the $\sigma_s^2$, $\sigma_g^2$, $\sigma_{g \times s}^2$ random effects was assessed by a one degree of freedom chi-square likelihood ratio test comparing the original model to a model excluding the corresponding random effect.

Within each trait, variance components were used to compute coefficients of variation $CV_{\sigma^2}$:
$$CV_{\sigma^2} = 100 \times \frac{\sqrt{(\sigma^2)}}{\text{mean of the trait in the considered data set}}$$

Differences in cushion traits were analysed using a two-way ANOVA (followed by Tukey tests when necessary) with site and phenotype as treatments.

For the reciprocal transplant competition experiment, we conducted separate analyses per type of target for survival and growth data, and one analysis grouping all types of targets for RII data. Differences in survival and total leaf number of transplanted targets were analysed using a two-way ANOVA model (followed by Tukey tests when necessary) with neighbour and site as treatments and survival and total leaf number as response variables. Differences in cushion's net effects on target survival and growth were analysed using a three-way ANOVA model with site, phenotype and target as treatments and RII<sub>total leaf number</sub> as response variables. Differences in re-established plant species richness and abundance were analysed using a two-way ANOVA (followed by Tukey tests when necessary) with site and neighbour as treatments and species richness and abundance as response variables. Richness and abundance data were treated with ANOVA models instead of generalized linear models, as they were not discrete but continuous, resulting from averaging per cushion genotype in each treatment combination. Differences in cushion's net effects on re-established plant species richness and abundance were analysed using a two-way ANOVA model with site and phenotype as treatments and RII<sub>Richness</sub> and RII<sub>Abundance</sub> as response variables. One-sample *t*-tests were used to analyse significant deviation of RII values from zero, which represents no cushion effect.

All dependent variables were checked for normality and log-transformed (cushion traits, target growth, and re-established plant species richness and abundance) or arcsine square-root transformed (target survival) before parametric tests. All statistical analyses were done using R (R Development Core Team 2013).

# Results

## RECIPROCAL CUSHION TRANSPLANT EXPERIMENT

After one growing season, *F. gautieri* cushion traits were significantly affected by both the phenotype and site treatments (Fig. 2 and Table S2). Cushions were twice bigger in the concave than in the convex site (site effect: *P* < 0.001). However, the increase in cushion surface from the convex to the concave site was slightly higher for loose than tight cushions (*S* × *P* effect: *P* < 0.05, Fig. 2a). Cushion leaves were 40% longer in the concave than in the convex site (site effect on maximum leaf length: *P* < 0.001) and 17% longer for loose than tight cushions (phenotype effect: *P* < 0.01, Fig. 2b). Cushion's penetration was 40% higher in the concave than in the convex site (site effect: *P* < 0.001), and 25% higher for loose cushions than for tight cushions (phenotype effect: *P* < 0.001, Fig. 2c). Leaves were 5% thicker in the convex than in the concave site (site effect: *P* < 0.05) and 15% thicker for tight than loose cushions (phenotype effect: *P* < 0.001, Fig. 2d). Cushions had overall a higher leaf density in the convex than in the concave site (site effect: *P* < 0.001), but this effect differed among phenotypes (*S* × *P* effect: *P* < 0.001, Fig. 2e): tight cushions had 15% more leaves per unit area than loose cushions in the convex site, but a converse effect was observed in the concave site.

The genotypic variance components were insignificant for three cushion traits [cushion surface, *h*² = 0.0239 (CL 0; 0.0728); maximum leaf length, *h*² = 0.0698 (CL 0.0152; 0.127); and leaf density, *h*² = 0 (CL 0; 0)] and significant for two traits [cushion penetration, *h*² = 0.1492 (CL 0.0588; 0.2288), *P* < 0.01 and leaf thickness, *h*² = 0.3598 (CL 0.2086; 0.4799), *P* < 0.001]. Environment × genotypic effects interaction plots, variance components as well as CV are provided in Supporting Information (Fig. S3 and Table S3).

## RECIPROCAL TRANSPLANT COMPETITION EXPERIMENT

In September 2013, after summer season, there was a high survival for *A. capillaris*, which was affected by neither the site and neighbour treatments nor their interaction (Fig. S2a and Table S4). In contrast, the two other types of targets had a much lower survival, in particular in the open of the convex site (significant *S* × *N* interactions, Fig. S2b,c). Thus, RII results showed an overall switch from facilitation in the convex site to competition in the concave site (site effect: *P* < 0.001, Fig. S4, also see Table S5), and in particular for the two fescue targets (significant *S* × *T* interaction, *P* < 0.05). In contrast, differences in effects of the two phenotypes were marginally significant at this first date (phenotype

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Means + SE (n = 8) of cushion surface (a), maximum leaf length (b), cushion penetration (c), leaf thickness (d) and leaf density (e) of tight and loose *Festuca gautieri* cushion phenotypes in the convex and concave sites. Significant results of two-way ANOVAS on the effects of site (S), phenotype (P) and their interaction are indicated. Tukey tests are shown on top of error bars when a significant site × phenotype (S × P) interaction is shown. *, *P* < 0.05; **, *P* < 0.01; ***, *P* < 0.001.</figcaption></figure>
effect: *P* < 0.1). In June 2014, 1 year after the beginning of the experiment, survival of *A. capillaris* was significantly higher without neighbours than within tight or loose cushions (neighbour effect: *P* < 0.001, Fig. S5a, also see Table S6), whereas survival of the tight fescue target was significantly higher without neighbour and within loose cushions than within tight cushions [neighbour effect: *P* < 0.01, Tukey results: open area (a), tight *F. gautieri* cushion (b), loose *F. gautieri* cushion (a), Fig. S5b]. For the loose fescue target, survival was higher in the convex rather than in the concave site (site effect: *P* < 0.05, Fig. S5c). Thus, RII results showed that competition for survival was stronger in the concave than in the convex site (site effect: *P* < 0.01, Fig. 3a and Table S7), but the most significant effect was the phenotype effect, with an overall increase in RII from tight to loose cushions (phenotype effect: *P* < 0.001, Fig. 3a). There were also significant differences in competitive responses for survival between targets [target effect: *P* < 0.001, Tukey results: *A. capillaris* (ab), tight *F. gautieri* target (b), loose *F. gautieri* target (a)] as the loose fescue target was the strongest response competitor and the tight fescue target the weakest. We also found a significant site × phenotype × target interaction (S × P × T, *P* < 0.05) due to a strong shift from positive to negative RII from the convex to the concave site, only observed for the tight fescue target within the loose cushions (Fig. 3a). This latter result shows the stronger plasticity of the effect of the loose cushion as compared to the tight one, as well as the lower competitive response of the tight fescue target as compared to the two other types of targets.

For growth, in September 2013, there was a highly significant neighbour effect for *A. capillaris* as there were more leaves without than within both cushion types (neighbour effect: *P* < 0.001, Fig. S6a, also see Table S8), in particular in the concave site (significant *S* × *N* interaction: *P* < 0.05). We also found a significant *S* × *N* interaction for the loose fescue target (*P* < 0.05, Fig. S6c) as there were more leaves in the open area and the loose *F. gautieri* cushions than within tight cushions in the concave site but no differences in the convex site (see Tukey results in Fig. S6c). In contrast, there was no significant effect of any treatment for the tight *F. gautieri* (Fig. S6b). RII<sub>total</sub> leaf number results showed significant differences in targets' competitive response, with *A. capillaris* being the weakest response competitor [target effect: *P* < 0.001, Tukey results: *A. capillaris* (b), tight *F. gautieri* (a), loose *F. gautieri* (a), Fig. 3b, also see Table S9]. There was an increase in competition, or a decrease in facilitation, from the convex to the concave site (site effect: *P* < 0.05) and no phenotype effect at this date. We also found a significant *S* × *T* interaction (*P* < 0.05) because there was a decrease in RII from the convex to the concave site for both *A. capillaris* and the loose *F. gautieri* targets but not change for the tight *F. gautieri* target. Specifically, the loose fescue target was facilitated only in the convex site and competition increased for *A. capillaris* in the concave site (Fig. 3b).

RE-ESTABLISHMENT OF SUBORDINATE SPECIES

The natural re-establishment of subordinates within tight and loose cushions (in both sites) was significantly affected by the site and phenotype treatments (Fig. 4 and Table S10). For species richness, there was a significant site effect (*P* < 0.001, Fig. 4a) with a higher richness in the convex than in the concave site. Additionally, RII results showed that competition for species richness was significantly higher in the concave than in the convex site (site effect: *P* < 0.001, Fig. 4c), but there was a tendency for this effect to be stronger for the tight than loose phenotype (marginally significant *S* × *P* interaction and highly significant competition within the tight phenotype in the concave site, see result of the sample *t*-tests in Fig. 4c). For abundance, there was a significant neighbour effect (*P* < 0.05, Fig. 4b), as individuals re-established more within loose than tight cushions with intermediate values in the open [Tukey results for the neighbour effect: open area (ab), tight *F. gautieri* (b) and loose *F. gautieri* (a)]. Thus, there was a significant phenotype effect on RII<sub>Abundance</sub> (*P* < 0.01, Fig. 4d) with tight cushions being highly competitive and loose ones neutral or weakly facilitative. Additionally, there was a significant site effect (*P* < 0.05) as RII<sub>Abundance</sub> became less negative for the tight cushion and shifted from negative to positive for the loose cushion from the convex to the concave site.

Discussion

We aimed to assess the heritability and plasticity bases of the
differences in morphological traits of the two phenotypes of
*F. gautieri* and of their effects on subordinates. Our results
indicated that both heritability and plasticity account for dif-
ferences in phenotypic traits and in effects on subordinates.
However, both results from our transplant competition experi-
ments and survey of naturally re-established subordinates
showed that heritable and plastic effects act in opposite direc-
tions with increasing stress. Heritable effects are in line with
an increase in competition with increasing stress, whereas
plastic effects are in line with a decrease in competition with
increasing stress.

Our reciprocal cushion transplant experiment indicated a
significant variation in morphological traits between the tight
and loose *F. gautieri* phenotypes for three of the five mea-
sured traits. Additionally, we found significant broad-sense
heritability for the two traits likely to be involved in cushion
effects on subordinates, cushion penetration and leaf thick-
ness. Tight cushions were more compact (lower penetration)
with shorter and thicker leaves than loose ones. The pattern
was consistent across topographic position (site treatment)
4 years after field collection, suggesting that a heritable com-
ponent for the intraspecific morphological differences
observed across natural habitats very likely exists. This is
consistent with the experiment of Al Hayek et al. (2014) who
found that differences in morphological traits between trans-
plants of both *F. gautieri* phenotypes were maintained 2 years
after being grown under the same environmental conditions.
<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Means ± SE (n = 8) of (a) Relative Interaction Index (RII)survival (June 2014) and (b) RIItotal leaf number (September 2013) of the three targets (*Agrostis capillaris*, the tight *Festuca gautieri* and the loose *F. gautieri*) calculated for both cushion phenotypes in convex and concave sites. Significant results of the three-way ANOVA on the effects of the site (S), phenotype (P), target (T) treatments and their interactions are indicated. Tukey tests are shown on top of error bars when a significant site × phenotype × target (S × P × T) interaction is shown. Also, results of one-sample *t*-tests are shown above error bars: (*), *P* < 0.1; *, *P* < 0.05; **, *P* < 0.01; ***, *P* < 0.001.</figcaption></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Upper panels: means ± SE (n = 8) of species richness (a) and abundance (b) of naturally re-established plant species in open areas and within tight and loose cushions of *Festuca gautieri* in the convex and concave sites. Significant results of the two-way ANOVAS on the effects of the site and neighbour treatments and their interaction are indicated: *, *P* < 0.05; ***, *P* < 0.001. Lower panels: mean ± SE (n = 8) of RIIRichness (c) and RIIAbundance (d) of naturally re-established plant species for the two *F. gautieri* cushion phenotypes in convex and concave sites. Significant results of the two-way ANOVAS on the effects of the site (S) and phenotype (P) treatments and their interaction are indicated, and significant results of one-sample *t*-tests on RII values are shown above (or below) error bars: (*), *P* < 0.1; **, *P* < 0.01; ***, *P* < 0.001.</figcaption></figure>

Other authors have shown heritable morphological variations within plant species [e.g. Rowland (2001) between four populations of *Populus deltoides*, Crutsinger, Strauss & Rudgers (2010) for erect and prostrate *B. pilularis* phenotypes, Michallet *et al.* (2011) for tight and loose *G. rossii* phenotypes]. Additionally, tight and loose cushions differed significantly in their optimal environmental conditions for two growth traits, surface and leaf density. We found higher performance for

these traits for tight than loose cushions in the convex site (high stress) and vice versa in the concave site (low stress). This suggests a phenotypic specialization and thus an adaptation of tight cushions to high-stress habitat, and loose cushions to low-stress habitat, consistent with their natural distribution. Heritable differences in traits among phenotypes reveal the occurrence of differences in functional strategies, the loose phenotype being more exploitative with long and
thin leaves and the tight one more conservative with short and thick leaves (Grime 1974; Tilman 1982; Liancourt, Callaway & Michalet 2005), consistent with the results of Michalet et al. (2011) for the alpine cushion species *G. rossii*. On the other hand, varying environmental conditions across sites had a significant effect on all morphological traits, with cushions in the concave site having longer leaves, larger surface and higher penetration, and cushions in the convex site having thicker leaves and higher leaf density. Thus, variation in morphological traits across natural habitats is also in part due to plasticity effects, which have a high ecological importance through changes in effects on dependent species.

Heritable and plasticity effects acted in the same ecological direction, with the tight phenotype from the convex site having cushions with the lowest penetration and the thickest and shortest leaves, and the loose phenotype from the concave site showing the opposite traits. Byars, Papst & Hoffmann (2007) also found patterns of cogridient selection within the alpine grass *Poa hiemata* along an elevation gradient, with both environmental and genetic factors contributing to the occurrence of short leaves at high altitude and long leaves at low altitude. Like in our study, phenotypes with shorter leaves may be selected in stressful conditions as a selection response to reduce water loss through transpiration and phenotypes with longer leaves in more benign conditions due to increasing biotic pressures (Arendt 1997).

Results of our competition experiment also showed that both heritable and plastic effects account for differences in competitive and facilitative abilities between *F. gautieri* phenotypes. Competition increased from the convex to the concave sites for both target survival and leaf number. Depending on the performance variable and types of targets, and following Michalet et al. (2014), this increase in competition with decreasing stress was due to either an environmental severity effect (increase in the performance without neighbours, e.g. Fig. S6a) or a neighbour trait effect (decrease in the performance with neighbours, e.g. Fig. S5c). In the former case, the decrease in RII was only due to the better conditions found by the target without neighbours in the concave than in the convex habitat, whereas in the latter case RII decreased across sites because the fescue neighbour increased its negative effect on the target. This shows that both environmental changes and plasticity of cushion effects contributed to this increase in competition across sites. For survival, the phenotype effect was even stronger than the site effect with a higher competitive effect for the tight than for the loose phenotype. This effect increased through time with only a marginally significant effect in September 2013 but a highly significant effect at the end of the experiment in July 2014. This asserts our prediction of higher competitive effects for tight than loose *F. gautieri* phenotypes and proves the heritability of these effects. Tight cushions had higher competitive effects than loose ones, probably due to lower penetration (related to higher leaf thickness and stiffness), inhibiting target species within the cushion's canopy. Interestingly, these two traits most involved in effects on subordinates were the traits showing the highest heritability. Contrasting effects of differ-

ent phenotypes of foundation species on subordinates have been evidenced in several studies (Callaway, Nadkarni & Mahall 1991; Pugnaire, Haase & Puigdefabregas 1996; Rudgers & Maron 2003; Crutsinger, Strauss & Rudgers 2010; Michalet et al. 2011; Cranston et al. 2012; Schöb et al. 2013; Al Hayek et al. 2014). However, when the different phenotypes occur in different environments, no previous study has shown whether the contrasting phenotypic effects are due to heritable or plastic trait variation in the foundation species.

Interestingly, we found that heritable and plastic competitive effects increased in opposite directions along the stress gradient. Plastic competitive effects increased with decreasing stress from the convex to the concave sites, consistent to the SGH (Bertness & Callaway 1994) and the results of several experiments (Choler, Michalet & Callaway 2001; Callaway et al. 2002; but see Maestre & Cortina 2004). In contrast, heritable competitive effects increased from the loose to the tight phenotypes, showing that heritable effects are in line with an increase in competition with increasing stress. This latter effect contradicts Grime's (1974) model and the SGH (Bertness & Callaway 1994) but is consistent with other theoretical models proposing either the occurrence of high competition in nutrient-poor soils (Tilman 1982) or even an increase in competition with decreasing resource availability (MacArthur & Wilson 1967; Taylor, Aarssen & Loehle 1990). This alternative prediction has been supported by several experiments, in particular in dry environments (Davis, Wrage & Reich 1998; Tielbörger & Kadmon 2000; Maestre & Cortina 2004; Saccone et al. 2009).

Our assessment of richness and abundance of naturally re-established subordinates brought additional evidence that differences in competitive and facilitative effects among *F. gautieri* phenotypes have both heritability and plasticity bases. As in the transplant competition experiment, we found a heritable increase in competition from the loose to the tight phenotype for both richness and abundance. Plasticity effects were also consistent with those of the transplant competition experiment for richness, with an increase in competition from the dry to the wet site due to a neighbour trait effect. Thus, our results overall go along with those of the transplant competition experiment and, therefore, reinforce their relevance for understanding interactions occurring in natural habitats. However, we acknowledge that there might be confounding factors in the experiment that have not been realized, and therefore, these results should be considered carefully. Yet, there were differences between the patterns of resident species re-establishment and the richness and abundance of subordinates within the two natural habitats. For the loose phenotype, the number of the re-established subordinates (richness) in the concave site was twice as high as the number of subordinates occurring in loose cushions in their natural habitat, and their abundance was similar (see Figs 1 and 4a,b). In contrast, for the tight phenotype, both the re-established subordinates richness and abundance were much higher in the convex site than in the natural habitat (ten times higher for richness and four times for abundance, see Figs 1 and 4a,b). This suggests that the effects of the loose cushions are almost completely established and similarly expressed as in their natural habitat, whereas the effects of the tight cushions
are far from being established and, thus, not identically expressed as in their natural habitat.

To conclude, our results showed that both heritability and plasticity play a role in the variation of the competitive effects between tight and loose *F. gautieri* phenotypes. The most important result of our study was certainly to demonstrate that heritable and plastic effects act in opposite directions, with the former enhancing competition and the latter decreasing it with increasing stress. This shows that, in contrast to cushion phenotypic traits, cushion competitive effects showed countergradient patterns of selection (Antonovics 1976; Sultan 1987; Eckhart, Geber & McGuire 2004; Byars, Papst & Hoffmann 2007). In order to assess the ecological outcome of phenotypic variation for the diversity of this subalpine system, it is worth trying to evaluate which of the heritable and plastic components of the competitive effects have a higher consequence on diversity. Diversity is thought to increase in semi-stressful environments due to the decrease in competition (Grime 1973) and to an increase in facilitation, and again to decrease at extreme stress level due to the collapse of facilitation (Michalet et al. 2006) and the too stressful conditions for most plants (Grime 1974), as shown by Le Bagousse-Pinguet et al. (2014) in the same subalpine system. Thus, as the habitat of the tight phenotype is located in semi-stressful conditions (Le Bagousse-Pinguet et al. 2014), we may predict that diversity should increase from the natural habitat of loose *F. gautieri* cushions (low stress and high competition) to that of the tight ones (high stress), as plasticity effects reduce competition. However, the opposite pattern is observed in our system, as diversity decreased from loose to tight cushion habitats. We argue that this is most probably due to the heritable higher competitive effect of tight cushions in the high-stress habitat, which counteracts plasticity effects. Thus, for diversity we conclude that heritable effects overcome plasticity effects.

## Acknowledgements

This research was funded by the by the French ANR 09 – STRA – 09 O2LA and the University of Bordeaux. PA.H. was financially supported by a Ph.D. fellowship of the Lebanese National Council for Scientific Research. We acknowledge the Experimental Unit of INRA-Pierroton (Cestas, France) for hosting the shadehouse at the Site de recherche Forêt-Bois Bordeaux-Aquitaine, and Mr Pierre Casabonne, Mayor of Arette, for giving us the permission to carry out experiments on Arette’s community lands. We are grateful to Jean-Baptiste Lamy for his help for data analyses and Guillaume Caro, Clément Lalait, Yoann Le Bagousse-Pinguet, Florène-Diane Richard, Charles Schoirfer and Thomas Whitham for their help in the field. Authors have no conflict of interest. The experiments comply with the current laws of France in which the experiments were performed.

## Data accessibility

Data deposited in the Dryad Digital Repository: http://dx.doi.org/doi:10.5061/dryad.6g9b7 (Al Hayek et al. 2015).

## References

Al Hayek, P., Touzard, B., Le Bagousse-Pinguet, Y. & Michalet, R. (2014) Phenotypic differentiation within a foundation grass species correlates with species richness in a subalpine community. *Oecologia*, **176**, 533–544.

Al Hayek, P., Maalouf, J., Touzard, B. & Michalet, R. (2015) Data from: Disentangling the heritable and plastic components of the competitive and facilitative effects of an alpine foundation species. *Dryad Digital Repository*, doi:10.5061/dryad.6g9b7.

Antonovics, J. (1976) Nature of limits to natural-selection. *Annals of the Missouri Botanical Garden*, **63**, 224–247.

Antonovics, J. (1992) Toward community genetics. *Plant Resistance to Herbivores and Pathogens: Ecology, Evolution, and Genetics* (eds R.S. Fritz & E.L. Simms), pp. 195–215. University of Chicago Press, Chicago, UK.

Arendt, J.D. (1997) Adaptive intrinsic growth rates: an integration across taxa. *Quarterly Review of Biology*, **72**, 149–177.

Armas, C., Ordiales, R. & Pugnaire, F.I. (2004) Measuring plant interactions: a new comparative index. *Ecology*, **85**, 2682–2686.

Bailey, J.K., Schweitzer, J.A., Ubeda, F., Koricheva, J., LeRoy, C.J., Madritch, M.D. et al. (2009) From genes to ecosystems: a synthesis of the effects of plant genetic factors across levels of organization. *Philosophical Transactions of the Royal Society B: Biological Sciences*, **364**, 1607–1616.

Bailey, J.K., Genung, M.A., Ware, I., Gorman, C., Van Nuland, M.E., Long, H. et al. (2014) Indirect genetic effects: an evolutionary mechanism linking feedbacks, genotypic diversity and coadaptation in a climate change context. *Functional Ecology*, **28**, 87–95.

Bennington, C.C., Fetcher, N., Vavrek, M.C., Shaver, G.R., Cummings, K.J. & McGraw, J.B. (2012) Home site advantage in two long-lived arctic plant species: results from two 30-year reciprocal transplant studies. *Journal of Ecology*, **100**, 841–851.

Bertness, M.D. & Callaway, R. (1994) Positive interactions in communities. *Trends in Ecology & Evolution*, **9**, 191–193.

Bronstein, J.L. (2009) The evolution of facilitation and mutualism. *Journal of Ecology*, **97**, 1160–1170.

Brooker, R.W., Maestre, F.T., Callaway, R.M., Lortie, C.L., Cavieres, L.A., Kunstler, G. et al. (2008) Facilitation in plant communities: the past, the present, and the future. *Journal of Ecology*, **96**, 18–34.

Byars, S.G., Papst, W. & Hoffmann, A.A. (2007) Local adaptation and cogradient selection in the alpine plant, *Poa hiemata*, along a narrow altitudinal gradient. *Evolution*, **61**, 2925–2941.

Callaway, R.M., Nadkarni, N.M. & Mahall, B.E. (1991) Facilitation and interference of *Quercus douglasii* on understory productivity in central California. *Ecology*, **72**, 1484–1499.

Callaway, R.M., Brooker, R.W., Choler, P., Kikvidze, Z., Lortie, C.J., Michalet, R. et al. (2002) Positive interactions among alpine plants increase with stress. *Nature*, **417**, 844–848.

Cavieres, L.A. & Badano, E.I. (2009) Do facilitative interactions increase species richness at the entire community level? *Journal of Ecology*, **97**, 1181–1191.

Cavieres, L.A., Brooker, R.W., Butterfield, B.J., Cook, B.J., Kikvidze, Z., Lortie, C.J. et al. (2014) Facilitative plant interactions and climate simultaneously drive alpine plant diversity. *Ecology Letters*, **17**, 193–202.

Choler, P. & Michalet, R. (2002) Niche differentiation and distribution of *Carex curvula* along a bioclimatic gradient in the southwestern Alps. *Journal of Vegetation Science*, **13**, 851–858.

Choler, P., Michalet, R. & Callaway, R.M. (2001) Facilitation and competition on gradients in alpine plant communities. *Ecology*, **82**, 3295–3308.

Choler, P., Erschbamer, B., Tribsch, A., Gielly, L. & Taberlet, P. (2004) Genetic introgression as a potential to widen a species' niche: insights from alpine *Carex curvula*. *Proceedings of the National Academy of Sciences, USA*, **101**, 171–176.

Connor, J.K. & Hartl, D.L. (2004) *A Primer of Ecological Genetics*. Sinauer Associates, Inc., Sunderland, MA, USA.

Cranston, B.H., Callaway, R.M., Monks, A. & Dickinson, K.J.M. (2012) Gender and abiotic stress affect community-scale intensity of facilitation and its costs. *Journal of Ecology*, **100**, 915–922.

Crutsinger, G.M., Strauss, S.Y. & Rudgers, J.A. (2010) Genetic variation within a dominant shrub species determines plant species colonization in a coastal dune ecosystem. *Ecology*, **91**, 1237–1243.

Crutsinger, G.M., Rodriguez-Cabal, M.A., Roddy, A.B., Peay, K.G., Bastow, J.L., Kidder, A.G. et al. (2014) Genetic variation within a dominant shrub structures green and brown community assemblages. *Ecology*, **95**, 387–398.

Davis, M.A., Wrage, K.J. & Reich, P.B. (1998) Competition between tree seedlings and herbaceous vegetation: support for a theory of resource supply and demand. *Journal of Ecology*, **86**, 652–661.

Eckhart, V.M., Geber, M.A. & McGuire, C.M. (2004) Experimental studies of adaptation in *Clarkia xantiana*. I. Sources of trait variation across a sub-species border. *Evolution*, **58**, 59–70.

Ellison, A.M., Bank, M.S., Clinton, B.D., Colburn, E.A., Elliott, K., Ford, C.R. et al. (2005) Loss of foundation species: consequences for the structure and
dynamics of forested ecosystems. *Frontiers in Ecology and the Environment*, **3**, 479–486.

Falconer, D.S. & Mackay, T. (1996) *Introduction to Quantitative Genetics*, 4th edn. Longman, Essex, UK.

Fletcher, R. & Stace, C.A. (2000) A new section and species of *Festuca* (Poa-ceae) naturalized in England. *Watsonia*, **23**, 173–177.

Gibson, D.J., Allstadt, A.J., Baer, S.G. & Geisler, M. (2012) Effects of foundation species genotypic diversity on subordinate species richness in an assembling community. *Oikos*, **121**, 496–507.

Goldberg, D.E. (1990) Components of resource competition in plant communities. *Perspectives on Plant Competition* (eds J.B. Tilman & D. Grace), pp. 27–49. Academic Press, New York, NY, USA.

Grassein, F., Till-Bottraud, I. & Lavorel, S. (2010) Plant resource-use strategies: the importance of phenotypic plasticity in response to a productivity gradient for two subalpine species. *Annals of Botany*, **106**, 637–645.

Grime, J.P. (1973) Competitive exclusion in herbaceous vegetation. *Nature*, **242**, 344–347.

Grime, J.P. (1974) Vegetation classification by reference to strategies. *Nature*, **250**, 26–31.

Johnson, M.T.J. & Stinchcombe, J.R. (2007) An emerging synthesis between community ecology and evolutionary biology. *Trends in Ecology & Evolution*, **22**, 250–257.

Joshi, J., Schmid, B., Caldeira, M.C., Dimitrakopoulos, P.G., Good, J., Harris, R. et al. (2001) Local adaptation enhances performance of common plant species. *Ecology Letters*, **4**, 536–544.

Kawecki, T.J. & Ebert, D. (2004) Conceptual issues in local adaptation. *Ecology Letters*, **7**, 1225–1241.

Lankau, R.A. & Strauss, S.Y. (2007) Mutual feedbacks maintain both genetic and species diversity in a plant community. *Science*, **317**, 1561–1563.

Le Bagousse-Pinguet, Y., Forey, E., Touzard, B. & Michalet, R. (2013) Disentangling the effects of water and nutrients for studying the outcome of plant interactions in sand dune ecosystems. *Journal of Vegetation Science*, **24**, 375–383.

Le Bagousse-Pinguet, Y., Maalouf, J.P., Touzard, B. & Michalet, R. (2014) Importance but not intensity of plant interactions relates to species diversity under the interplay of stress and disturbance. *Oikos*, **123**, 777–785.

Lekberg, Y., Roskilly, B., Hendrick, M.F., Zabinski, C.A., Barr, C.M. & Fishman, L. (2012) Phenotypic and genetic differentiation among yellow monkeyflower populations from thermal and non-thermal soils in Yellowstone National Park. *Oecologia*, **170**, 111–122.

Liancourt, P., Callaway, R.M. & Michalet, R. (2005) Stress tolerance and competitive-response ability determine the outcome of biotic interactions. *Ecology*, **86**, 1611–1618.

Liancourt, P., Viard-Cretat, F. & Michalet, R. (2009) Contrasting community responses to fertilization and the role of the competitive ability of dominant species. *Journal of Vegetation Science*, **20**, 138–147.

Liancourt, P., Spence, L.A., Song, D.S., Lkhagva, A., Sharkhuu, A., Boldgiv, B. et al. (2013) Plant response to climate change varies with topography, interactions with neighbors, and ecotype. *Ecology*, **94**, 444–453.

MacArthur, R.H. & Wilson, O. (1967) *The Theory of Island Biogeography*. Princeton University Press, Princeton, NJ, USA.

Maestre, F.T. & Cortina, J. (2004) Do positive interactions increase with abiotic stress? – A test from a semi-arid steppe. *Proceedings of the Royal Society B-Biological Sciences*, **271**, S331–S333.

Michalet, R., Brooker, R.W., Cavieres, L.A., Kikvidze, Z., Lortie, C.J., Pugnaire, F.I. et al. (2006) Do biotic interactions shape both sides of the humped-back model of species richness in plant communities? *Ecology Letters*, **9**, 767–773.

Michalet, R., Xiao, S., Touzard, B., Smith, D.S., Cavieres, L.A., Callaway, R.M. et al. (2011) Phenotypic variation in nurse traits and community feedbacks define an alpine community. *Ecology Letters*, **14**, 433–443.

Michalet, R., Schöb, C., Lortie, C.J., Brooker, R.W. & Callaway, R.M. (2014) Partitioning net interactions among plants along altitudinal gradients to study community responses to climate change. *Functional Ecology*, **28**, 75–86.

Pakeman, R.J., Beaton, J.K., Thoss, V., Lennon, J.J., Campbell, C.D., White, D. et al. (2006) The extended phenotype of Scots pine *Pinus sylvestris* structures the understory assemblage. *Ecography*, **29**, 451–457.

Pigliucci, M. (2001) *Phenotypic Plasticity: Beyond Nature and Nurture*. John Hopkins University Press, Baltimore, MD, USA.

Pugnaire, F.I., Haase, P. & Puigdefabregas, J. (1996) Facilitation between higher plant species in a semiarid environment. *Ecology*, **77**, 1420–1426.

R Development Core Team (2013) *R: A Language and Environment for Statistical Computing*. R Foundation for Statistical Computing, Vienna, Austria. ISBN 3-900051-07-0, URL http://www.R-project.org/.

Rowland, D.L. (2001) Diversity in physiological and morphological characteristics of four cottonwood (*Populus deltoides* var. *wislizenii*) populations in New Mexico: evidence for a genetic component of variation. *Canadian Journal of Forest Research*, **31**, 845–853.

Rudgers, J.A. & Maron, J.L. (2003) Facilitation between coastal dune shrubs: a non-nitrogen fixing shrub facilitates establishment of a nitrogen-fixer. *Oikos*, **102**, 75–84.

Saccone, P., Delzon, S., Pages, J.-P., Brun, J.-J. & Michalet, R. (2009) The role of biotic interactions in altering tree seedling responses to an extreme climatic event. *Journal of Vegetation Science*, **20**, 403–414.

Sambatti, J.B.M. & Rice, K.J. (2006) Local adaptation, patterns of selection, and gene flow in the Californian serpentine sunflower (*Helianthus exilis*). *Evolution*, **60**, 696–710.

Saule, M. (1991) *La Grande Flore Illustrée des Pyrénées*. Randonnées Pyrénées-Editions Milan, Toulouse, France.

Schöb, C., Armas, C., Guler, M., Prieto, I. & Pugnaire, F.I. (2013) Variability in functional traits mediates plant interactions along stress gradients. *Journal of Ecology*, **101**, 753–762.

Sultan, S.E. (1987) Evolutionary implications of phenotypic plasticity in plants. *Evolutionary Biology*, **21**, 127–178.

Taylor, D.R., Aarssen, L.W. & Loehle, C. (1990) On the relationship between r/K selection and environmental carrying-capacity – a new habitat templet for plant life-history strategies. *Oikos*, **58**, 239–250.

Tielbörger, K. & Kadmon, R. (2000) Temporal environmental variation tips the balance between facilitation and interference in desert plants. *Ecology*, **81**, 1544–1553.

Tilman, D. (1982) *Resource Competition and Community Structure*. Princeton University Press, Princeton, NJ, USA.

Whitham, T.G., Young, W.P., Martinsen, G.D., Gehring, C.A., Schweitzer, J.A., Shuster, S.M. et al. (2003) Community and ecosystem genetics: a consequence of the extended phenotype. *Ecology*, **84**, 559–573.

Whitham, T.G., Bailey, J.K., Schweitzer, J.A., Shuster, S.M., Bangert, R.K., LeRoy, C.J. et al. (2006) A framework for community and ecosystem genetics: from genes to ecosystems. *Nature Reviews Genetics*, **7**, 510–523.

Whitham, T.G., DiFazio, S.P., Schweitzer, J.A., Shuster, S.M., Allan, G.J., Bailey, J.K. et al. (2008) Extending genomics to natural communities and ecosystems. *Science*, **320**, 492–495.

Wilson, D.S. (1976) Evolution on level of communities. *Science*, **192**, 1358–1360.

Received 14 February 2015; accepted 24 June 2015

Handling Editor: Marina Semchenko

## Supporting Information

Additional Supporting Information may be found in the online version of this article:

**Figure S1.** Photo showing the two sites [convex (left) and concave (right)] of the reciprocal transplant experiment at La Pierre Saint-Martin (Atlantic-Pyrenees, France). The photo was taken in July 2014 by Patrick Al Hayek.

**Figure S2.** Survival (September 2013) of the three types of targets transplanted within and outside *Festuca gautieri* cushions.

**Figure S3.** Interaction plots between genotypes and the environment (site) for cushion surface, maximum leaf length, cushion penetration and leaf density.

**Figure S4.** Net effect (RII) of *Festuca gautieri* tight and loose cushion phenotypes on the survival (September 2013) of the three transplanted targets.

**Figure S5.** Survival (June 2014) of the three types of targets transplanted within and outside *Festuca gautieri* cushions.

**Figure S6.** Total leaf number (September 2013) of the three types of targets transplanted within and outside *Festuca gautieri* cushions.
**Table S1.** Statistical results of the effect of *Festuca gautieri* cushion phenotypes on the richness and abundance of subordinate species in the natural habitats.

**Table S2.** Statistical results of the reciprocal transplant experiment on the five measured cushion traits of *Festuca gautieri*.

**Table S3.** Heritability, variance components (genotype, site, interaction and residual error) and their coefficients of variation for each trait.

**Table S4.** Statistical results of the reciprocal transplant competition experiment on the survival (September 2013) of the three transplanted types of targets.

**Table S5.** Statistical results of the reciprocal transplant competition experiment on the RII<sub>survival</sub> (September 2013) of the three transplanted types of targets.

**Table S6.** Statistical results of the reciprocal transplant competition experiment on the survival (June 2014) of the three transplanted types of targets.

**Table S7.** Statistical results of the reciprocal transplant competition experiment on the RII<sub>survival</sub> (June 2014) of the three transplanted types of targets.

**Table S8.** Statistical results of the reciprocal transplant competition experiment on the total leaf number of the three transplanted types of targets.

**Table S9.** Statistical results of the reciprocal transplant competition experiment on the RII<sub>total leaf number</sub> of the three transplanted types of targets.

**Table S10.** Statistical results of the reciprocal transplant experiment on species richness, abundance, RII<sub>richness</sub> and RII<sub>abundance</sub> of the naturally re-established species.