SummaryFacilitative interactions are defined as positive effects of one species on another, but bidirectional feedbacks may be positive, neutral, or negative. Understanding the bidirectional nature of these interactions is a fundamental prerequisite for the assessment of the potential evolutionary consequences of facilitation.In a global study combining observational and experimental approaches, we quantified the impact of the cover and richness of species associated with alpine cushion plants on reproductive traits of the benefactor cushions.We found a decline in cushion seed production with increasing cover of cushion-associated species, indicating that being a benefactor came at an overall cost. The effect of cushion-associated species was negative for flower density and seed set of cushions, but not for fruit set and seed quality. Richness of cushion-associated species had positive effects on seed density and modulated the effects of their abundance on flower density and fruit set, indicating that the costs and benefits of harboring associated species depend on the composition of the plant assemblage.Our study demonstrates 'parasitic' interactions among plants over a wide range of species and environments in alpine systems, and we consider their implications for the possible selective effects of interactions between benefactor and beneficiary species.

International audienceQuestionsAre variable effects of different phenotypes of foundation plant species on subordinates across contrasting habitat conditions (with varying stress and disturbance levels) due to either varying environmental conditions or heritable differences in traits between phenotypes? To evaluate the contribution of environmental effects we quantified the effects of contrasting phenotypes of a foundation legume shrub on their subordinate species across exposure (drought stress) and grazing conditions. LocationA sub-alpine xerophytic community of western Mount Lebanon, Lebanon. MethodsFor two phenotypes of the spiny cushion shrub Onobrychis cornuta, a facilitative phenotype that occurs in concave topographies (mesic soil) and a competitive phenotype that occurs in convex topographies (xeric soil), we quantified cushion traits, environmental conditions and subordinate plant species abundances (within and outside the cushions) for the two phenotypes in northern (low stress) and southern (high stress) exposures, and with and without grazing. Relative interaction index (RII) for subordinate species richness and abundance was calculated in the eight treatment combinations and a correspondence analysis (CA) was conducted on species composition. ResultsDrought stress exacerbated phenotypic effects in southern exposure, with loose phenotypes being more facilitative and tight phenotypes more competitive than in northern exposure. This was related to both changes in cushion traits of the two phenotypes and to an increase in the pool of subordinate species sensitive to cushion effects. In contrast, grazing increased cushion positive effects of both phenotypes through the occurrence of indirect facilitation, with loose phenotypes becoming more facilitative and tight phenotypes less competitive. This was due both to changes in cushion traits of the two phenotypes and to their shared spiny phenotype limiting grazing effects. ConclusionsBecause increasing stress from northern to southern exposure did not increase competitive effects of either phenotype, and because the cessation of grazing did not cancel out differences in facilitative effects between phenotypes, neither drought stress nor grazing disturbance appeared to be the main drivers of the observed phenotypic effects on subordinate species across habitats. We conclude that differences in phenotypic effects of this legume shrub are very likely due to heritable differences in traits between phenotypes

Few studies have examined consequences of ecotypic differentiation within alpine foundation species for community diversity and their feedbacks for the foundation species' fitness. Additionally, no study has quantified ecotypic differences in competitive effects in the field and in controlled conditions to disentangle genetic from plasticity effects in foundation/subordinate species interactions. We focused on a subalpine community of the French Pyrenees including two phenotypes of a cushion-forming species, Festuca gautieri: tight cushions in dry convex outcrops, and loose cushions (exhibiting high subordinate species richness) in wet concave slopes. We assessed, with field and shadehouse experiments, the genetic vs. plasticity basis of differences in: (1) cushion traits and (2) competitive effects on subordinates, and (3) quantified community feedbacks on foundation species' fitness. We found that trait differences across habitats had both genetic and plasticity bases, with stronger contribution of the latter. Field results showed higher competition within loose than tight phenotypes. In contrast, shadehouse results showed higher competitive ability for tight phenotypes. However, as changes in interactions across habitats were due to environmental effects without changes in cushion effects, we argue that heritable and plastic changes in competitive effects maintain high subordinate species diversity through decreasing competition. We showed high reproduction cost for loose cushions when hosting subordinates highlighting the occurrence of community feedbacks. These results suggest that phenotypic differentiation within foundation species may cascade on subordinate species diversity through heritable and plastic changes in the foundation species' competitive effects, and that community feedbacks may affect foundation species' fitness.

Contrasting phenotypes of foundation species are known to differentially affect understorey plants. However, there is little knowledge on both the mechanisms of competition (resource competition versus interference) of stress‐tolerant phenotypes and the importance of indirect interactions. In an oromediterranean community from Mount Lebanon we assessed the effects on understorey forbs of two contrasting phenotypes, a tight competitive from stressful habitat and a loose facilitative from more benign habitat. In a dry south and a wet north exposure we assessed short‐term resource effects removing shrub canopy and long‐term soil effects (including litter interference) with the comparison of forb performances in adjacent naturally open areas vs no shrub. Indirect effects were quantified through the removal of grasses. Abundance, richness and biomass of forbs were measured in all treatments after one year of experiment, together with litter depth and soil moisture. We found strong direct negative soil effects of the tight phenotype on all forb performances and in particular in south exposure. These effects were due to litter interference on water availability, but not to resource competition. They were likely explained by the high hydrophobicity of organic matter accumulating in the stressful habitat of the tight phenotype. We also found an indirect competition of the loose phenotype for forb richness, due to its direct positive soil effect on competitive grasses, and in particular in south exposure. Our results improve our knowledge on the importance of litter interference in dry nutrient‐poor habitats and the role of indirect interactions in phenotypic effects on understorey species.

Summary1. 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.