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In alpine environments facilitator species buffer environmental extremes while building up soil resources above that of open areas. These modulating effects are critical for the persistence of species out of their optimal range and contribute to increase community-level plant species richness and phylogenetic diversity. We analyzed the effects of seven potential facilitator species with contrasting morphologies on subordinate plant species along a crossed environmental gradient, linking such effects to canopy effects. We also used these patches consisting of multiple shrub species to evaluate the effects of the whole shrub community on species richness and phylogenetic diversity, and whether such shrub community effects differed along the gradient. We used ecological and phylogenetic data of alpine plant communities along two altitudinal gradients on opposing aspects of the Sierra Nevada Mountains (Spain). As expected, shrubs buffered harsh abiotic conditions by decreasing mean temperatures and increasing relative humidity with regard to open areas. Composition of subordinate plant communities differed among shrubs and among sites, and correlated with relative humidity along the gradient pointing to the dependence of subordinate species on micro-environments created by shrubs. There were a variety of shrub effects on overall plant abundance and richness depending on shrub identity. In the most extreme sites we recorded generally positive effects of the shrub community, which promoted whole-community species richness and phylogenetic diversity despite species-specific effects ranging from clearly negative to positive. Our data therefore show that the effect of different shrub species on plant community richness and phylogenetic diversity is not redundant, as every shrub species may host unique communities, thereby affecting the structure and composition of the whole community. [ABSTRACT FROM AUTHOR]

• Nurse plants in alpine systems may enhance species survival under climate change. • We simulated climate change effects on an alpine cushion species. • We also assessed consequences of new climate on its facilitation effect. • Warming enhanced cushion performance but did not alter its facilitation effect. • Global change may alter carbon fixation and respiration, threatening plant survival. Climate change is more pronounced in high-elevation habitats than elsewhere, potentially causing disruptions in plant community structure and dynamics through changes in plant interactions. We tested the effect of warming and rainfall manipulations on growth and gas exchange of a cushion plant species, Arenaria tetraquetra , in the Sierra Nevada Mountains in SE Spain and assessed its consequences for plant interactions. Arenaria tetraquetra is a facilitator that often hosts a variety of beneficiary species within its canopy. Warming and drought could alter cushion growth patterns and canopy compaction, which in turn affect facilitation. We increased temperature using acrylic open-top chambers (OTC) and increased or decreased water availability with rainfall collectors. Our rainfall treatments did not have significant effects on cushion growth or facilitation. Three years of warming enhanced cushion photosynthesis and respiration, leading to larger cushions with thinner leaves, but did not change other cushion traits nor alter its facilitation effects. Some traits, however, readily responded to inter-annual climate variability. Global warming will change cushion plant physiology, as higher temperatures at times increase plant respiration above the level of carbon fixation. However, we did not find changes in interaction between benefactor and beneficiary species, perhaps due to the short-term nature of our experiment. Recorded physiological and morphological changes might be, however, signs of more pronounced, long-term consequences of warming on plant survival and community composition. [ABSTRACT FROM AUTHOR]