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Shifts between positive and negative plant interactions along environmental stress gradients can affect alpine plant performance. We removed neighbours around Salix herbacea, a common arctic and alpine dwarf shrub, along elevational and snowmelt gradients on three mountains in Switzerland. The objectives of our study were to determine the effect of neighbours on phenological, morphological, and fitness traits of S. herbacea, and to determine whether neighbour interactions shift from competition to facilitation along environmental stress gradients.Target plants without neighbours required less time for fruit production; however, they also were more likely to be damaged by caterpillar herbivory. Effects of neighbour removal changed along the environmental gradients: plants without neighbours had smaller leaves on earlier snowmelt sites, and increased fungal damage with increasing elevation. Without neighbour removal, damage generally led to reduced female flowering under later snowmelt conditions in the following summer.Our results indicate that the majority of neighbour interactions influencing S. herbacea are facilitative, particularly at stressful early snowmelt and high elevation sites. We suggest that neighbours moderate environmental conditions by protecting plants from temperature extremes, and reduce plant apparency to caterpillars. Neighbours also indirectly increase fitness by reducing damage. Facilitation by neighbours may become more important under climate change, as early snowmelt may increase stress.
