Hikers and livestock using mountain trails damage native vegetation and act as seed vectors, thus favouring the spread of non-native plants. We evaluated the effect of trails and livestock abundance on the success of non-native plants in the arid central Andes of Argentina. We surveyed six trails, covering elevations between 2400 m and 3570 m a.s.l. and recorded non-native and native vegetation using transects distributed along the elevational gradient and spanning distances up to 22 m from the trail. We assessed how non-native occurrence, richness and cover varied with distance from the trail, intensity of use by livestock, native plant community composition and elevation. We found that trails favoured nonnative occurrence, but did not in uence richness and cover, while livestock favoured non-native occurrence, richness and cover. Non-native richness and cover decreased with elevation and varied with native community composition. In addition, non-native richness was positively correlated with native shrub cover suggesting possible facilitative interactions. Our results show that despite strong environmental ltering that decreases non-native abundance with elevation, non-natives occur up to the upper limits of vegetation, and that trails and livestock favour their spread in the mountains.Aconcagua, where over 120 vascular plant species have been recorded (Méndez 2004(Méndez , 2007Méndez et al. 2006). SamplingWe carried out the eld surveys in the summer season (January-March) of 2018 and 2019. Selected sites were intensively used for mountaineering activities and had a broad elevational range, with an average difference in elevation of 650 m a.s.l. between the start and the end of the trail. The sampling followed the T-trail survey protocol developed by the Mountain Invasion Research Network (MIREN) (Liedtke et al. 2020). We surveyed twenty transects along each trail, located approximately every ca. 35 m of elevation starting at the trailhead, avoiding areas with secondary trails. Each transect consisted of three 2 m x 10 m plots arranged in a T-shape, for a total of 120 transects (360 plots) (Fig. 2). In each plot, we identi ed and estimated the cover of all vascular plant species, both native and non-native. To estimate the level of livestock activity (cows, horses and mules), we estimated dung density on each plot (Ender et al. 2017). We collected plant specimens that could not be identi ed in the eld and subsequently identi ed them with herbarium specimens and taxonomic keys in the Ruiz Leal Herbarium of the Argentine Institute for Dryland Research (IADIZA, CONICET Science and Technology Center, Mendoza). We classi ed species according to their origin and life forms using the database from the Darwinion Botanical Institute (Instituto de Botánica Darwinion 2018). We recorded the trail track, elevation and transect location with a GPS device and later processed them with QGIS and R software to determine the distance of each transect to the start of the trail.

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