Climbing invasions or climatic refugees: how many and to which extent non-native plant species could reach the Pyrenees mountains under climate change?

Under climate change, the status of invasive species is becoming complex, as taxa considered invasive in one region may become threatened in another, experiencing simultaneous expansion and decline across their range. In this context, mountain systems may act both as climatic refuges and/or as out-of-reach areas for these species. The Pyrenees, a longitudinal barrier separating the Iberian Peninsula from the rest of Europe, are increasingly exposed to alien plant species spreading from lowlands. Several invasive species have already reached high elevations in the massif, raising concerns about how many more may follow and to what extent under future climate change. This study assesses future invasion risks for 35 non-native plant species recognized as invasive on both sides of the Pyrenees. We project future dynamics of their bioclimatic niche suitability to identify which are likely to become widespread and where by 2100. Combining occurrence data with 19 bioclimatic variables at 1x1 km resolution, we model current bioclimatic niches and project its future distribution using an ensemble framework integrating five algorithms under four climate scenarios covering the 2021-2100 period. Projections indicate that 80 % of invasive plant species in the Pyrenees would not be climatic winners under future warming. The centroid of suitable climatic conditions is projected to shift upslope for more than 87 % of species. This upward movement would tend to stall around 2,000 m, as suggested by future hotspot patterns, effectively squeezing suitable areas and constraining future invasions within mountain zones, as climatic pressure increases at lower elevations. Such redistribution would not necessarily imply negative outcomes, and overlapping with endemic species would remain very limited. These results raise a broader question: could todays invasive species become tomorrows mountain flora?