Local Species Loss and Retraction Across the Alps and Apennines: Recent Thermophilization and Diversity Changes of the Alpine Flora

Aim: The European alpine life zone is a recognized biodiversity hotspot, currently impacted by warming temperatures. Here, we investigated temporal plant diversity changes and thermophilization processes on multiple summits' flora. Location: Italy: six mountain regions across the Alps and the Apennines. Methods: We evaluated two decades (2001–2024) of plant species presence and abundance across 23 summits, covering an elevation gradient from the treeline ecotone to the nival belt, monitored through permanent sampling areas. We investigated temporal trends of species gain and disappearance, species richness, alpha and beta diversity indices, species-specific abundance changes, and the thermic indicator of plant assemblages. Results: Three summits in the Central and Northern Apennines experienced net species loss, involving endemic taxa, although species richness significantly increased in all other summits. At summit level, 7% of species significantly declined in abundance (’losers’), and with the same rate across the upper alpine and subnival belts (7%). About 5% of the species significantly increased (’winners’), and colonization rates were highest in the uppermost vegetation belts. The thermic indicator significantly increased in all mountain regions but the Central Apennines, with increasingly higher rates from the lowermost to uppermost vegetation belts, and accelerated during 2015–2024 in the Texel Group. Main Conclusions: We provide evidence of recent increasing thermophilization trends across the Alps and the Northern Apennines. The local plant species loss, including endemic taxa, and the widespread trend of loser species (not yet disappeared) may signal future local extinctions across both the Alps and Apennines also at the high-elevated belts. The colonization rate observed in the uppermost vegetation belts further suggests a shift towards warmer thermic conditions at high elevations.