Rare and Vulnerable: Mires of the Western Alps Are Changing Rapidly Under Climate Pressures

Aims: Mires are important habitats that provide fundamental ecosystem services. Although they have traditionally been considered stable ecosystems, exhibiting little to no change in floristic composition over several decades to millennia, the effect of contemporary climate change is largely unknown. This study aimed to assess (i) how the vegetation of Sphagnum-dominated mires changed over the last two decades in the Western Alps, where mires are fragmented and at the southern edge of their European distribution, and (ii) whether climate change was a primary driver of these changes. Location: Western Alps, Italy. Methods: We resurveyed 139 plots across 14 sites, comparing historical vegetation data from 1998 and 2011 with new surveys conducted in 2023, including vascular plants and bryophytes. We analysed the shifts in species composition and the changes in community diversity and ecological indicators. We also evaluated climate trends over the past three decades and their relationship with vegetation dynamics. Results: Climate data analyses confirmed increasing air temperatures, decreasing precipitation and increasing evaporation across the study sites. Both short-term and long-term resurveys showed an increase in species diversity, in particular woody and non-specialist vascular plants, and a decrease in or local extinction of mire specialists, including Sphagnum species. These results suggest a lowering of the water table and a consequent drying. Longer term resurveys revealed signs of acidification and eutrophication, likely due to increased mineralisation. Attribution to climate change was supported by the higher increase in species richness in mires with a stronger decrease in precipitation. Conclusion: Our results revealed significant vegetation changes under the pressure of climate change, with mires at the southern edge of their European distribution exhibiting more rapid and pronounced vegetation dynamics than previous studies conducted at higher latitudes. Urgent conservation measures, including rewetting, are essential to preserve these mires and their ecosystem services.