Reduced early growing season freezing resistance in alpine treeline plants under elevated atmospheric CO2

The frequency of freezing events during the early growing season and the vulnerability to freezing of plants in European high‐altitude environments could increase under future atmospheric and climate change. We tested early growing season freezing sensitivity in 10 species, from four plant functional types (PFTs) spanning three plant growth forms (PGFs), from a long‐term in situ CO₂ enrichment (566 vs. 370 ppm) and 2‐year soil warming (+4 K) experiment at treeline in the Swiss Alps (Stillberg, Davos). By additionally tracking plant phenology, we distinguished indirect phenology‐driven CO₂ and warming effects from direct physiology‐related effects on freezing sensitivity. The freezing damage threshold (lethal temperature 50) under ambient conditions of the 10 treeline species spanned from ?6.7±0.3 °C (Larix decidua) to ?9.9±0.6 °C (Vaccinium gaultherioides). PFT, but not PGF, explained a significant amount of this interspecific variation. Long‐term exposure to elevated CO₂ led to greater freezing sensitivity in multiple species but did not influence phenology, implying that physiological changes caused by CO₂ enrichment were responsible for the effect. The elevated CO₂ effect on freezing resistance was significant in leaves of Larix, Vaccinium myrtillus, and Gentiana punctata and marginally significant in leaves of Homogyne alpina and Avenella flexuosa. No significant CO₂ effect was found in new shoots of Empetrum hermaphroditum or in leaves of Pinus uncinata, Leontodon helveticus, Melampyrum pratense, and V. gaultherioides. Soil warming led to advanced leaf expansion and reduced freezing resistance in V. myrtillus only, whereas Avenella showed greater freezing resistance when exposed to warming. No effect of soil warming was found in any of the other species. Effects of elevated CO₂ and soil warming on freezing sensitivity were not consistent within PFTs or PGFs, suggesting that any future shifts in plant community composition due to increased damage from freezing events will likely occur at the individual species level.