The frequency of freezing events during the early growing season and the vulnerability to freezing of plants in European high-altitude environments are both expected to increase under future atmospheric and climate change. Therefore, we tested early growing season freezing sensitivity of leaves in a total of ten species belonging to three plant functional types (PFTs) 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 leaf phenology, we distinguished indirect phenology-driven CO₂ and warming effects from direct physiology-related effects on freezing sensitivity. We hypothesized that (1) CO₂ enrichment would negatively affect freezing resistance but that this effect is intrinsic to CO₂ and not associated with a phenological shift induced by the treatment; (2) soil warming would advance leaf phenology, leading to increased sensitivity to early growing season freezing events; and (3) plants of different PFTs would have distinct freezing damage thresholds under current conditions and would respond similarly to the experimental treatments.

Results/Conclusions

The freezing damage threshold (LT50) under ambient conditions of the ten treeline species spanned from –6.7 +/- 0.3°C (Larix decidua) to –9.9 +/- 0.6°C (Vaccinium gaultherioides), but PFT did not provide a significant explanation for the observed 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. At the species level, the elevated CO₂ effect on freezing resistance was significant in Larix decidua, Vaccinium myrtillus and Gentiana punctata and marginally significant in Homogyne alpina and Avenella flexuosa. No CO₂ effect was found in Empetrum hermaphroditum, Pinus uncinata, Leontodon helveticus, Melampyrum pratense or V. gaultherioides. Soil warming led to advanced leaf expansion and reduced freezing resistance in V. myrtillus, whereas A. flexuosa showed greater freezing resistance when exposed to warming. No soil warming effect was found in any of the other species. Effects of elevated CO₂ and soil warming on freezing sensitivity were not consistent within PFTs, suggesting that any future shifts in plant community composition due to increased damage from freezing events will likely occur at the individual species level.