Bud freezing resistance in alpine shrubs across snow depth gradients

Despite the importance of spring freezing events for alpine species distribution, few studies have analysed the response of alpine shrub species to early spring freezes. It is also not known how snow cover gradients influence the process of de-hardening between individuals of the same species and their vulnerability to early spring frosts. We analysed early spring freezing resistance for the buds of eight alpine Ericaceae shrubs growing at 1 m snow depth at treeline in the Swiss Alps. Moreover, buds of Rhododendron ferrugineum and Loiseleuria procumbens were analysed for freezing resistance and sugar content along a snow depth gradient. The LT50 (lethal temperature for 50% of samples) of the eight species ranged from −25.1 ± 1.6 °C (Vaccinium vitis-idaea) to −11.1 ± 1.2 °C (Vaccinium uliginosum), with differences being related to the phenological stage in addition to shrub preferences for contrasting snow cover microsites. Although the effect of snow depth on the freezing resistance of plants was not significant, samples collected from 1 m to 1.5 m snow depth tended to be more vulnerable to freezing, particularly L. procumbens. Buds collected at shallower snow depths had higher sugar concentrations, indicative of their stronger physiological hardening. Consequently, we conclude that differences in snow cover may significantly affect the physiological hardening of plants during the onset of spring. Individuals growing at less than 0.5 m snow cover are hardier, i.e., show moderately higher freezing resistance than individuals from snow banks. Snow cover is a highly important aspect of climate change, and freezing resistance in alpine plants with respect to snow conditions can be a relevant driver of plant responses to climate change.