Low temperatures at higher elevations require plants to exhibit increased freezing resistance throughout the summer months

The frequency and severity of night frosts increase during the summer in the alpine environment as elevation increases. Intra-specific, temporal, and elevational variability in the freezing resistance (FR) of alpine plants during the active growing period has not been documented, especially in regards to the intra-specific distribution of plants at their upper elevational limits of growth. The FR of leaves of 13 species was investigated at regular intervals from June through August, 2017 at elevations between 600 and 3200 m. The examined species have different lower and upper elevational range limits, and either grow exclusively in alpine sites or extend into the subalpine life zone. Each species was sampled twice, close to its upper and lower range limit. For most of the species, leaves exhibited a higher FR (-0,17 K/100 m) at the upper elevational range limit. The intra-specific FR was substantial, covering an amplitude of 7-15 K (>15 K in Pinus). Fully-expanded leaves had a lower level of FR in June. Evergreen leaves had a higher FR than deciduous leaves throughout the summer. Frost hardening in evergreen and a decrease in FR in deciduous leaves was observed in August; whereas, deciduous leaves at that time had already begun to senesce. Ecotypes growing at their upper elevation limit were more frost hardy than individuals of the same species growing near the lower level of their elevation range. The most severe summer frosts occurred in June when leaves were fully expanded and their FR was lowest. Thus, the highest risk of frost damage to plants at alpine sites occurs in June, even though the most frost-susceptible phase of leaf expansion may have already passed.