Survival types of high mountain plants under extreme temperatures

Extreme temperatures are a main factor limiting plant growth in high mountain habitats. During winter, the risk of frost damage is highest at windblown and often snow-free sites. During summer, actively growing plants are particularly endangered by episodic cold spells, but also by short-term overheating. The current review gives an overview of extreme temperatures in the European Alps and observations of temperature damage on plants in their natural habitats. Furthermore, seasonal time courses of frost and heat resistance derived from laboratory tests on different plant growth forms are presented. Study species were the cushion plants Silene acaulis, Minuartia sedoides, Saxifraga oppositifolia and Carex firma collected on wind-exposed ridges; the rosette plant Soldanella alpina collected on snow-protected sites, and three Sempervivum species collected in xerothermic habitats.The temperature resistance of leaves, stems, rhizomes and roots were tested in two annual time courses. Frost treatments were conducted in controlled freezers by rapid cooling (10 K h−1, for current resistance) as well as by stepwise cooling (1–3 K h−1, for hardening capacity). Heat treatments followed a standardised procedure by exposing samples to heat for 30 min in hot water baths. The damage was visually estimated using the topographic tetrazolium test.During winter, cushion plants from exposed sites were fully hardened (LT50 below −70 °C). Rosette plants, which are protected by a constant snow cover, survived temperatures of down to −25 °C. During the growing period, foliage and shoot tips of the investigated species were damaged at −5 to −8 °C (LT50). Stems and rhizomes were only partially damaged by temperatures of −10 to −15 °C. Heat resistance in cushion plants generally reached 56–58 °C (LT50), shoots and rhizomes of C. firma could reach 60 °C. Succulent plants like Sempervivum arachnoideum and S. tectorum from hot and dry microsites were the most heat resistant among the tested species (62–64 °C). The investigated mountain plant species showed highly varying resistance patterns, with variation in maximum hardening capability and seasonal time courses of resistance. Plants were capable of rapid adjustments to extreme temperatures, which is crucial for survival in a high mountain climate.