Abstract
Changes in freezing resistance and its mechanisms (i.e., avoidance by supercooling or tolerance of ice formation) were studied in two deciduous (Vaccinium myrtillus, V. uliginosum) and two evergreen (V. vitis-idaea, Oxycoccus palustris) ericoid shrubs at two elevations (470 m and 1070 m a.s.l.) throughout the growing season. The temperature causing 50% damage (i.e.
Lt50) and nucleation temperature at which ice was formed in the tissues (i.e. exotherm) were measured. The aim was to investigate whether the freezing resistance varied during the growing season and whether the changes were determined by elevation.
Freezing resistance declined from the spring to the summer (late May to August) and then it increased toward the end of the season. In half of the cases the resistance remained comparable between the summer (August) and the early autumn (September).
The majority of plants were tolerant during most of the season and avoidance occurred only in early or late May. There was no distinct elevational difference in plant freezing resistance between the localities, since the plants from higher elevation were more resistant only in September.
Hence, the greatest risk of frost injury was found during the early weeks of the growing season when the ericoid shrubs develop new leaves.