SEASONAL-CHANGES OF FROST HARDINESS IN PICEA-ABIES AND PINUS-SYLVESTRIS IN FINLAND

The frost hardiness of 15- to 25-year-old Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) growing under central Finnish conditions was followed during 1985-1987. Shoots were subjected to artificial frost in the laboratory. Frost hardiness was assessed by the impedance method and by visual scoring. Frost hardiness varied during the years from -3.5-degrees-C to lower than -40-degrees-C. The rate of dehardening increased after about mid-April in both tree species when the daily mean temperature increased by several degrees above O-degrees-C. The maximum rate of dehardening varied slightly from year to year. In both species the frost hardiness of the previous year's shoot decreased during shoot elongation. This phenomenon was more prominent in pine than in spruce. Shoots were most susceptible to frost damage at the time when shoot elongation was ceasing. The onset and development of hardiness in autumn varied from year to year, especially in spruce. Some difference in hardening was found between the current and the previous year's shoots. The rate of hardening increased typically around mid-September in both species, when the mean daily temperatures decreased to within the range of 5-10-degrees-C.