The effects of long-term elevation of air temperature and CO2 on the frost hardiness of Scots pine

The frost hardiness of 20 to 25-year-old Scots pine (Pinus sylvestris L.) saplings was followed for 2 years in an experiment that attempted to simulate the predicted climatic conditions of the future, i,e, increased atmospheric CO2 concentration and/or elevated air temperature, Frost hardiness was determined by an electrolyte leakage method and visual damage scoring on needles, Elevated temperatures caused needles to harden later and deharden earlier than the controls, In the first year, elevated CO2 enhanced hardening at elevated temperatures, but this effect disappeared the next year, Dehardening was hastened by elevating CO2 in both springs, The frost hardiness was high (<-40 degrees C), even at elevated temperatures, in midwinter, at which time the electrolyte leakage method underestimated the frost hardiness compared with the visual scoring, In addition to the significant differences between treatments, there was also significant variation between trees in frost hardiness within treatments, These results suggest that the risks of frost damage are marked in the predicted climatic conditions in Finland, and, more specifically, they depend on how the occurrence of the frost episodes changes with respect to climatic warming during the annual cycle, especially in the autumn and spring, We also conclude that the conditions in midwinter are not critical for frost injury to trees in the future.