INFLUENCE OF THE NUMBER OF FREEZING CYCLES AND COOLING AND THAWING RATES ON FROST-RESISTANCE OF SPRUCE

In a wind tunnel, 7-year-old potted spruce from different Austrian high- and low-altitude provenances had been exposed to low temperatures during different periods in winter, and frost damages were determined. In all cases low-altitude spruce provenances suffered greater frost damage than trees of a high-altitude origin. The amount of damage increased with the number of freezing cycles. As to the amount of damage caused by low temperature, the number of freezing cycles was more important than the time of exposure to low temperatures. When cooling down to temperatures at which the water inside the needles was already freezing but no visible frost damage occurred, also repeated freezing cycles did not cause any damage. While there was no clear difference in the amount of damage when cooling and thawing rates were between 4K h-1 and 8 K h-1, an increase of the cooling and thawing rates from 5 K h-1 up to 20 K h-1 caused an increase of frost damage in spruce from high-altitude origin from 6% up to 100%. This increase of frost damage corresponds with a lowering in frost resistance (TL50) of about 3 K. Continuous monitoring of needle temperatures at the alpine timber line on Mt. Patscherkofel (1935 m) over a 3-year period clearly indicated that such freezing and thawing rates are in the average nearly reached during winter. The maximum is much higher (up to 130 K h-1). Biological frost alterations (freezing and thawing of water in the needles) during winter occurred on several days a month, during February in the average on 10 days. During the monitoring period there were many days with two freezing cycles and even some days with three cycles. For judging the possibility of frost damage to spruce at timberline it is therefore necessary to run frost resistance experiments with freezing and thawing conditions similar to those in the open field.