Freezing dehydration damages roots of containerized Scots pine (Pinussylvestris) seedlings overwintering under subarctic conditions

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Abstract
The survival of conifer roots under winter conditions is not well understood. The effect of long-lasting freezing at extremely low temperatures on the survival of the roots of Scots pine (Pinussylvestris L.) seedlings from a timberline seed source was studied. One-year-old containerized seedlings were kept for 4 months under winter conditions typical of the subarctic region. The freezing treatment was accelerated by preventing snow accumulation on half of the seedlings. The proportion of living root tissue and the amount of sugars (glucose, fructose, and sucrose) and starch in the needles and roots were measured at weekly intervals. The temperature within the growth medium without snow cover fell on a number of occasions to −30 °C. The lowest temperature recorded within the snow-covered growth medium was −20 °C. The sugar content increased faster and to a higher level in the seedlings without snow cover, but it did not affect root survival. The proportion of living root tissue without a cover of snow decreased by 80% by the end of December, and the roots were completely injured by the end of February. There was only a slight decrease in the proportion of living roots among seedlings covered by snow. The reduced root survival was not directly related to exposure to the lowest freezing temperatures; instead, it was manifested some weeks after exposure. This suggests that dehydration resulting from prolonged severe freezing caused root damage. The presence of a cover of snow reduces subzero temperature extremes and the severity of dehydration; together, these result in enhanced root survival.

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