Seasonal changes in soil temperature and in the frost hardiness of Scots pine roots under subarctic conditions: Comparison with soil temperature and snow-cover under different simulated winter conditions

Seasonal changes in soil temperature and in the frost hardiness of adult Sc ots pine (Pinus sylvestris L.) trees were studied between August 1991 and M ay 1993 in a pine forest growing on dry heathland soil (67 degrees N, 29 de grees E). The temperature in the humus Layer varied between +21.2 degrees C and -3.2 degrees C and in the mineral soil (down to 10 cm) between +21.6 d egrees C and -2.4 degrees C. The temperature in the humus layer was continu ously slightly colder than in the mineral soil from late August until May. The frost hardiness of the pine roots was lowest (about -5 degrees C) in Ma y and during the first week of September. Frost hardiness stayed at its max imum of about -20 degrees C during November and December and slightly decre ased in January and February. During most of the sampling lime, the frost h ardiness of the roots in the humus layer was greater than in the mineral so il. There was a clear relationship between the soil temperature and the fro st hardiness of roots. Soil temperature and precipitation as snow in differ ent winter conditions were simulated using the SOIL model. The simulations show that the insulating effect of the snow cover is crucial for the frost survival of Scots pine roots even during a moderate winter.