SEASONAL-CHANGES IN SOIL-TEMPERATURE AND IN THE FROST HARDINESS OF SCOTS PINE (PINUS-SYLVESTRIS) ROOTS UNDER SUB-ARCTIC CONDITIONS

The seasonal changes in soil temperature and in the frost hardiness of adult Scots pine (Pinus sylvestris L.) trees was studied between Sept ember 1991 and February 1993 in a pine forest located in Finnish Lapla nd. Air and soil (humus layer and 5 cm depth of mineral soil) temperat ures were measured continuously every second hour. The frost hardiness of the roots in the humus layer and in the mineral soil (down to 10 c m) was measured by means of the electrolyte-leakage method. The temper ature in the humus layer varied between 21.2 degrees C and -3.2 degree s C and in the mineral soil between 21.6 degrees C and -2.4 degrees C. The temperature in the humus layer was continuously slightly colder t han in the mineral soil from late August until May. The frost hardines s of the pine roots was lowest (about -5 degrees C) in May and during the first week of September. Frost hardiness stayed at its maximum of about -20 degrees C during November and December and slightly decrease d in January and February. During most of the sampling time, the frost hardiness of the roots in the humus layer was greater than in the min eral soil. There was a clear relationship between the soil temperature and the frost hardiness of roots.