TEMPERATURE-VARIATIONS IN PLANTING MOUNDS DURING WINTER

Soil temperatures were measured at a depth of 8 cm in top, middle, and bottom positions of 30 cm high mineral and organic mounds and at 8 cm depth in scarified patches during winter and spring 1987-1988 and 1988-1989. At low air temperature, frozen mounds without snow cover showed much lower temperatures than snow-covered mounds, the maximum difference being 16 degrees C. During the coldest period of the two winters, when minimum air temperature was -26 degrees C, soil temperature in the top of a snowless mineral mound remained within -16 to -10 degrees C for 3.5 days and -8 to -5 degrees C in a snowless scarified patch. Minimum temperatures were lower, duration of low temperature freezing was longer, and temperature changes were more rapid in mineral than in organic mounds. Large temperature differences were found between the top and the bottom of mounds. In dry conditions during early spring, the upper part of the mineral mound thawed and froze repeatedly with daily maximum and minimum temperatures of 5 degrees C and -6 degrees C. Soil temperature patterns during the winter period are discussed in relation to root freezing tolerance of conifer seedlings. Mounding as a scarification method should be used with care as winter temperatures may injure seedling root systems.