There is a marked difference in soil morphology between winter and summer in eastern Hokkaido, where the soils are frozen for the considerable part of the year. In the cold season, the rising vapour from the underground is caught by the upper soil horizon, and condensed. During the soil freezing period the frozen soil horizon takes off the moisture, and the soil aggregates are kept under dry conditions, and then the structure formation is accelerated. By frost column formation the platy structure, characteristic of the frozen soil, is made. Maintaining the excess moisture the soil passes the thawing period and later the dry period in spring, and the humid soil formation proceeds under the cool humid climate in summer. In such a natural condition the hydrophytes (Ulmus propinqua, Fraxinus mandshurca var. japonica, Syringa reticulata, Cercidiphyllum Japonicum, Phellodendron amurense var. sachalinense, Juglans ailanthifolia, etc.) grow and are distributed regardless of the topography and soil morphology.
