Thufur and turf exfoliation in a subalpine grassland on Mt Halla, Jeju Island, Korea

Thufur, 42-200 cm long and 9-27 cm high, are distributed in a summit crater of Mt Halla where freeze-thaw days occur over 70 times annually. The cryogenic mounds are frozen as a hard solid mass during winter and their lower part remains frozen until late April even though they begin to thaw in mid-March. Since the water content of thufur increases and exceeds a liquid limit during March and April, thufur soils may be subject to cryoturbation. An exfoliating terrace in a northwestern grassland of Mt Halla retreated at a rate of 22.2 mm/year from 2002 to 2004. Frost action is the process contributing most to turf exfoliation, with the maximum retreat rate observed from late March to early May. Thufur and turf exfoliation indicate that the subalpine grassland of Mt Halla is likely to be situated in a periglacial environment. However, global warming has affected these periglacial features through vegetation changes. The withering of alpine shrubs is partly responsible for the formation of crater-like thufur and subsequent rupture of thufur, while the rapid spread of Sasa quelpaertensis checks the retreat of turf terraces.