Patterned Ground Activity in a Southern New Zealand High-Alpine Cushionfield

Abstract

Recent measurements confirm the current activity of some solifluction terraces in high-alpine cushionfield, under periglacial conditions on the Old Man Range in southern New Zealand. Downslope movement amounts to some 3.5 cm a decade. Extensive areas of fully vegetated earth or soil stripes on gentle slopes (3–7°) and similarly vegetated soil hummocks on adjacent flatter sites are much less obviously active and have been assumed by previous researchers to be relicts of a more glacial climate. Three years of over-winter temperature measurements at up to four depths (5, 20, 40, 60 cm) beneath an adjacent crest and hollow of well-developed stripes (ca. 1.5 m between crests and 30 cm high), and similar measurements over one winter from a nearby hummock site revealed consistent differential temperatures, and usually differential freezing. Crests of both stripes and hummocks remained continuously frozen at least to 20 cm depth over much of the winter while soil in the depressions remained unfrozen at 20 cm depth and only intermittently frozen at −5 cm. Differential freezing would permit water transfer along a free energy gradient from the nonfrozen to frozen regions, where expansion associated with its freezing probably is sufficient to maintain the microtopographic patterns of both stripes and hummocks.