Temperature and moisture trends in non‐sorted earth hummocks and stripes on the Old Man Range, New Zealand: implications for mechanisms of maintenance

Abstract

The mechanisms of maintenance of earth hummocks and non-sorted stripes in seasonally frozen ground on the Old Man Range, Otago, New Zealand were investigated. These landforms are hypothesised to be active periglacial landforms maintained by seasonal movement of moisture down an energy gradient. We tested three hypotheses: 1) freezing should occur predominantly in the crests of the stripes and hummocks; 2) differential freezing patterns should be consistent between years; and 3) in the presence of a temperature gradient (i.e. in winter), soil moisture content should be greater in crests than in furrows. An array of 39 thermistors at each of two sites was used to monitor soil temperature gradients during three winters, and replicate soil cores were taken in autumn and winter to determine soil moisture gradients. Freezing occurred mainly in the crests and sides (heavily influenced by aspect), and patterns of freezing showed strong interannual consistency, supporting our first two hypotheses. Soil in crests had a higher moisture content in both freezing and thawed seasons, which was inconclusive relative to the expected water gradient. Figure 1 (A) Stripe study site (about 1590 m a.s.l.) on the Old Man Range, viewed looking northeast. Data logger was housed in the large (47 cm  47 cm  22 cm) steel box. (B) Hummock study site (about 1640 m a.s.l.) viewed looking due west with the Garvie Mountains in the distance. (C) Schematic of the soil profile beneath a hummock or stripe transect.

Earth Hummocks in West Dartmoor, SW England: Characteristics, Age and Origin

“…Hummock development under conditions of shallow ground freezing is supported by hummock formation in northern England on low ground since the early 19 th century AD (Pemberton, ) and on high ground since AD1950 (Tufnell, ), by hummock formation in Iceland within 10–20 years on ground levelled by tillage (Schunke and Zoltai, ) and by formation of small hummocks on artificial slopes at 150–300 m asl in Germany (Kelletat, ). Hummocks similar to those on Dartmoor occur in areas of shallow (20–40 cm) ground freezing in Lesotho (Grab, , , ) and New Zealand (Mark, ; Scott et al ., ). Collectively, such evidence suggests that hummocks may develop and persist in areas where frost penetrates periodically no deeper than 20–40 cm into the soil.…”

“…Though differential frost heave has previously been identified as a cause of hummock initiation (Lewis et al, 1993), the mechanism of subsequent hummock growth from an initial regularly spaced pattern of gentle swells and depressions is less well understood. It has been shown, however, that under conditions of shallow ground freezing, hummock crests and sides freeze first, and that depressions may remain unfrozen during shallow freezing cycles (Mark, 1994;Grab, 1997Grab, , 2005bScott et al, 2008). Assuming that laterally non-uniform frost heave creates an initial pattern of swells and depressions (Figure 9a), these findings suggest that frost heave of swells will be amplified by earlier and deeper freezing (Figure 9b), with ice lens growth within Figure 9 Model of hummock growth by migration of silty soil in advance of inclined freezing planes.…”

“…The term earth hummock describes patterned ground comprising vegetated mounds, typically 0.1–0.6 m high and 0.5–2.0 m wide, separated by a network of troughs. Earth hummocks are widespread in arctic tundra environments (Tarnocai and Zoltai, ; Kojima, ; Figure a), in subarctic permafrost areas south of the treeline (Kokelj et al ., ), on subarctic, alpine and subalpine mountains (Scotter and Zoltai, ; Grab, ; Mark, ; Scott et al ., ) and in maritime periglacial environments such as Iceland, where they are referred to as thúfur and occur in permafrost‐free areas (Schunke, ; Gerrard, ; Van Vliet‐Lanoë et al, ; Figure b). Earth hummocks usually occur as ‘hummock fields’ on flat or gently sloping terrain.…”

Linking energetics and overwintering in temperate insects

“…Furthermore, snow cover is a combination of local and regional patterns of precipitation (Henry, 2008;Kreyling and Henry, 2011), in combination with localised topography that influences snow drift formation and therefore both snow depth and persistence (e.g. Scott et al, 2008). These microtopographical influences are hard to predict (Scherrer and Körner, 2010) and may mean that small changes in microhabitat selection could have a big influence on exposure to overwinter energy drain, just as behaviour is a key determinant of climate change impacts in the summer (Sunday et al, 2014).…”

Effects of Cushion Plants on High-Altitude Soil Microarthropod Communities: Cushions Increase Abundance and Diversity of Mites (Acari), but not Springtails (Collembola)

“…The crests of hummocks and stripes are occupied by ultradwarf cushion shrubs; the furrows between hummocks contain some herb and lichen species, as well as bare soil (Mark and Bliss, 1970). This topography is hypothesized to be an active periglacial landform maintained by seasonal differential freezing (Mark, 1994;Scott et al, 2008). The patterned ground landforms are not unique to New Zealand, and similar microtopographic structures are found in polar or high-altitude periglacial environments in North America, Africa, Greenland, Iceland, and Fennoscandia (Grab, 2005;Walker et al, 2008).…”