Repeated freezing and thawing may lead to the formation of needle-ice, which results in the lifting up of the upper few mm or cm of soil.
Needles of 10 to 50 mm length develop during one night of freezing and total needle length can reach up to 400 mm after several nights.

When does needle-ice occur?

Several conditions must coincide to allow the development of needle ice. These include temperature fluctuations around the freezing-point, a sufficient supply of moisture and sparse vegetation cover.

1 - Needle-ice (109K)

Where does needle-ice occur?

Needle-ice can be observed on all continents. In the tropics, it is restricted to high mountain areas. At moderate latitudes, it is particularly common in the alpine environment because of the higher frequency of freeze-thaw cycles compared to lowlands and the often higher soil moisture.

Needle-ice formation is a frequent phenomenon in the tropics and may occur throughout the year (diurnal climate). In the temperate zone of the northern hemisphere, it mostly occurs at the beginning and end of the winter period, and only infrequently at the height of summer. At very high latitudes, needle-ice formation is restricted to the summer months.

2 - Altitude-Latitude Relationship

Why is needle-ice important?

Intense freeze-thaw cycles of water-saturated soils result in high internal soil disturbance and promote small-scale erosion. The unstable substrate is unfavourable for plant colonization because seedlings may be damaged by frost heaving Recently disturbed surfaces are susceptible to needle-ice formation that may delay development of plant cover in these bare patches.

The intensity and frequency of needle-ice activity are usually not sufficient to affect established vegetation with a dense ground cover. However, locations exposed to frequent and intense frost soil phenomena may be bare of vegetation.

3 -
Different disturbances favour needle-ice formation (green arrows), e.g. by creating bare ground (see text at arrows)

A closed vegetation cover impedes needle-ice formation (red arrow)
Needle-ice may prevent or delay the formation of a closed vegetation cover

During a freezing and thawing cycle particles of different size may be moved to different extents. This leads to a process of frost sorting which, over a long period of time can produce different types of patterned ground, e.g. stone stripes or polygons. These provide a variety of microhabitats which may persist for many years and are colonized by different plant species.

4 - Stone stripes (163K)

Frost heave is also a major cause of solifluction found on many steep slopes in alpine areas. Solifluction lobes and terraces may creep down at speeds of several cm per year. In areas of very intense solifluction, this ground movement may even impede vegetation development. In the afro-alpine zone, solifluction seems to be the most serious factor limiting higher plant growth.
Solifluction lobes and terraces can overrun the vegetation, especially in very wet years, whereas in warm and dry years, the lobes may be stabilized by the roots of plants.
The form of solifluction lobes and terraces provides different microclimatic conditions ranging from the more exposed sites to the sheltered lee of the terraces. These are associated with pronounced vegetational differences.

29 August 2011
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