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Frost heaves in the cold arid Leh–Ladakh region: observations on their morphological variability and patterns as indicators of pastureland degradation

This study analysed frost heave lands in the cold arid Leh–Ladakh region of the Himalaya, India through semi-detail field survey and remote sensing. Most of these sites are highland grasslands occurring along the alluvial plains of rivers in the Leh and Nubra valleys. The study identified the occurrence of some unique frost heave patterns whose morphometry varied from simple, isolated to complex ones with elongated, coalesced and superimposed bedforms. These lands can be further subdivided into vegetated and degraded types. The degraded forms exhibited several cracks, dry, collapsed and slump structures. Invasion of Cirsium arvense over native grass species, soil salinity and feeding pressure by livestock have been identified as major land degradation factors. C. arvense has replaced the native vegetation on frost heaves and has allowed Carex species to grow, which is known to degrade and deplete the pasture ecology.

FROST heaves are typical land features of cold arid regions. A 'heave' is often described as a winter-time uplift (swelling) or displacement of the ground surface in cold climates. This phenomenon leads to development of uneven roads, cracked foundations and also causes damage to infrastructure 1 . Similar to frost heaves, many other land features such as patterned grounds, pingoo, palsa and lithalsa have also been reported from peri-glacial environment in the Arctic, Antarctica, northern Canada, Alaska, Siberia and Iceland. All these surfaces have definite geometric forms. Palsas are typical forms of discontinuous permafrost regions commonly occurring in areas where the winters are long and snow cover tends to be thin 2,3 . Pingos are dome-shaped mounds that are up to 50 m high, consisting of a good concentration of ice at the core and a layer of soil on the surface often having vegetation 4 . There are reports about Mars having patterned ground.
The process of heaving or soil water accumulating and then freezing is well documented 2,5,6 . Taber, discredited the assumption that frost heaves result due to expansion of freezing water, but stated that it is caused due to water supply that helps ice growth inside the soil. Due to the weight of overlaying soil, vertical growth of ice is restricted and ice lenses are formed which are capable of lifting soil. Beskow 5 recognized few factors like type and size of soil particles, amount of water availability, rate of freezing and overburden pressure determining frost heaving. This process has a direct impact on the soil, as it tends to disturb the frost layer when ice lenses on the soil accumulate and protrude. During thawing, ice lenses increase the water pressure on the soil pore spaces that results in soil destabilization, thus enhancing the potential for mass movements. Frost heaves when uniform, have no adverse impacts on land quality, but unequal heaves may be destructive.
In India, cold arid regions are spread over 0.70 m ha mainly in the Himalayan ranges. About 11.4 m ha area in this sector is covered by grasslands 7 . Frost heaving as a process of land degradation has affected about 7428 ha in the Jammu and Kashmir region 8 . Here we present an inventory of occurrences of frost heaves and documentation on their variability in the cold arid Leh-Ladakh region of the Himalaya and the impact on pasturelands of this region.

Study area
The study area is part of Leh (Ladakh) region (geographical area, 45,110 sq. km) in the northernmost part of India lying between lat 32°-36°N and long 75°-80°E. It is located at the western end of the Himalaya, flanked by Tibet in the east, Kargil in the west, China in the north and Himachal Pradesh in the south. River Indus, high mountain ranges, elevated plateaus, fan surfaces and rocky gorges are important geomorphic features in the Leh region 9 . The elevation of observation sites in this study ranges from 2600 m to 5030 m amsl. Leh city is situated in a large valley of River Indus lying between two Himalayan ranges -Zanskar in the west and Ladakh in the east. Nimoo, Shey, Thikshey, Choglamsar, Ranbirpur, Nang and Chushot are the major towns in the Leh valley while Diskit, Hunder, Lakjung, Khalsar and Sumur are the important towns of Nubra valley located at 3100 m elevation north of Leh. Changthang, situated at an elevation of 4000 m amsl in the south of Leh is a highaltitude pastureland. This cold arid region is characterized by low atmospheric pressure (493 mm Hg), low partial pressure of oxygen, high wind velocity (8-10 km h -1 ), very low annual precipitation (80-300 mm), sub-zero temperature (up to -40°C) and intense sunlight 10,11 . Three months (December-February) witness subzero temperature, while rest of the winter months till April experience 0°C temperature. May to October is the cropping season. Much of the surface water is through snowmelt. Streams during summer flow for a few hours per day when ice melts helping in the growth of vegetation, including grasses, shrubs and trees. Despite such harsh climate and rocky/glaciated terrain, settlements are located at various elevations of hills and valleys. Sheep and goat-rearing is an important component of people's living, particularly for the nomadic tribes.

Methodology
Frost-heave sites were identified through field surveys carried out at different altitudes in Leh, Nubra, Durbok, Kharu and Nyoma administrative blocks ( Figure 1). Area covered under frost-heave lands was mapped from the Indian Remote Sensing (IRS) LISS IV satellite imageries of 2017-18 with background information viewed on Google Earth images. Morphological parameters of frost heaves (length, width and height) were measured in the field. Soil samples were collected up to 15 cm depth in the profiles of frost heaves. Vegetation composition at each site and biomass of grass species were computed through detailed assessment in grids of 1 m × 1 m size at several sites.

Identification and mapping of frost-heave lands
Frost-heaving processes create a type of patterned surface that has typical undulating morphology. These lands have been found to occur near stream channels, riverside, stagnated water bodies and lakes. Such associations have helped identify the frost-heave features on the false colour composites (FCC) of IRS LISS IV satellite image which has a spatial resolution of 5.8 m. Field surveys and ground truthing conducted in the Leh-Ladakh region from 2013 to 2017 helped match the identified frostheave sites and their mapping at Saboo, Stakmo, Tangtse-Changthang, Giya-Tso Kar and Sakti in Leh and Diskit-Hunder in Nubra valley ( Figure 1). Table 1 summarizes the types, morphology and patterns of frost heaves at these locations.

Soil characteristics of frost-heave sites
Tables 2 and 3 present the physico-chemical characteristics of soils at Leh and Nubra valley. Analysis shows that the soils are loamy sand to sand, with sand content varying from 74% to 91%. The silt and clay contents are comparatively high varying from 5% to 16% and 3% to 10% respectively, except in Tso Kar-Giya valley where silt content is 20.45%. The higher silt and clay contents may be because of valley side location and deposition of alluvial soils formed by riverine deposits. Soils are alkaline with pH values ranging from 7.9 to 9.1. Sand dunes near Shey, Thikshey and Choglamsar towns indicate the     Here C. arvense is found to replace the native vegetation in frost-heave lands and has allowed Carex species, which can deplete and change the pasture ecology. In Tso Kar saline areas, Kobresia, Glaux, Astragalus are the major species along with weedy species of C. arvense and Carex. The dry and green biomass on heaves range from 0.05 to 0.62 kg sq. m at Saboo, Tangtse, Tso Kar and Nubra areas. In all pastures, feeding pressure posed by nomadic livestock and other wild animals has impacted the plant mantle significantlyabout 30%. C. arvense, an invasive weed has been observed to capture the frost-leave mounts, thereby reducing the endemic flora ( Figure 2 a and b).

Occurrences and morphological variability
In the study area, frost-heave lands occur under three topographic situations: uplands (on the hill slopes and pediments), wetlands (plains) and lower plains (riverbanks). Morphological patterns of the heave surfaces are shown in Figure 3.    Biomass of grass species has also been measured (Table 4).

Site V (Sakti village in Kharu block of Leh district):
This is one of the bigger settlements in this region.  (Figure 3 e), which can be the result of freezing and thawing, and (b) complex pattern which demonstrates an example when smaller heaves have expanded and gradually joined together to form compact and massive structures (Figure 3 d). In contrast, some degraded heaves are also noticed ( Figure 5 b-d). Such features indicate impact of desiccation when a compact, rounded or oval heave mound gradually flattens. Subsequently, heaves develop cracks and get eroded.
Grasses growing on such surfaces dry up and indicate gradual dying of the plant sections above and below the ground and decay of root litter that expands the surface cracks. Frost heaving accompanied with invasion of C. arvense, soil salinity and feeding pressure by livestock are the major processes identified for pasture degradation. In Changthang grasslands, the survey showed few invasive and exotic weeds like C. arvense. Such species are known to survive, establish themselves under harsh conditions and invade the heaves and start affecting the native vegetation due to allelopathy. This may be one of the reasons why interspaces between two heaves are devoid of any vegetation or remain barren at several locations, as in Tso Kar, Stakmo and Giya.
Since all these sites are grazing lands, continuous and regular grazing by nomadic livestock and other wild animals reduces the grass cover enabling the invasive species like C. arvense to dominate the vegetation cover.
Frost heaves require a frost susceptible soil, continuous supply of water from the water table and freezing temperature penetrating the soil. Studies have indicated that not all soils are susceptible to frost heaving. It is found more in soils that facilitate capillary flow. Thus frost-susceptible soils are fine-textured, with silt, loam and very fine sand 13 . Perkins 14 reported that frost also helps sort the sediments in the ground. We observed similar conditions in the surveyed sites. For example, at Saboo the outer portion of heaves contained finer sediments and coarse particles remained clinging to the mantle or inner core. Under wetland conditions, frost-heave surfaces act as excellent grazing lands; but under dry and water-deficit conditions, the topsoil breaks and gradually disintegrates to amorphous or powdery form and heaves become devoid of any grass or vegetation.

Conclusion
In cold arid regions of Leh-Ladakh and Nubra valleys, frost heaves occur along the river banks, in the valleys and near water points. Based on their shape, size and morphological pattern, frost heaves can be classified into simple and complex types. Recently formed frost heaves are single and small, while older ones are compact, elongated and massive with distinct soil profiles. The patterns vary from single to network type. Soil-water content, flowing water in the rivers and stones are important factors for variability in the morphology and degradation of frost-heave lands. In dry condition, the morphology and soils of frost-heave features show degradation like dryness, shrinkage, erosion, disintegration and collapsing of forms, leaving behind amorphous or powdery sediments and dried vegetation. Such processes have been found to affect the status of vegetation growing on frost heaves. In this study we also found invasion of weeds like C. arvense on these heaves which has not only thrived well, but has replaced the native vegetation and allowed Carex species to grow, which is known to degrade and deplete the palatable plants or grass species and affect pasture ecology. The present study can serve as a basic inventory of frost heaves and their morphological patterns in the Leh-Ladakh region. It also shows how pasturelands of the region are undergoing degradation due to frost-heaving processes.