In the last five centuries, the inappropriate management of the Vel’ká Fatra Mts. sub-alpine and alpine areas has led to the development of different forms of surface destruction. For evaluation of the dynamics and variability of surface degradation the territory of the Hornojelenská valley was chosen. It is a significant avalanche area. It has clearly been destroyed by avalanches, water erosion and cryogenic erosion as well as anthropo-zoogenic processes. The forms of destruction were mapped on a scale of 1:200 based on the aerial photographs and satellite images taken in 1961, 2003, 2009 and 2012. The total area of degradative morphogenetic forms (DMF) in 1961 was 5.5780 ha, 4.0650 ha in 2003, 4.5752 ha in 2009 and 4.9431 ha in 2012. The DMF reached its peak in 1961. In the mid-1960s, there were ambitions to reforest the highest areas of the study area that led to the decrease of DMF and the development of vegetation. The present exogenous geomorphologic processes are causing a gradual increase of the total destructed area.
Specific geoecological conditions of the areas above the timber line are the reason for the high sensitivity of environmental components and ecosystems situated within subalpine and alpine levels to natural or anthropogenic disruptions. In relation to these areas, the most obvious disruption seems to be the risk of vegetation cover and soil stability [1, 2]. A significant reduction of timber line in subalpine zone associated with the different forms of turf destruction led to the formation of wasted lands [3, 4]. Wasted lands are defined as intensively eroded soils (up to the stage of bedrock denudation), the formation of which was partially caused by humans´ acceleration of natural and anthropo-zoogenic processes. Wasted lands are typical for horizontal or vertical discontinuity of the soil mantle. In conditions of the Carpathians, the soil mantle is most often destroyed by water erosion or slope movements. Other exogenous relief-forming factors with their erosive impact, such as gravity, water, wind, snow, frost - ground ice, organisms and humans must also be taken into consideration [5, 6]. The soil mantle was removed, thinned or substantially physically altered by means of destructive processes with a significant contribution of the differentiated exposure climate. On the other hand destructed surfaces can be perceived as a heterogenous environment providing available habitats, food and shelter and thus increasing the biodiversity [7, 8]. Destructive processes lead to degradation of the original ecosystem and create a new one at the same time. The processes destruction-degradation-creation cannot be separated, so we are using all the three terms in the meaning of alteration/change of the original ecosystem by geomorphic phenomena.
The timber line in Vel’ká Fatra Mts. was shifted downwards by 400-450 m, by the deforestation impact and subsequent cattle grazing. It resulted in a significant intervention to the spruce level while the timberline at some places was cut even as far as the beech forest vegetation level . The reduction of the original vegetation cover contributed to the reduction in performance of forest stand functions and provided space for a wide range of destructive phenomena to develop above the timber line. The forms of total soil destruction affect 11% of the Velká Fatra Mts. subalpine and alpine area .
2 Study Area
The end part of Hornojelenská valley was chosen in order to analyse the dynamics of surface degradation development in Velká Fatra Mts. The study area (Figure 1)at the end of the Hornojelenská valley extends to 34 ha. It is one of the most typical locations with the continuous occurrence of glide avalanches in the conditions of Slovakia. The study area is situated within the range of 1,090-1,510 m a.s.l. Its upper half is situated above the break-off avalanche point, which is currently afforested mostly with dwarf mountain pine, Swiss stone pine and Norway spruce . The inclination angle of this part of the area is relatively slight, rarely exceeding 25° . The lower part includes the avalanche transport zone with the destroyed areas. Its inclination angle ranges from 26° to 55°, locally up to 70°. The study area is directed mostly to the east (60% of slopes) and less to the north-east and south-east (25% and 11% - see Figure 2). The end of the Hornojelenská valley is crossed by a significant tectonic fault , which is found in the rocks of geological formation composed of greenish and grey marly slates, and also less resistant marly limestones, marls and marlstones. In the deposition areas of the slopes, rocks consist mostly of clay-rocky deluvium and debris . Regarding shallow rendzinas and calcaric cambisols formed on steep slopes, the whole locality is characterised by the mosaic disruption of the soil and vegetation cover integrity. In many places, uncovered bedrock can be seen on the surface. Denudation processes have had the greatest effect in the avalanche chutes and nearby surroundings. The smooth parts of the slopes between the chutes are destroyed in a different way due to abrasion during the avalanche release or snowpack movement. These factors are followed by the destructive impact of water erosion processes in the summer and partially by cryogenic processes during regelation which may occur for a period of up to 70 days in this area .
The Hornojelenská valley ends in a relatively massive avalanche kettle, which is intersected by seven avalanche chutes (Figure 3). Below them, gravel cones with partially tough deposits are formed. Huge snowpacks are loaded on the avalanche slopes and due to changes of weather and wind conditions they fall as avalanches even several times a year. Permanent avalanche hazard and relatively frequent snowslides into the lower-situated Rybô village were the basis of the projects for biological and technical avalanche control measures. The first detailed draft of avalanche control project was in the 1920s. However, the project was not fully accomplished because of the lack of funds. In the 1960s, several projects were successfully implemented with the 196 soil terraces and several small stone terraces built in the avalanche area . Moreover, in order to control snow accumulation in the forest zone, technical barriers (such as snow and windbreak fences with either horizontal or vertical paling, snow targets )as well as stakes that control snow cover movements (shifting or sliding down) were installed. Furthermore, the zone within the altitude of 1,300-1,400 m a.s.l. (above sea level) was afforested with dwarf mountain pine, Swiss stone pine, Norway spruce and willow Silesia .
The study area has largely been destroyed by snow-gravitational erosion and water-gravitational processes. One of the typical destructive forms is a nival abrasion due to avalanches, shifting and sliding-down snow movements . The critical surface destruction area is in fact a tectonic fault at the altitude range from 1,300 to 1,400 m a.s.l., where the greatest tension in the snow cover arises. The study area has suitable preconditions for the cryogenic destructive processes as well. Needle ice and frost heaving causes the elevation of fine-grained disintegrated matters by 94 mm in the bare soil surface layers .
The idea of this paper was to present the results of the dynamic and variability of surface destruction in the subalpine and alpine zones. The analysis of development and changes of surface destruction could provide detailed information on strength of individual geomorphological processes and intensity of their driving forces.
2.1 Material and Method
The object of research and analyses were degradative morphogenetic forms  (DMF) over the timber line at the end of Hornojelenská valley. DMF are perceived as a result of erosion-denudation system, which is caused by the whole complex of exogenous morphogenetic processes influencing soil mantle. DMF are used for surface degradation development and monitoring of the Western Carpathians since 1970´s .
The extent of DMF in time periods (1960, 2003, 2009, 2012) was determined from satellite and aerial images. The interpretation of these images can help get the information, which is hard to obtain by field measurements and survey, especially in inaccessible mountain terrain.
The following cartographic materials have been used as an input: aerial images from 1961 (resolution 2400 dpi, height of flight 750 m - Military Topographic Institute) and satellite images from 2003, 2009 and 2012 (raster resolution 30 cm Military Topographic Institute). These materials have been georeferenced for further processing in the ArcGIS 9.3 program which was used for all procedures applied in this study. ArcGIS 9.3 played a major role in producing geo-spatial data in digital form, as well as in storing digital data, thus accessibility of data modification and manipulation. GIS application was also used in producing thematic maps (location of the study area, map of slope inclination, slope orientation and geological units of the study area, map of spatial representation of the evaluated DMF categories and image of avalanche chutes within the study area – Figures 1, 2, 3 and 5) as well as those geospatial data derived from satellite images processing. The formations of DMF have been vectorised on a scale of 1:200 and classified into following categories:
Surface with the tendencies to be destroyed (or threatened) by snow abrasion (slope surface exposed to the influence of avalanches, but more or less overgrown by continuous turf cover with no obvious signs of destruction);
Surface in the initial stage of destruction (disrupted vegetation cover with bare bedrock up to 30% of the area);
Surface with partially bare bedrock (significantly degraded surface taking 31-70% of the area, turf fragments);
Surface with a total bare bedrock (continuous vegetation cover is missing almost completely, bare bedrock taking 71-100% of the area);
Erosive-avalanche chutes completely abraded by snowpack (7 significantly carved out erosive-avalanche chutes with total absence of soil);
Erosive-avalanche chutes with debris (erosive-avalanche chutes filled mainly with loose and partially toughened debris, occurring soil and vegetation cover);
Debris covered area (the area with piled up debris localised off erosive avalanche chutes).
We have also focused on the area, currently covered with wood vegetation and turf that are exposed to the permanent influence of polygenetic processes of surface destruction and are bound to avalanche kettle holes. Within the vegetation category, we have also evaluated the subalpine and alpine meadows, yet treeless in 1961.
A detailed analyses of the individual DMF categories may give a clearer vision of the current geomorphological processes. Their transformation into other categories provides data about the slowing or intensification of destructive processes. The transformations were observed in 1961-2003,2003-2009 and 2009-2012. Within each DMF category, we have evaluated the relative (percentual) change into a different category. For this purpose, the DMF layers from individual periods were converted into raster. In order to capture changes as precisely as possible, the cell size of 1 dm2 was chosen.
Special attention was paid to characteristics of seven avalanche chutes. Our goal was to analyse their changes at the same time periods as the dynamics of development of individual DMF was examined. The subject of the research was to examine changes in their lengths, which can thus provide the data about the intensity of present geomorphological processes. Their greatest lengths, measured from the beginning of the avalanche chute to the end of the last destructed structure in their gravitational unit, were vectorised in ArcGIS 9.3.
To understand the dynamics of the development of destructive morphogenetic forms at the end of Hornojelenská valley, it is necessary to analyse data on the relief, exposure, geological bedrock and soil types in detail. Therefore, the first part of this paper mostly deals with the abiotic characteristics of the study area. The data about the relief was acquired by means of the digital elevation model analyses with a spatial resolution of 10 m. Equally, maps of slopes and slope exposures were acquired from the model as well. The data of geological bedrock were obtained from geological map in the scale 1:50 000 . The characteristics of soil types were obtained from field survey .
The basis for assessing the dynamics of DMF was the aerial survey photo showing the condition of avalanche kettle in 1961. It provides data about the area of greatest DMF extension at the end of the Hornojelenská valley just before planting avalanche control woody vegetation and taking biological-technical measures. The current geomorphological processes can be seen on satellite images taken in 2003, 2009 and 2012. The photos from 2003 were chosen on purpose to capture the development of DMF approximately 40 years since the time avalanche control measures were put into practice. During that period, the planted woody vegetation took over the basic avalanche control function and its impact should become obvious regarding the intensity of soil degradation processes.
3 Results and Discussion
Development and dynamics of DMF categories
Destructed formations within the research area were generated out of the map data in the form of polygons using four time horizons (1961, 2003, 2009, 2012). Subsequently, they were classified according to destructive morphogenetic forms. Detailed information about the extent and relative representation of individual DMF in selected time horizons is presented in Table 1. A spatial representation of the evaluated categories according to individual years is illustrated in Figure 3.
|Degradative morphogenetic forms (DMF)||1961||2003||2009||2012|
|Total area of the study territory||34.000||100.00||34.00||100.00||34.00||100.00||34.00||100.00|
|Total area of DMF||5.5689||16.38||4.0650||11.96||4.5752||13.46||4.9431||14.54|
Legend: 1 - surface with the tendencies to be destroyed (or threatened) by snow abrasion, 2 - surface in the initial stage of destruction, 3 - surface with partially bare bedrock, 4 - surface with a total bare bedrock, 5 - erosive-avalanche chutes completely abraded by snowpack, 6 - erosive-avalanche chutes with debris, 7 - debris covered area, 8 – woody vegetation below the avalanche chutes.
The largest area of destructed surfaces was recorded on the aerial images from 1961. Based on the analysis of de–structed formations in 2003, their total area has decreased by 27%. The significant reduction in the area of DMF is referred to as the result of biological-technical avalanche control measures taken . The results from other time horizons, compared with conditions in 1961, show a gradual expansion ofDMF to almost 90%. The gradual increase in degraded areas is predominantly being caused by climatic factors induced geomorphic processes [4, 12, 14]. Especially the presence of heavy and wet snow in the spring period , which disrupts a continuous vegetation cover , opens the way for the realisation of other destructive processes .
Woody vegetation above the avalanche kettle consists mostly of dwarf mountain pine and spruce. Although the woody vegetation is old enough to fulfil its avalanche control function [17, 18], it still cannot fully prevent the break-off and formation of avalanches in case of large snow-pack. To fully serve the avalanche control function, the forest stands above the avalanche break-off line do not yet have the corresponding structure to meet the natural forest structure of timber line. The plants of arborescent stature lack the sufficient thickness of , and thus decrease the avalanche control efficiency of vegetation. The results of the analysis dealing with surface degradation covering the last 10 years reveal that the DMF area tends to expand.
The factors that directly affect the morphogenesis of high mountains surface have already been monitored in the study area for several decades [4, 19]. Their indirect effects on geomorphological and sedimentary processes are less known . To take a measurement of meteorological factors (such as the snowpack depth, amount and intensity of precipitation, air temperature, wind speed, etc.) as well as their manifestations depending on the geoecological characteristics of the monitored area (which include the attributes of snowpack in situ), is very complicated particularly due to extreme conditions and their considerable spatial and time variability . Alpine geomorphological hazards such as avalanches, landslides and other demonstrations of slope instability may result from human activities as well as climatic variability factors, acting alone or in combination . While the management of both landscape and human activities is important when considering the standpoint of the anthropic impact on the adverse effects of natural hazards, climatic factors play a significant role in the formation of geomorphological risks by affecting all the landscape elements .
Detailed analysis of Table 1 revealed the gradual expansion of DMF within the categories of the areas with completely and partially exposed surfaces, areas at the initial stage of soil and vegetation destruction, as well as the increase in the area of erosive-avalanche chutes with debris completely abraded by snowpack. Figure 4 shows their common characteristic - even though the area of these DMF categories in 2003 was smaller relative to the extent observed in 1961, they have been gradually enlarging. Regarding the areas with partially exposed surface and areas with erosive-avalanche chutes completely abraded by snowpack, the area has even enlarged when compared with the conditions monitored in 1961. Figure 4 illustrates the percentual changes of area of the evaluated categories during years 2003, 2009 and 2012, compared with year 1961. The percentual coverage of DMF categories in year 1961 was used as the standard. Figure 4 shows the fluctuation of the evaluated categories and their trend since 1961. The zero value on the “x” axis presents the state of the evaluated categories in 1961. The “y” axis shows the percentual change of the categories in years 2003, 2009 and 2012.
Erosive-avalanche chutes completely abraded by snow-pack. The first monitoring of aerial image from 1961 already suggested that the series of seven avalanche chutes would be classified as the areas with the most destroyed surfaces. When monitored in 2003, their area had decreased by 335 m2. However, due to the influence of polygenetic destructive processes the area enlarged again in 2009 by 449 m2. Compared with 1961, the present area is larger by more than 1,575 m2. The enlargement of avalanche chutes can be observed predominantly in the direction of gravity, while the significant extension can be also seen in the upper parts - so called collecting areas which slope down the chutes in a funnel shape.
The enlargement of this DMF category might have been caused mainly by water-gravitational and snow-gravitational processes. During the course of the last 40 years, forest stands planted above the avalanche break-off line have taken over the avalanche control function  and the major part of snowpack in situ remains fixed in their area. Among the snow-gravitational processes, glide avalanches (e.g. release of the entire snow cover), which are formed after overcoming the friction between the snowpack and the bedrock, represent the main reason of the chutes´ destruction. The increase in weight of a snow slab and consequent initiation of avalanches in chutes can be caused by additional precipitation or the load of other material coming from unstable snowpack from the upper parts of the slope. The small areas of the chutes hold relatively large masses of snow which completely abrade their surface. Their presence in the study area after afforestation can be explained by the observations of climate trends in mountain areas during the period from 1981/82 to 2007/08 . The observations revealed that the altitude range of 1,000-1,500 m a.s.l. records significant decrease of solid precipitation at the expense of mixed type. It results in a heavier and thicker snowpack which, after being broken off, is referred to as a glide avalanche 
The kettle at the end of the Hornojelenská valley is primarily perceived through the prism of avalanche events. Water-gravity processes seem to be significant for the surface development of this area (the period since the stabilisation of effective avalanche control vegetation). These processes become severe especially in the warm half of the year, when their destructive forces demonstrate themselves in the form of splash erosion processes and concentrated surface run-off (coming from rainfall and melting snow ). The phenomena bound to the running water activity in conditions of cold climate are actually polygenetic forms of surface destruction . They are created simultaneously by water from snowmelt, rainfall, solifluction, ground needle ice and by wind eventually. Therefore, it is difficult to separate demonstrations of water erosion itself from other erosion and gravitational-erosion processes; and to subsequently determine the proportion of water erosion to the soil destruction objectively. However, the surface run–off is currently the dominant factor of surface destruction .
During the course of observed time horizons, both the area and length of chutes had been changing. Geomorphological processes caused formation of the seven major avalanche chutes at the end of the Hornojelenská valley, the longest of which is 366 m long (2012). Their detailed characteristics in individual years are presented in the Table 2. Avalanche chutes (Figure 5) have been numbered from bottom to top.
|The number of a chute||Lengt (m)|
Table 2 shows that chute no. 3 is subject to the greatest influence due to the present geomorphological processes. Compared with 1961, its actual length has extended almost three times. Partially, it is a result of a local destructive process that led to the revealing of partially tough deluvial sediments at the bottom of the chute . The vegetation masking/camouflage function plays a significant role in the chute’s length identification. Luxuriant vegetation can cover relatively narrow forms of surface destruction and prevent their precise identification from satellite images. Thus the quasi-extension of the chute was visible on the image from 2003. In addition, its bottom deepens as a result of geomorphological processes. Compared with an adjacent slope, the bottom is carved in its deepest point by up to 12 m. Considering the deforestation of the study area in the 15th century (at the beginning of the 16th century at the latest ), the average intensity of the chute’s vertical erosion is 20-24 mm/year during the 500-600 year long absence of the forest. Lower values of denudation in the same location (the average of 16.74 mm/year) are provided . The higher speed of erosion furrows formation was observed in the Kopské saddle area in Belianske Tatry Mts., which is comparable to the geological conditions of the study area .
Erosive-avalanche chutes with debris. These are erosive grooves that are found on less steep slopes where the accumulation of material mostly occurs . From the first monitoring, their area had been slightly increasing. However, in recent years (2009 and 2012), we can talk about their stagnation. Their further development strongly depends on following the climatic conditions . In case of longer periods with no significant avalanche events and/or with no torrential rain precipitation, the condition of erosive-avalanche chutes with debris is expected to stay the same, or more precisely a slight decrease in their area can be expected. Another option is the increase of their area due to intense destructive processes in the header of avalanche chutes, and thus the chutes are silted with debris. The third possibility is a reduction/decrease in their area at the expense of erosive-avalanche chutes completely abraded by snow cover category, or more precisely their shift in the direction of the slope.
Area covered with debris. Similarly to the former category, there is a strong uncertainty of further development in case of this category. Table 1 shows that the increase of the area in 2003 as well as the decrease in subsequent years is evident. Nowadays (2012), the area is smaller by 218 m2 compared with the beginning of the monitoring.
The reduction in size of the area with debris is explained by its overgrowth of luxuriant vegetation. Mainly, it is the result of successfully performed avalanche control measures which eliminate the influences of the major destructive factors - avalanches. Consequently, they form favourable conditions for the vegetation growth, especially the woody one. Compared with 1961, the presence of vegetation (debris overgrowing) indicates the presence of a higher proportion of nutrients and soil components in the debris, which were shifted from the header of avalanche chutes with active soil degradation processes. The frost heaving caused by the needle ice in combination with processes of snow patch and water-gravitational erosion results in disrupting the cohesion of the soil and its subsequent transport through the chutes into the debris location .
Surface threatened by snow abrasion. This category covers the slopes under the line of avalanche break-off point covered with turf, without visible signs of disturbance. Their area reaches the peak in 2003 within the observed period from 1961 to 2012. The area of this surface type is reduced at the expense of extensively destroyed formations which are classified into categories of surface destruction at different stages. Table 1 shows a significant increase of the area (an increase by 1.1683 ha in 2003) after the avalanche control measures. After the effects of the main modelling factor - avalanches moderate, and the surface becomes consequently destroyed by crawling snow, ice and water-induced erosion. The trend of the increasing density of the snow cover  and its more intensive melting at the end of a winter season , probably contributed to the gradual expansion of DMF with areal forms of surface destruction. Smooth parts among the chutes are affected not only by a destruction caused by avalanche abrasion or snow cover movements . These modelling factors are followed by the destructive influence of water erosion processes in a spring-summer period and cryogenic processes in the regelation period [4, 31]. From the point of potential water erosion of soil, the observed area is ranked among the most exposed areas. The average intensity of potential soil erosion by running water is 7.2 mm/year. The measured elevations of bare soil due to cryogenic processes at this locality reached 94 mm, while the elevation under the intact turf covers only 8 mm . These phenomena is closely related to the frost melting of the destroyed turf cover and the enlargement of areal DMF categorised into the areas in the initial stage of destruction, DMF with partially exposed surface and DMF with totally exposed surface.
The expanse of all three categories of areally destroyed morphogenetic forms was decreasing after 1961. However, it has been gradually increasing since 2003 (Table 1).The area of DMF with partially exposed surface category started to expand. Between 2003 and 2009, the area reached the expanse it used to have in 1961. At present, the area is larger by 0.32 ha. That is probably due to the fact that this category represents a transition between the surfaces at the initial stage of destruction and total vegetation removal. This category is the most sensitive to any changes influencing the surface destruction. Its area is affected by the processes stimulating vegetation development as well as the increasing intensity of soil degradation processes.
The significant importance of avalanche control measures and their influence on the vegetation development is obvious in Figure 4. The development of predominantly grass-herbaceous vegetation is significantly reflected in decreasing areas in the initial stage of destruction. Compared with 1961, their area decreased by 9,787 m2 in 2003, by 8,459 m2 at present. It is also reflected in the development of surfaces threatened by snow abrasion. However, no major destruction of the surface can be seen. Moderation of an avalanche eroding effect has not a negligible influence on the reduction of areas with totally exposed surface. The reduction in the area by 0.5 ha was recorded in 2003. However, the decrease of such areas is possible only with the soil residues remaining and/or at the slopes with the inclination angle up to 25°.
Variability assessment of individual DMF categories
The qualitative and quantitative variability of individual categories are shown in Tables 3, 4 and 5. The horizontal direction shows the percentage changes of individual categories in favour of others. The values in the vertical direction show how much the category increased at the expense of others (expressing the percentage increase of each category).
|The condition of categories in 2003|
|The condition of categories in 1961||Category||1||2||3||4||5||6||7||8||9||10||%|
Legend: 1 - erosive-avalanche chutes with debris, 2 - alpine meadows, 3 - surface with the tendencies to be destroyed (or threatened) by snow abrasion, 4 - surface with a total bare bedrock, 5 - surface with partially bare bedrock, 6 - debris covered area, 7 - afforested area, 8 - erosive-avalanche chutes completely abraded by snowpack, 9 - Surface in the initial stage of destruction, 10 - woody vegetation located below the avalanche chutes.
|The condition of categories in 2009|
|The condition of categories in 2003||1||79.64||0||15.43||0||3.17||0||0||1.24||0.39||0.13||100|
Legend: 1 - erosive-avalanche chutes with debris, 2 - alpine meadows, 3 - surface with the tendencies to be destroyed (or threatened) by snow abrasion, 4 - surface with a total bare bedrock, 5 - surface with partially bare bedrock, 6 - debris covered area, 7 - afforested area, 8 - erosive-avalanche chutes completely abraded by snowpack, 9 - Surface in the initial stage of destruction, 10 - woody vegetation located below the avalanche chutes.
|The condition of categories in 2012|
|The condition of categories in 2009||Category||1||2||3||4||5||6||7||8||9||10||%|
Legend: 1 - erosive-avalanche chutes with debris, 2 - alpine meadows, 3 - surface with the tendencies to be destroyed(or threatened) by snow abrasion, 4 - surface with a total bare bedrock, 5 - surface with partially bare bedrock, 6 - debris covered area, 7 - afforested area, 8 - erosive-avalanche chutes completely abraded by snow cover, 9 - Surface in the initial stage of destruction, 10 - woody vegetation located below the avalanche chutes.
The period from 1961 to 2003
Regarding the first evaluation period, the changes in the DMF categories occurring between 1961 and 2003 were analysed. It is the longest monitored period with the most perceptible changes within the individual categories.
From among the categories showing signs of surface destruction, only erosive-avalanche chutes with debris and areas covered with debris increased in their extent. This extent is as little as 0.1683 ha for both categories. The enlargement of these areas in 2003 resulted from the change (debris loading) of original surfaces threatened by snow abrasion. The presence and increase of debris areas suggest that despite the avalanche control measures , the development and transfer of eroded material in erosive-avalanche chutes can be observed. The image from 2003 shows these areas as new fields of coarsegrained disintegrated matter. The intensity of the surface abrasion or deepening of snow depressions in the study area varies from 1.6 to 2.7 mm/year . This can be considered a result of current morphogenetic processes as well as geoecological characteristics of the area . The geological formation of grey and charcoal limestone, which is formed by marly limestones, marls, marlstones and marly limestones, is prone to destruction . Similarly, soils developing on such a substrate produce fine-grained material by means of weathering. The soil sample (calcaric cambisols) from the study area at an altitude of 1,405 m a.s.l. contained 50.6% of particles with the diameter smaller than 0.01 mm . The amount and structure of fine-grained material is crucial for most soil degradation processes . More intensive soil degradation processes are related to its high proportion in the soil .
The area of crucial DMF within the study area, the erosive-avalanche chutes completely abraded by snow-pack, reduced during the monitoring period just minimally, by only 3.7% (Table 1).The changes within this category occurred predominantly in favour of the surfaces threatened by snow abrasion. A further reduction of this area is the result of transformations to the DMF categories of erosive-avalanche chutes with debris and surfaces with partially, or more precisely totally exposed surface (Table 3). Their common characteristic is a significant up to total absence of soil and vegetation cover that refers to considerable surface destruction. The further development of vegetation is limited by relief-forming processes and the absence of soil [30, 32]. By means of synthesis of former findings, we can conclude that despite the efforts made during the establishment of technical and biological avalanche control measures, their soil protective effect in these categories DMF will not significantly take effect. The effect of avalanche control measures on the DMF characterised predominantly by linear destruction (category 1, 8 and partially 6) is not as evident as on the areally destroyed DMF.
The areal decrease within DMF categories (by 1.5141 ha), which are characterised by areal surface destruction, indicates the decrease of ongoing geomorphological processes on the slopes. The reduction of destructive effects of avalanches in the study area is the result of the positive effects of avalanches control vegetation . The planted woody vegetation took over the soil protective function in the period of forty years from planting. The influence of avalanche control measures within the beginning of avalanche chutes is evidently manifested by the accumulation of snow in their area . Therefore, less snow is blown away from the upper parts of the slope load into the area of avalanche kettle. In the spring season, the slopes are covered with less snow in firn snowfields, which are considered to be the source of snow abrasion . Fewer surfaces being damaged by snow abrasion reduces the area where the impact of other morphogenetic processes linked to an avalanche deposition zone is recorded.
The significant enlargement of surfaces with no signs of surface destruction (the category of surfaces threatened by snow abrasion) and woody vegetation situated below avalanche chutes (Figure 4) indicates favourable conditions for the development of vegetation . One of the major factors is also the termination of intensive land use which is referred as the dominant factor in re-stabilisation of woody vegetation within a timber line ecotone . Overgrowing lower-situated debris areas by woody vegetation refers to a longer period of their stability, which might suggest that they are not loaded with new material. In 2003, debris areas enlarged their areas, but only in places situated in the upper parts of the slope, which refers to the weaker carrying strength of transport forces within erosive chutes . This particularly refers to chutes no. 2, 6 and partially chute no. 7.
Moderating the activity of destructive factors en bloc indicates an evident change of areally destroyed categories (areas with partially and completely exposed surface) into the areas classified as DMF categories in the initial stage of destruction or with no destruction signs (Table 3). The reduction of soil destructive factors leads to a lower nutrient loss due to erosion. Thus the conditions for the development of vegetation will likely be created  and a gradual overgrowth of areally destroyed DMF arises. The expansion of turf cover is visible in Table 3, especially in categories no. 5 and 9. Within the categories destroyed by morphogenetic factors, an area of 1.4951 ha was transformed into surfaces with no signs of destruction (category no. 3).
Several evaluated categories (no. 1, 4, 5, 6 and 9) showed a relatively high variability. This was evaluated as a tendency to transformation into other categories. Table 3 shows that at least a half of the area of the mentioned categories did not remain without changes at the end of the monitoring period. During the period from 1961 to 2003, these DMF categories were the most sensitive to any changes that occurred within the avalanche kettle. The highest tendency to changes was recorded with the surfaces in the initial stage of destruction. Their vegetation cover is not compact and so is open to the influence of a variety of geomorphological processes [4, 16]. On the other hand, they are still covered with relatively dense vegetation, which, in case of eliminated destructive factors, is able to cover an exposed substrate to a certain extent , especially if rich in nutrients.
The category of afforested areas (no. 7) was established by planting the avalanche control vegetation at the beginning of avalanche chutes, which were classified as the category of alpine meadows (no. 2). It partially appears on the surfaces, which in 1961 showed signs of initial surface destruction, as well as on the surfaces with a partially exposed base.
The period from 2003 to 2009
The changes of evaluated categories, which took place in the period of seven years, can be found in detail in Table 4. A synthesis of partial findings leads to the conclusion that the changes in the evaluated categories are not so obvious in comparison with the previous period. A substantial part of the area of most categories remained unchanged, except for the areas covered with debris and surfaces in the initial stage of destruction. This indicates their sensitivity to any changes in the intensity of geomorphological processes, resulting particularly from the impact of climatic factors. The fact, that the period of monitoring the changes over seven years is long enough to acquire data about the variability of evaluated categories, was confirmed by comparable studies . This refers particularly to the intact turf and woody vegetation (categories 3 and 10), which indirectly point to the changes in categories characterised by the surface destruction.
The enlargement of surfaces at different stages of destruction (totally by 0.5102 ha) and the decrease of previously intact surfaces (by 0.8795 ha) indicates the development of destructive processes [4, 36]. However, the enlargement of woody vegetation located below erosive-avalanche chutes by 0.3749 ha suggests that soil degradation processes destroy the surface in situ, but do not have an impact on lower-situated locations. It may be the case of the processes determined by the presence of heavy and wet snow in the spring period [14, 15], which disrupts a continuous vegetation cover and so opens the way for the realisation of other destructive processes .
The fact that the area of erosive-avalanche chutes (categories 1 and 8) was enlarged by 0.0953 ha and at the same time it shortened slightly (Table 2) refers to their widening. The reduction of debris areas located below the chutes implies that there are geomorphological processes that do not produce coarse-grained products by means of weathering. The nature of geomorphological processes in the mountains  infers that the fine-grained products produced by weathering are mainly the result of processes of water gravitational erosion, cryogenic processes and partially also aeolian destructive processes. Since it is the case of the surfaces with no signs of destruction in 2003, geomorphological processes disrupted the integrity of vegetation and obtained eroded soil particles while moving in the course of the following years. This fine-grained material was transported by gravitational processes (especially by water) and enriched the debris area . The increase of nutrients caused the development of woody vegetation in particular . The intensity of destructive processes is supported by the fact, that in the course of the seven year monitoring, this area has changed from the surfaces with no signs of destruction (category 3) into the completely destructed areas. There are relatively intense geomorphological processes within the study area . An average annual loss of soil from the area above the timber line reached up to 0.27 mm. At the slope angle inclination of 3-5°, the average soil erosion of 2.7 mm/year can be observed at the exposed surface of calcaric cambisols. During the study of destructive cryogenic processes, the soil elevation up to 94 mm was recorded and up to 70 of such uplifts per year can occur. The extent of soil elevation under turf cover reached up to 8 mm. These processes are related mostly to the gelifluction of destroyed turf cover.
At the end of the previous monitoring period in 2003, the decrease in the total DMF areas was approx. 1.5039 ha. This area became a part of areas with no signs of destruction and vegetation as a whole (mountain meadows, the woody plants planted at the beginning of avalanche chutes and below them). Compared with the previous monitoring period, the area of DMF between 2003 and 2009 enlarged by 0.5102 ha and at the same time, the area of surfaces threatened by snow abrasion became smaller by 0.8795 ha. The area of woody vegetation below avalanche chutes (category 10) enlarged by 0.3749 ha. The increase within this category was caused mainly by joining in the areas of surfaces threatened by snow abrasion as well as the surfaces in the initial stage of destruction. The processes of spontaneous vegetation expansion refer to the favourable climatic conditions and the attenuation of natural and/or anthropogenic disturbances . The results of such processes can be seen within the study area, particularly at the bottom of the avalanche kettle and outside the impact zones of erosive-avalanche chutes (Figure 3).
The period from 2009 to 2012
The data that record the changes within the evaluated categories during the four year monitoring period are shown in Table 5. A parallel to the evaluation of DMF variability with the monitoring period from 1961 to 2003 can be seen. Several evaluated categories (particularly the category no. 1, 4, 6 and 9) show a relatively high level of variability. Table no. 5 shows that at least a half of the area of the categories undergoes some changes by the end of the monitoring period. These DMF categories remain highly sensitive to changes in the intensity of geomorphological processes also between 2009 and 2012. The highest tendency to change was recorded with the surfaces in the initial stage of destruction. Their vegetation cover is not compact anymore and thus is open to the influence of a wide range of geomorphological processes [4, 16], which can be shown as polygenetic forms of soil destruction. On the other hand, they are still covered with relatively dense vegetation, which, in case of eliminated destructive factors, is able to cover an exposed substrate to a certain extent , especially if rich in nutrients.
The category with the woody plants planted at the beginning of avalanche chutes is definitely the most stable one. Their relative stability proves that, in case of the study area, they were planted into rather favourable environmental conditions for growth. Their future existence is essential for fulfilling ecosystem services  focused on the soil protection, namely an avalanche-control protection. In addition, the recent research of their biometric indicators refers to the stability of planted forest stands of dwarf mountain pine. After an initial rapid growth (a height increment up to 9.4 cm ), a present average increment was set to 3.3 cm. The reason of a growth slowdown is probably a retreating phase of the rapid early development of the forest stand . It is one of the reasons we do not expect any significant growth increments in the following years.
During the monitoring period the enlargement of DMF areas by 0.3679 ha was recorded. From 2009 to 2012, the areally destroyed surfaces at different stages of surface exposure as well as debris areas, enlarged in their areas by 0.2378 ha. This happened at the expense of intact turf vegetation (category 3), and mostly woody vegetation located below the avalanche chutes (category 10). This development indicates a gradual progression of destructive processes [4, 24]. A continuing decrease of the intact turf vegetation area indicates the influence of destructive snow impact is becoming more significant. Snow abrasion is the most significant geomorphological process influencing the vegetation compactness within the study area . The snow-rich winters in 2009/2010 and 2011/2012 were a significant driver of snow abrasion [39, 40].
Erosive-avalanche chutes (linear destructive forms - category 1 and 8) became subjects to significant changes. Compared with the previous period, their total length has increased by 60 m to the current length of 1,304 m (in 2012). However, their area enlarged even more significantly by 0.1301 ha (by approximately 12%). This happened at the expense of debris areas as well as areas with partially up to totally exposed surface. Areal enlargement within this category results from the joint influence of snow abrasion processes, water gravitational erosion, cryogenic processes and partially aeolian soil degradation processes [4, 6]. These processes are the reason for the prolongation of erosive-avalanche chutes, as depicted in the picture from 2012.
By means of synthesis of the findings about the dynamics and variability of the evaluated DMF categories during the monitoring period, we will try to outline the expected future development. The area of DMF en bloc is likely to expand. It is evident from the time series of all the destroyed categories. Regarding the further development of areally destroyed DMF, the categories of areas in the initial stage of destruction and partially exposed surfaces will play an important role. These two categories with the total area of 3.1542 ha have the potential to reverse the trend of areal surface destruction. In case of the attenuation of destructive processes, these areas will have favourable conditions for the expansion of vegetation. The intensity reduction of locally affecting destructive factors seems to be improbable. In case of snow-induced processes, the continuation of snow-abrasion processes is expected. Functional avalanche-control measures prevent the avalanches from forming and also avoid snowdrifts in the avalanche kettle. However, the climatic trends of snow cover in the mountains indicate that the percentage of solid precipitation decreases significantly within the altitude range from 1,000 to 1,500 m a. s.l.This affects the higher density of snow cover . The destructive activity of heavy, water-saturated snow is manifested directly by its own pressure with simultaneous downslide movement. It causes a mechanical disruption of soil cover and disintegrated matters, or more precisely the abrasion of the bedrock and sweeping the vegetation. The indirect impact of destructive snow activities is connected with the presence of water when the snow is melting and its consequent destructive effect . The system of snow cover in the recent period is characterised by a later beginning and more intensive thaw at the end of the season . The observed trend may lead to prolonging the regelation period during which the snow-unprotected surface is exposed to the influence of the needle ice. The recorded levels of frost heaving are very high (94 mm). Even the frost heaving of soil mantle covered by turf (8 mm) may contribute to the disruption of the grass-herbaceous vegetation root system. More intensive thaw at the end of winter season leads to synergic devastating of water erosion effects. The surface insufficiently covered by vegetation might be additionally affected by heavy long-term rainfall and wind.
The outlined development of geomorphological processes, especially cryo-niveo-fluvial destructive activities, entails the intensive formation of disintegrated matters [4, 24]. In conditions of the avalanche kettle at the end of the Hornojelenská valley, it is the case of the formation of predominantly soil-rocky products by means of weathering. At present, these coarse-grained accumulations can be seen on the surface as debris areas, especially below the erosive-avalanche chutes. The geological bedrock of the avalanche kettle bottom part consists of Pleistocene and Holocene deluvial sediments . The enlargement of debris areas is possible only on condition that the turf cover is removed and the subsequent surface erosion occurs in a debris mantle. In case of long-term or intense rainfall combined with a significant areal removal of vegetation cover, the formation of debris slides may occur. During the evaluation of their geomorphological effects in the Tatras, debris slides have been characterised  as the most dynamic and intense acting exogenous processes. However, in case of gravitational processes, or more precisely water-gravitational processes, steep slopes covered with sliding rocks seems to be more likely to occur . One erosion furrow/debris slide of such a kind can be currently observed just a few hundred meters from the study area. The expected continuing destructive processes will have a significant impact on the weakening of a landscape′s hydricpotential . The reduced ability of landscape to retain rainfall may be reflected in an additional threat to vegetation and soil integrity. The danger of sudden floods at lower parts of the basin has been somewhat eliminated by damming the drainage stream of Jelenec in years 1926-27.
When evaluating the obtained results, some inaccuracy must be taken into consideration. Significant data misrepresentations may be caused by different levels of quality of the input data. Other inaccuracies may be associated with different vegetation aspects while obtaining the source data. Vegetation with its camouflage ability could partially cover destructive morphogenetic forms and thus the presented results might be distorted. Similarly, hardly recognisable transitions between the juvenile woody vegetation and turf vegetation may sometimes slightly distort the area size of one or the other type of vegetation cover.
The end of the Hornojelenská valley, located at the Vel’ká Fatra Mts., has been exposed to the soil degradation effects of natural and anthropogenic factors for at least five centuries. These factors represent the basis for accelerated soil erosion, manifestations of which persist until today. Endangered soil alongside a high avalanche danger led to the afforestation of the threatened area.
Within the study area, a massive avalanche kettle with seven erosive-avalanche chutes was mapped. Exogenous geomorphological processes contributed to the formation of the destroyed areas at different levels of surface distortion and occupy an area of approx.14% of the avalanche kettle. The evaluation of their dynamics and variability in four time sections brings us to the conclusion that the avalanche-control measures have a significant effect on the area reduction of the most destroyed surfaces. What is more, they create favourable conditions for the development of vegetation. In the period after 2003, a gradual expansion of all destructive morphogenetic forms was recorded. Compared with the beginning of monitoring, some of them enlarged their areas. Areas covered with debris alongside the areally destroyed surfaces on different levels of surface disturbance (categories 2-4) are the least resistant to changes related to both an increase and decrease of the intensity of the surface destruction. Among the signs of the intensity of surface degradation are the erosive-avalanche chutes the most significant. Extension of five chutes was recorded during the period 1961-2012. The average intensity of the chute′s vertical erosion is 20-24 mm/year.
By means of synthesis of the findings about the dynamics and variability of the evaluated DMF categories during the monitoring period (1961-2012), we expect that the area of DMF as a whole is likely to expand. It is evident from the time series of all the DMF categories. Regarding the further development of areally destroyed DMF, the categories of areas in the initial stage of destruction and partially exposed surfaces will play an important role. These two categories have the potential to reverse the trend of areal surface destruction. In case of the attenuation of destructive processes, these areas will have favourable conditions for the expansion of vegetation.
This research was supported by the project VEGA no. 1/0186/14. The author thanks Mr. Michael Harlan Lyman for proof reading of the manuscript.
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