GEOMORPHIC RESPONSES TO LANDUSE CHANGES ON STEEP SLOPES IN TIMBERLINE ENVIRONMENT; CENTRAL ALPS, AUSTRIA1

ERICH STOCKER ∗

Key-words: Alpine soil erosion, alpine gullies, alpine pastures, blaiken, snow avalanches, Austrian Alps.

Veränderungen geomorphologischer Prozesse und Formen als Reaktion auf den Nutzungswandel an steilen Almflächen im Bereich der Waldgrenze; Zentralalpen, Österreich. Seit Jahrhunderten wird die Höhenzone im Bereich der Waldgrenze für Weide und als Mähwiesen genutzt. Damit wird ein weites Spektrum von Abtragsprozessen (Frostverwitterung, Solifluktion, Rasenabschälung, Abspülung, Massenbewegungen, Schnee- und Lawinenschurf) durch die Tätigkeit des Menschen modifiziert, und teils sogar gesteuert. Die Wirkung der natürlichen Variablen auf die Prozesse des Abtrags wie Geologie, Klima, Relief, Boden und Vegetation kann sich dadurch deutlich verschieben. Eine der am deutlichsten sichtbaren Phänomene an Steilhängen dieser Höhenstufe bezeichnet man in den Ostalpen als „Blaiken“. Sie treten als Kahlflächen oder nur schwach von Vegetation bedeckte Flächen hervor und stellen typische Formen beschleunigter Abtragung dar. Viele Arbeiten haben sich damit beschäftigt, mit Hilfe der Blaiken als Indikator eine Relation zwischen menschlichem Einfluss und geomorphologischen Wirkungen nachzuweisen, da Blaiken mit Hilfe von Luftbildern hinsichtlich ihrer Ausdehnung und Verteilung sehr leicht erfasst und quantifiziert werden können. Allerdings ergaben schon die Untersuchungen über die Prozesse der Entwicklung dieser Erscheinungsformen beschleunigter Hangabtragung sehr unterschiedliche Resultate: vor allem auf Grund von Untersuchungen in den Nördlichen Kalkalpen wurden sie als einfache Bodenrutschungen oder durch Schnee- und Lawinenschurf erklärt. Messungen südlich der Hohen Tauern (Kreuzeckgruppe) ergaben, dass viele dieser Kahlflächen durch ein Zusammenwirken mehrerer Prozesse entstanden, wobei jene der Frostverwitterung kombiniert mit Kammeisaktivität und sommerlicher Bodenabspülung hier als wirksamer eingestuft wurden als jene durch Lawinenschurf. Die Arbeit versucht daher anhand von Dokumentationen von 26 Steilhängen im Umkreis der Hohen Tauern eine Typisierung der Blaiken, aus der hervorgeht, dass Blaiken nördlich des Hauptkammes der Hohen Tauern primär im Zusammenhang mit Grundlawinen erklärbar sind, die Vorkommen in den Zentralalpen südlich der Hohen Tauern dagegen vor allem durch Rasenabschälung entstanden oder sich ausgehend von Runsen als Nischen mit rascher Rückverwitterung entwickelten. Ein Monitoring der Blaikenentwicklung im Bereich Lenkengraben (Kreuzeckgruppe, Kärnten) zeigte, dass sich die Aktivität der unter der Waldgrenze liegenden nischenartigen Blaiken seit 1980 rasch, jene der über Waldgrenze liegenden flachen Blaiken nur langsam verringerte. Dies wurde mit der Dokumentation der Landnutzung seit 1950 in Beziehung gebracht. Zunächst wurden Mähwiesen an Steilhängen eingestellt. Erst als 1968 auch die durch Hirten gewährleistete flächenhafte Schafweide und Ziegennutzung eingestellt wurde, kam es zu einem schrittweisen Überwachsen der Kahlflächen durch Grünerlengebüsch, sowie Jungwald aus Lärchen und Fichten. Die Schafweide verringerte sich und bei freiem Wandern der Schafe wurden vorwiegend die Areale oberhalb der Waldgrenze genutzt. Die Steilhänge darunter fielen über Jahrzehnte nahezu brach.

INTRODUCTION

Features of accelerated erosion in the Eastern Alps are named often as “blaiken” or “plaiken”, a term which was used for studies since STINY (1910). They were described as bare surfaces created by sliding, snow avalanche scour and/or rain-wash, favoured by slope angles steeper than 35° and by weak subsurface rock conditions which provide ample and fine weathering material. As the dominant process in generation of blaiken most studies from the Northern Calcareous Alps quoted translational

∗ Assoc. Prof. (retired), Department of Geography and Geology, University of Salzburg, Hellbrunnerstrasse 34, A 5020 Salzburg, Austria. 1 Paper presented at the IAG Regional Conference on Geomorphology Landslides, Floods and Global Environmental Change in Mountain Regions, Braşov, September 15–26, 2008.

Rev. Roum. Géogr./Rom. Journ. Geogr., 53, (1), p. 91–106, 2009, Bucureşti. 92 Erich Stocker 2 slips, caused by rainstorms during the summer season. Natural predispositions for accelerated erosion are mostly supported by alpine farming which implies clearing of forests on slopes below the natural timberline, growing of bushes and scrubs which favour avalanche erosion (shortcomings in the preservation of pastures) and a high intensity of animal treats. The increasing areas of such patches of bare surface in the last decennia’s has been generally explained as a consequence of the reduction in staff of the Alpine farming because the former labour- intensive measures and treatments of soil conservation were more and more neglected. In the last decades investigations (e.g. Schauer 1975, Kelch, Drexler & Zech 1977, Riedl 1982, Stehrer 1987, Blechschmidt 1990) showed strong evidences for a relationship of blaiken with the change of land utilization in alpine areas. As an indicator for human impact, blaiken can be easily identified on aerial photographs and their distribution and extension can be measured exactly. By studies performed in the subalpine and alpine belt of the Central Alps of Carinthia (Stocker 1971, 1985, 1996) a succession of processes has been observed on steep slopes beginning with turf exfoliation and needle ice action on sharp and frayed turf borders. Developing bar surfaces underlie rain-wash during the summer season and procure an abundant erosion of loose material along the border on which the blaiken-surface cuts the weathering horizons. Other basic differences in comparison with the described blaiken from the Northern Calcareous Alps were found in on-slope process interactions and interactions between blaiken slopes and ravine channels. Accelerated erosion of larger extent contains often both regolith and subsurface rock layers and creates a sequence of form elements and interacting processes. In addition, weathering processes play a more important role than snow avalanche scouring. The actual study focuses on comparisons of features of accelerated erosion from 26 different sites north and south of the main chain of the Alps (Hohe Tauern) which could help to create a typology in which it is shown that “blaiken” can be explained not only by one typical process or a typical set of processes, in fact they are resulting from different geomorphic processes and process-interactions influenced by human impact. Land use change can trigger effects which vary in dependence upon the types of accelerated erosion. A monitoring in the study area of Lenkengraben, Kreuzeck-Group, Carinthia, over 30 years shows a gradually decrease of blaiken activity, which proceeded faster below the natural timberline and very slowly above the timberline. The different stages of stabilization can be attributed as a result of general reduction of pasture on steep slopes in the timberline environment.

STUDY AREAS

For reasons of comparability the slopes were selected in terms of similar altitudinal ranges and slope angles (generally between 30° and 45°); further the slopes are predominately turf covered, suitable for alpine pastures, and situated around the timberline. Areas created by glacial scour, such as ice-scoured and rocky trough valley sides and areas consisting of roches mountonnées were excluded. The high number of sites makes it possible to study also the influence of a wide range of comparable crystalline rock conditions and climatic conditions both on the north- and the south side of the main chain of Hohe Tauern. The studied slopes of the northern Hohe Tauern comprise areas of the geologic units of the “Tauern Window” which belong to the Austroalpine crystalline complexes with the central gneiss- zone. On these very resistant rocks, accelerated erosion is found only on steep slopes which are not primarily dominated by glacial activity. The Northern part of the Sonnblick- and Glockner-Group belongs to the Austroalpine Cover and consists of a series of schist’s, the “Bündnerschiefer” (Neubauer & Handler 2000). In average, the resistance of these metamorphites (sericite-mica-schists, calcareous mica-schists, greenschists and gneisses) is varying from mainly soft to partially hard. Extended slopes of these areas with a relative relief of about 1000–1200 m, show a minor amount of glacial origin in landscape development. The steep slopes underlie primarily gully erosion and ravine development. Study areas in the south of the Hohe Tauern are belonging to the Middle Austroalpine basement and consist

3 Geomorphic responses to land use changes in Central Alps, Austria 93 primarily of a wide range of phyllites and schists and show a high variability of resistance, tending largely to rock deformation by gravitation (gravitational sagging, toppling, “Bergzerreissung”).

Fig. 1 – Study area, distribution of sites.

CHARACTERISTICS OF ANALYSED SLOPES

17 of the analysed slopes are situated north of the main chain of Hohe Tauern and Niedere Tauern, 9 are located on the southern side of the main chain. The mean altitude of the crest is about 2300 m a. s. l. , the base of the analysed slopes has an average altitude of 1600 m a. s. l. ,it means that generally 350 m of height level differences are considered both above and below the mean timberline. Most of the analysed slopes are orientated between southeast and west. Slopes of the north-sectors, which were controlled primarily by glacial erosion, show different conditions. The inclinations of the slopes were measured by 100 m contour-lines beginning on the watersheds. The results give an overall view of slope angels beginning with those for 100 200 m... difference in altitude, measured from the watershed. The data show only a small decrease of slope angles from 37–40° for the first 100 meters on the upper slope to 34° on the whole slopes including the lower slopes (Fig. 2). Slope angles depend clearly on geologic conditions (strikingly steeper slopes in areas of “central gneiss”). Glacial undercut of slope bases on stable slopes over gneiss result in distinctively steeper slopes (Fig. 3). Most of the analysed slopes are characterised by ravine-systems with dense gully networks (Fig. 4).

TYPOLOGY OF “BLAIKEN”

The generally turf covered pastures (or forests below the timberline) contrast clearly with features of accelerated denudation visible as areas of absent or thin vegetation (blaiken). Mostly the borders between turf and blaiken are sharp especially on the upper parts and build distinct scarps; however their lower borders can form small transitional areas of pioneer vegetation. Dimensions and features of accelerated denudation vary from patches of square meters to areas of hectares.

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Study area crest base r or. Typ s 200 s 400 s 600 s 800 s1000 s1200 1 Schafelkopf 2200 1450 750 SE D 40 37 35 2 Schreckalmhöhe 2100 1550 550 SW D 44 40 3 Breitebenkopf 2200 1100 700 SE D 39 34 32 4 Baukgl. Bergmähder 2200 1900 300 SE D 36 5 Geißbach Sladinkopf 2300 1450 850 SE D 42 40 38 35 6 Krumlschafweide 2700 1500 1200 S D 43 41 40 39 37 40 7 Ritterkopf NW 2400 1600 800 NW B; C 34 36 36 34 8 Zitterauer Tisch 2400 1200 1200 S D 40 37 35 33 33 36 9 Ortberg 2400 1250 1150 S D 46 48 47 45 46 10 Lafenbach 2500 1700 800 S D 39 32 34 27 11 Glingspitze 2400 1500 900 SW D 39 38 35 34 12 Gurenstein 2200 1600 900 W B, D 36 33 27 13 Spielkogel E 2000 1600 400 E D 33 33 14 Spielkogel N 2200 1800 400 N B, C 32 28 15 Filzmooshöhe 2100 1700 400 S B 32 33 16 Draugstein 2300 2000 300 SW D 36 17 Lanschützgraben 2350 1700 650 SW D 40 30 27 18 Vd. Waschkarsp. 2200 1200 1000 W D 41 37 38 39 37 19 Lenkengraben 2200 1200 1000 SW A, B 37 34 38 29 28 20 Lackenbichl 2300 1900 400 W A, B 34 30 21 Kl. Grakofel 2400 1600 800 NE B, C 31 31 29 22 Hoher Bolz 2400 1900 500 S B 33 33 23 Reisachspitze 2500 1500 1000 W A, B 42 36 36 34 24 Paterskopf 2700 2000 700 SW B 37 34 26 25 Leitenkopf 2450 1800 650 S B, C 34 35 33 26 Kluidhöhe 2600 2400 200 SW B 23 Fig. 2 – Study areas: crests-altitudes, slope base (meter), r...relief (m), orientation, erosion types, values of slope angles measured from crest for differences in altitude from 200 m to 1200 m (s 200 to s 1200).

Fig. 3 – Slope angles for selected slopes measured from crests (for differences in altitude from 100 m to 1 200 m). The slopes Krumlschafweide (6), Zitterauer Tisch (8) and Ortberg (9) developed over gneisses and steepened by glacial undercutting of their bases stand out clearly from the others, which are developed over schist’s and phyllites.

5 Geomorphic responses to land use changes in Central Alps, Austria 95

Fig: 4 – Typical steep valley side slopes with differences in altitude of more than 1000 m. At the left “Krumlschafweide” (6) with an average inclination of 40° used for sheep pastures in the northern Hohe Tauern and on the right the W-faced slope of Reisachspitze (23) in the Deferegger Mts., south of the Hohe Tauern.

Type A: Considering only steep slope areas, features are limited on sites of the southern Central Alps. Analysis of blaiken in the area19 (Lenkengraben) showed relatively extended erosion features on the steep sides of deeply incised ravines cut as niches both in the solid rock and in the overlying regolith and turf (Fig. 5). A phase of channel incision in the ravines led partially to a strong coupling between the blaiken-erosion on channel sides and channel erosion. Accelerated back wearing of the channel walls has led to a far retreat of gully heads (Fig.4). Finally niches- development ends in a reduction of the cliffs, an expansion of basal debris cones and a stabilisation by pioneer vegetation. Geomorphological mappings showed that this type of blaiken is distributed primarily near the timberline and below, and also in sufficient distance from the watershed, were the incision of ravines reaches values between 50 to 100 meters. From 54 measured active features in a study area of 27,4 km², the average area of type A amount to 1540 m², their mean slope angle is 41,3 degrees. Blaiken of this type occur mostly in phyllitic rocks of lesser stability and therefore underlie deformations due to gravity with effects like toppling, deep sited creep or overturning. These kinds of loosening rock structure favour upward retreat of cliffs. The activity of cliff retreat is favoured further by a maximum of freeze-thaw-cycles during the late autumn and the early spring seasons. Human impact with the clearing of forests in the Middle Age and later led to a lowering of timberline favouring snow avalanche scour on the sides of ravines and as a consequence the disturbing of revegetation of the channel walls.

Type B: The features occur also primarily on sites of the southern Central Alps. North of Hohe Tauern they were found only on slopes, which are not affected primarily by snow avalanche scouring with slope angles generally below 33–34°. They differ from type A by their shallower form which generally affect only soil and regolith cover, but often they are cut in the rock surface and form shallow niches and small rocky cliffs. In contrast to type A the coupling to channels occurs only partially or it is absent, their occurrence include all slope surfaces and is much denser ( n = 22, 4 / km² ) with singular areas between 10 to 500 m² (Stocker 1996). The peak of their distribution lies clearly above the

96 Erich Stocker 6 treeline between 2100 m and 2400 m altitude in the Kreuzeck Group. Resistance of rocks against weathering processes on schists, phyllonites and phyllites is controlled primarily by jointing, cleavage and local mineral composition, further by the angle of which the slope cut these structures. The local slope angle plays another important role. Therefore a local remove of soil and regolith arise from different controls by structure, land surface shape and local weathering influences (exposition, orientation, snow cover conditions, humidity). Human land use plays further an important role. Pasturing implies variation in vegetation cover, development of steps, local destruction of turf cover, soil compression. The blaiken type B can arise from small bare areas by enlarging of turf-cliffs by needle ice action and removing of fine material by wind and rain-wash. Depending on the local conditions, the features can become a variable pattern consisting of patches and strips.

Fig. 5 – Type A and type B on Lenkengraben area (19).

7 Geomorphic responses to land use changes in Central Alps, Austria 97

Type C: The development of type C is to a great extent similar to those of type B. Black phyllites and carbonatic micascists of the Austroalpine Cover and also phyllites of the Middle Austroalpine Basement produce largely fine weathering materials. Under the condition of a moderate slope- steepness which does not exceed slope angles of about 35 degrees, the regolith cover underlies processes of solifluction. The environmental conditions permit grazing for cattle and sheep’s. Their pattern of steps are crossing the pattern of well developed turf banked solifluction lobes giving rise to ruptures, turf cliffs and the breaking down of parts of the solifluction lobes. Such kinds of destruction of the turf bounded solifluction lobes lead to further turf exfoliation and enlargement of bare patches and stripes, which underlie accelerated denudation (Fig. 6).

Fig. 6 – Type C: Kl. Grakofel (21) left and right, NW Ritterkopf (7) and type D: Baukgl. Bergmähder (4).

Type D: On steep slopes on the northern side of Hohe Tauern and also on the Northern Calcareous Alps snow avalanche scouring gives rise to a wide spread and effective reduction of soil- and regolith cover. Frequently the areas of avalanche scouring are extended below the timberline both by natural avalanche activity and human impact by deforestation for purposes of expansion of pasture areas. Avalanche scouring can easily be identified by their typical dense pattern of straight stripes and scratches, small, regulary distributed patches or larger areas of bare surfaces, all structured in downward direction. This type occurs nearly on all the analyzed slope-sites north of the Hohe Tauern but it is found only on small restricted areas south of the Hohe Tauern, especially on lee-sided, concave slopes. The occurrence of type D is limited therefore to slopes with full-depth avalanches (Gardner 1983). In the northern Hohe Tauern during winter season snow accumulation reaches normally more than 2 m causing full-depth snow avalanches primarily during late winter and spring with effect of snow scouring or shifting of turf. The resulting features were described especially in the

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Northern Calcareous Alps by Laatsch & Grottenthaler (1972). Stahr (1996) found abundant evidences that the development of that kind of blaiken generally can be explained by removal of turf under the pressure of a slowly gliding thick cover of wet snow and not by gravitational processes due to exceptional rain intensities. The examples of the northern Hohe Tauern show, that snow scouring areas predominate clearly areas on which snow-gliding take place.

PROCESSES OF ACCELERATED DENUDATION

Type A: As a consequence of gully incision, steep sidewalls have been incised in a former V-shaped ravine. This undermining of steep slopes of about 37° has led to the formation of shear planes which can cross the cleavage of phyllites and phyllonites, creating curvatures of cleavages, overturning, dilatation of joints and loosening of the rock structure by deformation (Fig. 7). As a consequence, on areas of weakness, and under the conditions of strong frost-weathering, an accelerated upwards retreating of rocky cliffs takes place. Series of stages of such retreating and widening niches on the sides of ravines in the Lenkengraben (19) can be observed (Fig. 4, 5). The first stage begins with a rocky scar on which rock weathering and rock fall lead to a retreating cliff with a small debris cone which feeds coarse materials to the ravine channel. As the frost cliff retreats upward it becomes mostly smaller and develops as a steep gully-head, the foot slope with debris scars then becomes longer. On the upper side of the gully head the cutting of regolith and turf cover produces a turf-cliff which may progress more rapidly creating a bare area on which soil erosion dominates (Fig. 9). Denudation of type A shows typical seasonal cycles. Measurements in a 5-year period (1974–1979) at an altitude of 1820 m show, that the number of frost-change days (air temperature) reaches a maximum during a season with partial snow-cover at an altitude between 1700 m and 1800 m (Fig. 8). The steep south-faced rocky gully heads underlay effective frost-weathering, supported by snow- melting and thawing conditions. Measurements of the rockfall-activity (Fig. 9) over a period of 1598 days between 1971 and) 1978 show that the production of coarse material during later autumn and early spring widely exceeds those during the summer (Stocker 1985).

Fig. 7 – Cross section of “blaiken” type A (left).

9 Geomorphic responses to land use changes in Central Alps, Austria 99

Fig. 8 – Snow cover and frost-change days on a site in1820m altitude, Lenkengraben (right).

Fig. 9 – Rock-fall intensity on a gully head in 1820 m altitude, Lenkengraben (19).

From May to September, on the bare and steep gully surface, rainfall removes loose materials by building rills on the gully sides and small central debris flows with basal debris cones. On the erosion area between the turf-cliff and the rocky gully-head, during the winter season, needle ice solifluction takes place and turf cliffs are undermined. During summer season rain wash and rill development can be observed. The soil erosion areas show principally the same dynamics as can be seen on type B and C.

Types B, C: Accelerated denudation predominantly takes place in regolith. The thickness of regolith cover and the portion of fine sizes of material are essential for the efficiency of erosional processes. The starting points of development can be small breaks of slope, but for the most part local destructions of turf cover can enlarge under conditions of high slope angles, increased weathering (by local weaknesses of rocks), unimpeded material remove. On wind exposed slopes with diagonal orientation of garlands and lobes, the movement of loose material can change in downward direction in case of destruction of the vegetated garland-borders giving rise to strips of bare surfaces (Fig. 6). During the cold seasons and under partially snow free conditions, needle ice supplies material from turf cliffs; during summer the loosened material then underlies transportation by rainwash. High rain intensities increase entrainment of fine material, but generate crusts and stone pavements, which reduce simultaneously material remove, whereas pavements can be destroyed only by hail. During the summer seasons between 1977 to 1981 measurements on two plots of about 10 m² yielded values between 23 and 33 g/m²/d for a period of 244 days. Maximum erosion of 866 g/m² has

100 Erich Stocker 10 been recorded on 4th August 1980 after a hailstorm (Stocker 1985). With increasing altitude the susceptibility for development of type B rises. Bare areas become enlarged offering a vast amount of weathering material disposable for entrainment by debris flows. Their channels may connect erosion areas and procure a down slope transmission of material (Fig. 10). Blaiken type B and C which are not coupled with basal gully or debris channel, storage material downslope by fixing on vegetation (Fig. 6) and also give rise to solifluction tongues.

Fig. 10 – Enlarged bare areas of type B, which build source areas for intense development of debris flows on SW-slope of Paterskopf (24).

Grazing of sheep generally intensify denudation, especially on type C were the tracks of animals cross borders of solifluction lobes giving rise to ruptures, compaction of treats, mobilization of loose material, steepening of the borders of tracks.

11 Geomorphic responses to land use changes in Central Alps, Austria 101

Type D: Under the influence of gliding of heavy wet snow and full-depth snow avalanches, patterns of patches and strips without vegetation cover appear. Densely arranged gullies which form the drainage system, act together as avalanche furrows (Sekiguchi & Sugiyama 2003). Even if the gullies are not generated primarily by avalanche scouring, full-depth-snow avalanches play the most important role in the material remove. A high percentage of the gully system has perennial runoff. Gullies begin on small sources on the gully heads (Fig. 11). Despite of the weakness of the phyllites and the fast production of weathering materials, both slopes and gully channels lack of regolith. The turf cover lies directly over the rock-surface. Any local storage of thicker covering of regolith underlies removing by avalanche scouring so that all the channels are nearly free from channel deposits.

Fig. 11 – Lafenbach (10) south-faced valley side with dense gully pattern on black-phyllites. In an altitudinal belt between 1700 m and 2200 m strong full-depth snow avalanche scouring occurs.

Patches of bare soil develop on exposed positions, were scouring processes are frequently acting, on positions on which abundant material for removal is available, on positions on which scrubs (mainly alders or young spruces) offer further obstacles to remove. Development of type D by avalanche scouring, however, is improbable in case of very short gliding- distances on positions near the crest… STAHR (1996) argued that deep wet snow covers collapse instantly, with the tear down of the snow masses giving rise to a compression of the existing high water content on the top layers of soil. On the base of the moving snow masses, turf layers begin to “swim” losing the frictional resistance and to curl up; the turf layers can be deposited downwards nearby (Fig. 12). Therefore the pattern of bare surfaces changes in dependence on the episodic avalanche events and local slope conditions. Especially concave foot slopes below maximum slope segments underlie stronger scouring because of their exposition and their higher availability of accumulated material (Fig. 6).

102 Erich Stocker 12

Fig. 12 – Pressure of heavy wet snow leads to a compression of turf which under high pore water content favours gliding and curling up of parts of turf, especially on loamy soils (after A. Stahr 1996).

The erosion pattern of type D exhibits therefore a snow avalanche controlled denudation system. The produced weathering materials are integrated into slope processes only to a small amount. In fact, bare patches due to avalanche scouring or gliding underlie rain-wash during summer, but the removed fine material can be estimated as modest. Although the available material is limited, transports of materials deposited during winter and spring, take place during summer in the gully drainage network. Generally during summer vegetation colonization tend primarily to a stabilization of bare surfaces.

GEOMORPHIC RESPONSES TO LAND USE CHANGE

Since the Middle Age the Alpine farming has a big importance in agriculture in alpine areas. For this purpose and partially for industrial and mining purposes extensive areas of forests below the natural timberline have been cleared with the effect that the treeless belt extended into the subalpine forests for at least 100 to 200 meters differences in altitude. This implies also that the subalpine dwarf scrub was modified in favour of grassland. The human impact in the timberline belt generated significant changes of hydrologic and geomorphic processes (increase of runoff, channel incision, accelerated erosion, stronger frost action and solifluction, extension of snow avalanche source areas). To prevent a deterioration of the used areas maintenance of pastures and meadows efforts have been carried out with a high input of staff. In the last 60 years by lack of staff and for reasons of cost on steep slopes various land use changes took place. As an example, a monitoring has been carried out in the study area of Lenkengraben (19), Kreuzeck-Group, Carinthia, where the following changes of land use took place: • Until 1950 the abandonment of mowing steep meadows has been finished by giving up also the lowest locations of the meadows. • 1950–1968: all the former meadows were included into the pasture. The pastures of sheep were attended by shepherds; they ensured that the whole area of the pasture was grazed as evenly as possible.

13 Geomorphic responses to land use changes in Central Alps, Austria 103

• 1969–1995: Enclosures for cattle were put up on flatter and lower positions on the margins of the steep slopes. The livestock of sheep and especially the goats were reduced. As a consequence of pasturing of sheep without shepherd and enclosures, the grazing of sheep concentrated on the higher located areas of the pasture above 1900 m of altitude. Over this period of 26 years the grazing of sheep affected lower positions of the pasture only marginally and for short term periods. In the meantime the young forest advanced below the natural timberline and further impeded the pasturing.

Fig. 13 – Strong temporal change and stabilization of blaiken type A (altitude 1650–1800 m) between 1975 and 2005 in the Lenkengraben area (19).

• 1995–2002: This period can be evaluated as occasional. Changing of owners led to a short period of intensification of sheep pasturing. The number of sheep increased drastically. For extension of pasture fire clearing of dwarf scrubs on the timberline was performed. However, for lack of

104 Erich Stocker 14 shepherd, the grazing without enclosures continued. Only the pastures above the timberline have been strongly affected during this intensification period. • After 2002: The livestock of sheep has been reduced again. The remaining flock of sheep grazes without enclosures. Nearly all the pastures below 1900 m of altitude are nowadays without use of pasturing. Geomorphic responses to these changes of land use took place 15 years with delay, as a response to the vegetation-development. The first aerial photographs (1954) show a maximum extension of blaiken type A and B. The series of aerial photographs from 1975 show, that pioneer vegetation as alders (Alnus viridis), young larches (Larix decidua) and spruces (Picea abies) began to grow on limited positions. Only after 1980 the ancient active cliffs and ‘blaiken’-areas have been largely covered by annual pioneer plants, alders and larches, which progressively impede the natural processes like frost action, gravitational movement and impact of rain and hail. Widespread natural growth of young forest in abandoned meadow areas below the natural tree line has led to a decline of snow avalanche activity and created more favourable conditions for covering the steep areas with young forest which became successively dense. At the same time grass cover were suppressed by growing and expansion of dwarf scrub areas (Vaccinietum myrtilli, Rhododendretum ferruginei, Vaccinio- Callunetum). Above the timberline (1900–2000 m) essential changes of land use did not happen. However, the grazing flocks of sheep were gradually reduced, interrupted only by the short period after 1995, erosional features (mostly type B) changed more slowly because many of the most efficient plants (e.g. Alnus, Larix) colonizing steep and bare surfaces at lower elevations are absent in this altitudinal belt although the global warming allow a beginning growth of bushes of alders or young larches and spruces. However in the last decades only a slow increase of the growth of clumps of grass and other pioneer plants led to a partial stabilization respectively a slight decrease of activity of the features of type B and C. Comparison of aerial photographs from 1954 and detailed photographs from 1981 (Fig. 14) make the small differences of these features over a period of 27 years visible. The former full activity (supposed by the absence of plants) is reduced to smaller areas. Partial activity has occupied many of the marginal areas by a successive increase of pioneer plants and tussocks. Some of the former active areas are completely stabilized by an increase of density of tussocks to a nearly closed vegetation cover. Furthermore it is in evidence that a number of turf cliffs deactivate during this period, identifiable by a reduction of the erosion zone below the turf cliff and the degradation of the step of the cliffs.

Fig. 14 – Small changes on features of “blaiken” type B (altitude 2030 m – 2060 m) between 1981 and 2008 in the Lenkengraben area (19) in the first place by denser growth of tussocks.

15 Geomorphic responses to land use changes in Central Alps, Austria 105

CONCLUSIONS

Accelerated denudation as a response to human impact in the timberline environment can arise from different geomorphic processes. The features of “blaiken” type A show mostly a strong coupling between deep incised ravines and develop on their side slopes within unstable and loosened rock material as retreating niches and gullies. Type B and C represent erosion features developed on steep slopes with abundant regolith cover and extend by retreating turf-cliffs. Type D, distributed north of the main chain of Hohe Tauern, results from external affects of scouring by full-depth snow avalanches. Long term effects of snow avalanche scouring result in a general reduction of regolith. Therefore, processes of type B could act only partially, on patches on which regolith is sufficient. Generally type D is characterised by a fast change of the outlines of features (documented in the Northern Calcareous Alps by Stehrer, 1987); further their shapes are strongly induced by the processes of snow-avalanche scouring, which indicate a negligible influence of type B-processes. There are evidences that land use by pasturing on steep alpine slopes generally favoured accelerated slope denudation. A monitoring in the Lenkengraben area actually shows a wide range of different stages of development. Models deduced from former measurements of scar-retreat, suggested that the niches, scars and side-slope gullies of the Lenkengraben-ravines have been developed over a period of hundreds of years (Stocker 1983). It can be assumed, that erosion type A initiated by a former additional incision of ravines (Fig. 7), triggered by the medieval clearing of forest below the natural timberline in order to expand the use of meadows and pastures. The probably medieval incision which created sidewalls of up to 10 meter have been the starting point for retreating scars and accelerated slope development. Further snow avalanche activity was favoured by extension of their source areas. By considering, that the ravines are used as the main avalanche tracks, snow avalanche scouring along their sidewalls causes episodically the removal of young trees, mainly larches and spruces supporting the activity of scar retreat by mechanical attack through removal of these obstacles. Already before 1980 on the Lenkengraben area some of the most advanced blaiken showed re- vegetation and stabilization. In those examples the cliff-percentage (of the whole length of gullies) has become very small and the average slope angle has decreased below 40°. Ancient stages of development show extended basal debris screes and increasing areas stabilized naturally by re- vegetation; the coupling between the basal channel and the gullies becomes weaker. Stabilization probably can take place when erosion intensity is reduced below a critical amount (Harvey 1994). Consequently the reduction of land use to a great extend (from intense sheep pasture to a nearly unused area) is sufficient for stabilization by re-vegetation. Further is taking into account the giving up of pasturing of goats since 40 years. The goats prefer to climb on gully heads pasturing preferably pioneer plants like shrub, bushes and herbaceous plants. The abandonment of the use of meadows after 1945 led to a change from grassy steep slopes to dwarf shrubs dominated by Vaccinio-Callunetum. These areas around 1700–2000 m of altitude, on which actually the growth of larches and spruces can be observed, has never been affected by accelerated soil erosion (e.g. Karre 2003). The increase of temperatures has been recorded beginning from 1980. Until 2000 the mean temperature increased about 1degree Celsius, in the decade between 1990 to 2000 0,76° Celsius, corresponding to a rise of timberline of at least 100 m of altitude, equivalent to a noticeable better growing conditions in altitudes around 1700 m. Actual development of blaiken Type A since 2000 in an altitude around 1500 m in vicinity show, that damage of forest by storms can trigger events of rock avalanches, which develop to gullies with accelerated scar-retreat. Thick covers of moraine material contribute to high erosion intensity which prevents re-vegetation.

106 Erich Stocker 16

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Received May 28, 2009