**4. Discussion**

The goal behind all restoration activities following interventions is to establish a dense vegetation cover as fast as possible. First and foremost, vegetation protects the soil from erosion by intercepting raindrops and absorbing their kinetic energy harmlessly. If rain drops reach the ground unimpeded, the kinetic energy damages the soil aggregates. This also reduces the water receptivity of the soil. Water not infiltrated the soil is running down the slope, causing erosion. A higher surface runoff is not definitely associated with higher soil erosion. Not only the amount of surface runoff but also other factors (type and coverage of vegetation, soil conditions) determine the extent of soil erosion (Stocking & Elwell 1976). Mosimann (1984) calculated a clear connection between vegetation cover and intensity of erosion. Up to altitudes of 1,600 m a minimum of 70 % vegetation cover is required to avoid erosion. Above timberline, more dense vegetation with a cover of about 80 % is recommended. Results of our EU-project Alperos clearly showed multiple positive ecological effects up from the 2nd year after sowing, if indigenous species were used. To reach sustainable vegetation with a cover exceeding the minimum requirement of 80 %, the use of indigenous seed mixtures is a precondition (Krautzer & Wittmann 2006).

The best period for restoration activities in high altitudes would be the first 4 weeks after snow melt (Lichtenegger 1994). During this period, most soils have a satisfying water content, also on exposed sites. In alpine environments, vegetation has a growing season of two to three months to establish. Especially the generally slow growing indigenous species need 4 to 6 weeks of satisfying growing conditions to germinate and to establish

Fig. 9. Soil losses referring to 500 mm precipitation, comparison of all sites (1999-2001)

use of indigenous seed mixtures is a precondition (Krautzer & Wittmann 2006).

The best period for restoration activities in high altitudes would be the first 4 weeks after snow melt (Lichtenegger 1994). During this period, most soils have a satisfying water content, also on exposed sites. In alpine environments, vegetation has a growing season of two to three months to establish. Especially the generally slow growing indigenous species need 4 to 6 weeks of satisfying growing conditions to germinate and to establish

The goal behind all restoration activities following interventions is to establish a dense vegetation cover as fast as possible. First and foremost, vegetation protects the soil from erosion by intercepting raindrops and absorbing their kinetic energy harmlessly. If rain drops reach the ground unimpeded, the kinetic energy damages the soil aggregates. This also reduces the water receptivity of the soil. Water not infiltrated the soil is running down the slope, causing erosion. A higher surface runoff is not definitely associated with higher soil erosion. Not only the amount of surface runoff but also other factors (type and coverage of vegetation, soil conditions) determine the extent of soil erosion (Stocking & Elwell 1976). Mosimann (1984) calculated a clear connection between vegetation cover and intensity of erosion. Up to altitudes of 1,600 m a minimum of 70 % vegetation cover is required to avoid erosion. Above timberline, more dense vegetation with a cover of about 80 % is recommended. Results of our EU-project Alperos clearly showed multiple positive ecological effects up from the 2nd year after sowing, if indigenous species were used. To reach sustainable vegetation with a cover exceeding the minimum requirement of 80 %, the

**4. Discussion** 

(Urbanska & Schütz 1986). Our assessment on vegetation cover of the plots showed, that under average conditions of high altitudes this minimum cover can be reached the second vegetation period at the earliest. This requires application techniques with additional protection of soil surface for the first vegetation period. From an economic point of view, restoration companies will always try to reach minimum requirements with a minimum of costs. Therefore it is important to give clear answers and stipulations for successful application techniques under average conditions. It is evident that a direct comparison between trials and years is not possible. Hence only clear differences or correlations are discussed.

With our first trial 1999, we also wanted to measure the influence of different seed mixtures on erosion. Due to the faster germination and early growth of commercial varieties, an increase of surface runoff and soil losses the weeks after sowing was expected for indigenous seed mixtures. However, the harsh conditions in high altitudes (low soil and air temperature, short vegetation period, frequent frost) are causing environmental stress to the vegetation, reducing the competitiveness of commercial forage grasses and herbs and neutralizing their greater productivity (Jones et al. 1989) Therefore, results obtained during the investigation period did not show substantial differences between seed mixtures on erosion processes during the first weeks after restoration.

The use of cover crops and in recent time also nursery grasses as additional protection against erosion is often used for restoration activities. Due to positive, longstanding experiences of restoration companies, those techniques were compared to normal hand sowing. Again we noticed that the admixture of fast growing components did not have positive influence on surface runoff and soil erosion. Compared to normal hand seeding, the reduction was poor. Once again, the environmental stress compensated the capability of fast early growth, reducing the positive effects towards zero. Results obtained clearly showed that the use of cover crops and nursery grasses did not have positive influence in view of a necessary reduction of surface runoff and soil losses during the first weeks after restoration.

Hydroseeding is described as one of the best application techniques for steep slopes with good properties in order to prevent erosion. To our surprise, the comparison of hydroseeding to hand sowing plus cover crop showed comparable results. The hydroseeding was carried out by a professional restoration company. Therefore, conditions close to practice can be assumed. One hour after application, we were faced with a raining event of two hours with a precipitation of app. 15 mm. This could have caused some wash out of not yet stabilized gluten, reducing the effect of building a protective layer on soil surface. Even if we take this possible problem into account, results obtained at least indicate a big risk in using this application technique without additional protection of soil surface.

Depending on soil physical properties, climate and altitude, varying characteristics of runoff, infiltration and erosion can be expected (Markart & Kohl 1995). Especially in high altitudes, the main goal behind the choice of a certain application method has to be a reduction of surface runoff and soil erosion to an acceptable degree. A comparison of all used application techniques during our assessments shows clear results. Only an additional cover of soil surface is able to reduce surface runoff and soil losses to an acceptable degree. For our trials in 1999 and 2001-2002 we used a straw mat. But there are a lot of different techniques available that guarantee a sufficient protection of soil surface. Straw mulching,

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hay mulching, different mats, nets made from jute or coco, three-dimensional mats etc. With the first series of trials, we were not able to work out differences between the materials. But a comparison can be made to results gained from field trials in South Tyrol (Waldner 1999, Graiss 2000). There, erosion was measured for different application techniques with and without covered soil surface, regarding to precipitation. A measurement of surface runoff was not possible. However, differences between the used techniques with covered soil surface (straw, hay, with or without bitumen emulsion to glue the organic matter) were low. The proportion between soil losses of covered plots to hand sowing plus cover crop (average proportion of 1:110) is comparable to the results of our project.
