The impact of land use- and climate change on the managed eco-geomorphic balance in the Alps

During the last decades, agricultural development in European mountain regions has caused considerable changes in land use intensity and management and as a consequence, in land cover (Brugger et al. 1984; MacDonald et al., 2000; Bätzing, 2005). The changing economic conditions for mountain farmers are causing a trend towards intensification of centrally located areas where farm machines can be used, whereas remote areas unsuitable for mechanization of farming experience a marginalization. Additionally, future climate scenarios predict a reduction in summer average rainfall accompanied by an increase in short, but potentially devastating heavy rainfall events (Beniston, 2006). Mountain ecosystems are fragile and highly sensitive to environmental alteration like land use or climate change (Steinwidder et al., 2011). The “managed eco-geomorphic balance” in mountain landscapes depends on physical site factors, ecological patterns controlled by organisms, the human impact and on geomorphologic processes. Soils are at the interface of the spheres forming the landscape, and represent a crucial parameter in alpine ecosystem services providing water (Meusburger and Alewell, 2008; Sutter, 2009), nutrients (Sutter, 2009), substrate and habitat for flora and fauna (MacDonald et al., 2000). Soil and slope stability are limited due to low soil depth and soil formation rates. In addition, the steep slope angles are associated to large erosion and mass wasting rates (Alewell and Bebi, 2011). In awareness of the vulnerability to land degradation and due to the dependence on fertile soil resources, alpine landscapes have been actively managed for about 5000 years to ensure ecosystem services for subsistence farming (Bätzing, 2005). During the last decades, several studies report an increase of soil degradation processes in the Alpine region (Tappeiner and Cernusca, 1993; Dommermuth, 1995; Newesely et al., 2000; Tasser et al., 2003; Meusburger and Alewell, 2008, 2014). Considering the limited soil depth (Alewell and Bebi, 2011) and soil formation rate in Alpine regions (Sutter, 2009; Alewell and Bebi, 2011), an assessment of the effect of land use and climate change on the managed eco-geomorphic balance is essential to prevent soil loss and to promote a sustainable development of the Alpine region.
In order to determine the effects of land use and climate change on the managed eco-geomorphic balance, a reconstruction of the environmental history is needed, including the history of land use, mass wasting, and climate. Understanding the past interactions of land use, climate change and soil degradation processes can support the assessment of future risks of land degradation processes and is essential for directed measures to prevent the loss of soil resources. The Ursern Valley in the central Alps of Switzerland offers an ideal opportunity for studying the history of land degradation in the 20th century. Land tenure lies largely with the Korporation Ursern, owning 90% of the agricultural area in the valley (Wunderli, 2011). The Korporation Ursern stores detailed reports on pastures condition and mass wasting events reaching back to 1900. Further on, the reports provide information on extreme weather events, land use intensity and land use management, which can be analyzed in the context of the mass wasting history. In addition, the study area experienced an increase in land degradation since the 1950s. According to Meusburger and Alewell (2008), the eroded area in the Ursern Valley nearly doubled between 1959 and 2004. Thus, a distinct research on the managed eco-geomorphic balance and its controlling factors is necessary.
The first study, published in Die Erde, (Caviezel et al., 2010), aimed at determining the applicability of historical data for the purpose of detecting triggering factors for mass wasting processes and at reconstructing mass wasting and land use history. Based on a pre-analysis of the reports, a checklist for the qualitative and semi-quantitative document analysis of classifiable parameters with relevance for mass wasting processes and land-use management was developed. The analysis of historical data showed, that mass wasting events are not distributed uniformly in time and space. A concentration of mass wasting events was found on geological sensitive areas of the Mesozoic layer shortly after the abandonment of use restrictions.
In order to distinguish between the effect of land use intensity and the change in rainfall characteristics on the increased surface degradation, a second study, published in Geoöko (Caviezel and Kuhn, 2012), was performed. Reported mass wasting events recorded in the historical archives were set in context to rainfall data of the “Swiss Meteo” station in Andermatt. The analysis of rainfall data allowed defining a threshold magnitude for triggering mass wasting events. Extreme rainfall events can therefore be considered as trigger for mass wasting events. However, the frequency analysis of rainfall events above the threshold magnitude and of mass wasting events revealed that the period of highest mass wasting frequency does not correlate with the period of highest frequency of potentially triggering rainfall events. Thus, land use change has affected the susceptibility of landscape towards mass wasting processes as shown by the increased frequency of mass wasting events that coincides with significant changes in land use and the abolishment of use restrictions.
A third study prepared for publication (Caviezel et al., 2015, in prep.), concentrated on the quantification of land abandonment. In the Unteralptal, a side valley of the main Urserntal, landscape characteristics show considerable changes primarily by the heterogeneous encroachment of green alder (Alnus viridis) and Alpine rose (Rhododendron ferrugineum) on former pasture areas. The area encroached by shrubs was analyzed using a series of air photographs. The results showed an increase of green alder cover of 63% between 1959 and 2007. The study also illustrated an interesting effect of the way land cover change is calculated in mountain areas. Assuming the conventional planimetric view, the area covered by shrubs increased by 72.8 ha, while the true area generated values of 86.7 ha. The latter difference is of particular importance when assessing issues such as net primary production or nutrient turnover in a biogeochemical context. Additionally, the analysis of the topographic and geomorphic landform characteristics of newly encroached areas since 1959 shows that green alder shrubs colonize noticeably areas with less geomorphic activity, more gentle slopes and south aspect, showing that the habitat spectrum of green alder is much wider than assumed. Thus, the previous green alder cover was mostly controlled by the former intensive land use on the adjacent areas
A final study in the Unteralptal, published in Earth Surface Processes and Landforms (Caviezel et al., 2014), was performed in order to assess the effects of land abandonment and shrub encroachment on soil stability. Along a chronosequence of shrub encroachment, identified by air photograph analysis, an index for soil stability was generated based on measuring shear resistance, penetration resistance, soil bulk density and rooting density. Soil properties, relevant for soil erosion caused by water runoff and mass movement, show two signals of change towards a greater erosion risk: i) a decreasing shear and penetration resistance after 15 years of shrub encroachment and ii) an increasing porosity, associated with an increase in infiltration capacity, after 40-90 years of shrub encroachment. The increased porosity decreases the stability of the less compacted soil potentially leading to greater rates of soil creeping; the increased infiltration capacity reduces soil erosion by running water. The complex interaction between soil and changing vegetation reveals that neither a general association of land cover with surface processes nor a single soil parameter are sufficient to assess the impact of land use change on slope stability.
The four studies in the central Alps showed that both, land use intensification as well as land abandonment, have a remarkable effect on the managed eco-geomorphic balance, and thus, on soil stability. Land use intensification on pastures, especially on slopes that are prone to mass wasting due to their geology, increases the frequency of solitary high magnitude landslide events. High magnitude rainfall events affected the managed eco-geomorphic balance predominantly in connection with the mentioned land use intensification. This result shows that the effect of land use change has a high relevance for the managed eco-geomorphic balance in Alpine regions comparable to the effect of climate change. Land abandonment changes surface processes towards continuous soil creeping processes of loose soil prone to liquefaction. Beside climate change, it is therefore necessary to consider the changes in soil properties and vegetation composition as well as their interaction with land use, to understand the current and future change in the managed eco-geomorphic balance.