Erosion in subalpine grasslands of the Alps – State-of-the-art, remaining questions and current research
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In many regions of the Alps grassland in the subalpine elevation zone is affected by shallow erosion. This group of mechanical denudation processes includes shallow landslides and abrasion by snow gliding and avalanches, which are displacing patches of turf and regolith. The resulting bare earth areas and the dislocated material are susceptible to secondary erosion and further particle transport by water or wind. On the one hand, several studies report an increasing occurrence of such eroded areas over several years to decades. They partly attribute this to agricultural extensification or abandonment and related vegetation changes, and a few studies also suggest a possible link to climate change. On the other hand, periods of accelerated and reduced erosion, respectively, have been observed to alternate irregularly and, for some areas and time periods, the recovery of eroded areas by vegetation succession seems to counterbalance the development of new eroded areas. The complexity of these spatio-temporal erosion dynamics is likely an effect of the manifold, interacting factors that are controlling erosion activity. Some of these factors are highly variable in space and/or time at various, often multiple scales. This concerns for instance (i) the small-structured spatial patterns of the natural predisposition for erosion (depending amongst others on slope morphology and geology), (ii) variable natural and anthropogenic influencing factors (such as land use and vegetation) and (iii) the spatial and temporal occurence of different triggers for erosion activity (e.g. precipitation events or snow conditions). Hence, an identification of trends and patterns of erosion and succession or a reliable estimate of erosion rates is challenging, both at the hillslope scale and regionally. In the first part, this contribution seeks to summarize the current knowledge on the main geomorphological processes for Alpine grassland erosion as well as on estimated process rates and to point to the most important unanswered questions. The dependence of erosion on various natural and anthropogenic factors, such as geology, slope morphology, agricultural land use and climate is discussed. Case studies at different test sites in the Alps indicate that abrasion by snow gliding and avalanches must be rated as the main process for initial development of shallow eroded areas. We argue that a systematic monitoring is needed (i) in order to localize and quantify the areas that are affected by erosion, (ii) to improve the understanding of opposing erosion and stabilization processes and (iii) to progress towards estimating rates of shallow erosion in Alpine grasslands at various spatial and temporal scales. Hence, we suggest detailed process-oriented analyses at the hillslope and at the plot scale, but also spatio-temporal analyses at catchment to regional scale, to evaluate robustly how the process activity depends on (changing) land use and climate. In the second part, we provide an overview of our current interdisciplinary research activities, which are systematically addressing some of the issues outlined above. In several study areas in North Tyrol (Austria) and South Tyrol (Italy), we pursue an approach that combines remote sensing and field work at plot scale to catchment scale. These investigations comprise for instance the stability of the regolith depending on its components and properties (including plant roots), or the mechanisms and dynamics of vegetation succession on eroded areas. Moreover, spatio-temporal dynamics of erosion are mapped and quantified, and identified patterns are analyzed considering also small structured patterns of potential drivers, such as historic and contemporary land use.