Widening rate of glacier forelands: the case study of the Forni Valley (Stelvio National Partk, Italian Alps)

Alpine glacier shrinkage is accompanied by a progressive expansion of deglaciated areas, both on the valley slope and in the bottom valley. Glacier retreat provides new habitats for plants and animals, that can colonize the glacier foreland. Chrono-sequences and ecesis have been already studied in order to assess the biological response to climate change in glacial environments, but the expansion of proglacial areas and their evolution over time are also topics of great interest in the context of global change. The aim of this study is to evaluate the expansion of the proglacial area of the Forni Glacier due to the glacial fluctuations occurred since the Little Ice Age (LIA). This research was conducted in the Forni Valley (Stelvio National Park, Italy), where the Forni Glacier past fluctuations are well documented by at least four moraine ridges, from the LIA until the last advance occurred in the end of the 1970s. Using aerial images, orthophotos and field data, the moraine ridges have been georeferenced, and the expansion rate has been estimated in GIS environment for each time interval defined by the dated moraines. Assuming an expansion of 100% of glacier foreland between the end of the LIA and the current position of the glacier, our results show that 24,1% of the glacier foreland expanded between 1859 and 1914; 6,7% between 1914 and 1926; 47,3% between 1926 and 1981; 21,9% between 1981 and nowadays. The velocity of expansion was 11.000 m2/year between 1859 and 1914; 14.300 m2/year between 1914 and 1926; 21.700 m2/year between 1926 and 1981; 19.200 m2/year between 1981 and nowadays. The results obtained in this study show that i) the mean velocity of area expansion of the glacier foreland during the last 88 years was nearly double than the mean velocity during the 1859-1926 time interval, thus featuring an ongoing phase of acceleration starting from the beginning of the last century; ii) the linear glacial retreat (measured by the operators of the Italian Glaciological Committee), instead, calculated for the same four time intervals shows a remarkable acceleration only starting from 1981. The increasing rate of expansion of the glacier foreland needs to be taken into account not only for a better understanding of the dynamics of biological forms in newly formed proglacial areas, but also for the evaluation of the physical processes involved in the landscape changes, related for instance to the action of melting water on unconsolidated debris. Future perspective of the research aims at analyzing and comparing different trends of expansion of glacier foreland at the regional scale.