Novel indicators of environmental change from trees in the debris-covered glacier foreland: the case study of the miage glacier (Mont Blanc Massif, Italian Alps)

Supraglacial trees are a useful source of data for reconstructing past glacier surface movements and debris-coverage instability. Proglacial trees also represent a useful tool for the identification and dating of changes in the glacial stream discharge and wide-spreading of melting water. Dendroglaciology is currently applied not only for reconstructing glacier fluctuations but also for investigating glacier surface dynamics at decadal scale. Trees and dated tree-ring characteristics such as scars, growth rate and reaction wood may provide information about glacier movements, discharge and hydrology. The Miage Glacier in the Mont Blanc Massif (Italy), represents a unique situation in the southern side of the Alps, due to the presence of abundant supraglacial vegetation. The density and distribution of trees is strictly linked to glacier surface velocity, thickness of debris-coverage, ablation rate, grain size distribution, slope and ice thickness, as documented by the results obtained during field surveys, data analysis and remote sensing techniques. The most recent studies show that supraglacial trees can also be considered environmental and climatic stress indicators. Leaf VOC (Volatile Organic Compounds) emissions and tree-ring carbon and oxygen stable isotopes show significant differences in trees located on the supraglacial debris with respect to trees on the lateral moraine, and these results suggest the possibility to apply these techniques in the identification of areas affected by glacio-geomorphological and climatic stress. Tree-ring characteristics may also be analyzed in order to reconstruct the past hydrology of debris-covered glaciers with annual resolution. In particular, trees fed by glacial meltwater of the Lago Verde (Miage Glacier) show that tree-ring cellulose is more depleted in δ18O compared to trees fed by other water sources and, moreover, tree-ring width is narrower in trees affected by lake-level fluctuations.