Supraglacial trees as environmental stress indicators at the Miage debris-covered Glacier (Italian Alps)

Supraglacial trees are becoming a common feature on debris-covered glaciers (DCGs). The Miage Glacier (Mont Blanc Massif) is the only DCG on the Southern side of the European Alps characterized by the presence of a supraglacial forest. Trees are negatively affected by glacier surface movements, and growth disturbances are recorded in tree rings. We have analyzed the glacier parameters influencing the establishment and growth of supraglacial trees at the Miage Glacier and we have compared the tree-ring growth patterns and stable isotopes, as well as the needle volatile organic compounds (VOCs), of supraglacial trees ("glacier" site) with respect to trees located on a lateral moraine ("control" site). We found that supraglacial trees are present where debris is thicker than 19 cm, slope does not exceed 10°, surface velocity is lower than 7.0 m/year and vertical changes due to glacier dynamics are positive. The analysis of tree-ring growth patterns reveals that trees on the supraglacial debris are affected by a major geomorphological stress compared to trees on the moraine. Tree-ring δ13C and δ18O stable isotopes are different in trees at the glacier and control sites, probably due to differences in soil water retention, temperature excursion, time exposure to direct solar radiation and water source. Needle VOCs show significant differences in some terpenes at the two sites, probably due to the extreme temperature conditions on the supraglacial debris. These results show that supraglacial trees at the Miage Glacier may be used as indicators of environmental stress, and that monitoring their distribution and characteristics can contribute to better assessing the factors controlling the environmental evolution on a typical DCG. Future research will aim at analyzing the geomorphological features characterizing the Miage Glacier, and at evaluating what are the main hazards on a DCG in the context of a changing climate.