Assessing glacier features supporting supraglacial trees: the case study of the Miage debris-covered Glacier (Italian Alps)

The number of debris-covered glaciers featuring supraglacial trees is increasing worldwide, as a response of high mountain environments to climate warming. Generally, their distribution on the glacier surface is not homogeneous, thus suggesting that some glacier parameters influence germination and growth of trees. In this study, we focused our attention on the widest Italian debris-covered glacier, the Miage Glacier (Mont Blanc massif). We analyzed the ablation area in the range from 1730 m to 2400 m a.s.l. where continuous debris coverage is present and trees are found. Using data obtained by remote sensing investigations and field surveys we defined a record of glacier parameters to be analyzed with respect to the presence and abundance of trees. We found that supraglacial trees are present at the Miage Glacier: i) whenever exceeding a debris thickness threshold (≥19 cm); ii) with a gentle slope (≤ 10°); iii) with a low glacier surface velocity (≤ 7.0 m/y); and iv) where the vertical changes due to glacier dynamics are positive (i.e. prevalent increase due to both slow debris accumulation and preservation of ice flow inputs that we found ranging between +7 m and +28 m over 28 years). The statistical analysis supports our findings. The analysis of the same parameters might be conducted on other debris-covered glaciers featuring supraglacial trees, in order to evaluate if such conditions are local ones or if they are general factors driving germination and growth of trees. By identifying the features supporting the presence and growth of trees in these environments, and their thresholds, a contribution is given for a better understanding of the importance of debris-covered glaciers and, in general, of debris-covered ice, as a refuge for trees during warm intervals of the Holocene.