Glacier features influencing the presence and abundance of supraglacial trees: the case study of the miage debris-covered glacier (Mont Blanc Massif, Italian Alps)

The number of debris-covered glaciers featuring supraglacial tree vegetation is increasing worldwide, as a response of high mountain environments to the current climate warming. At the debriscovered surface of these glaciers, trees can be found thus giving peculiar landscape and ecosystems. Their distribution is not homogeneous, thus suggesting that some glacier parameters influence germination and growth of trees. This study was performed on the widest Italian debris-covered glacier, the Miage Glacier in the Mont Blanc massif, where herbaceous and tree vegetation is present at the surface of the glacier tongue. We analyzed the ablation area in the range from 1730 m to 2400 m a.s.l., where a quite continuous debris coverage is present and colonized by trees (mainly Larix decidua Mill. and Picea abies Karst), also reaching an age of 60 years close to the terminus. By remote sensing investigations and through field surveys we obtained a record of glacier parameters (debris thickness, debris-surface temperature, slope, aspect, elevation, ablation rate, surface velocity, debris-NDMI, variation in ice thickness over several years) to be analyzed with respect to the presence and abundance of trees in 15 plots (plot size: 15 m x 15 m). Our results show that supraglacial trees are present at the Miage Glacier: 1) whenever exceeding a debris thickness threshold (≥19 cm), 2) with a quite gentle slope (≤ 22°), 3) with a low glacier surface velocity (≤ 7.0 m/year), 4) where the ice thinning due to surface ablation is moderate (ranging between -1.8 m/year and -0.7 m/year) and 5) where the vertical changes due to glacier dynamics are positive (i.e. prevalent increase due to both slow debris accumulation and then preservation of ice flow inputs that we found ranging from +7 m and +28 m over a period 28 years long). The analysis of the same parameters, conducted on other debris-covered glaciers featuring supraglacial trees, may provide new data in order to evaluate if such conditions are local ones or if they are actual and general factors driving germination and growth of trees.