The influence of glacier surface processes on the short-term evolution of supraglacial tree vegetation : the case study of the Miage Glacier, Italian Alps

Supraglacial debris cover allows vegetation to colonize glacier surface, and whenever it is enough stable and thick, also shrubs and trees can germinate and grow. Supraglacial tree growth and distribution patterns on the glacier are closely connected with the debris-covered glacier dynamics and evolution. The aim of the research reported here was to evaluate the tree age and tree distribution patterns on the glacier tongue and the influence of ice-cliff backwasting, close to glacier terminus, on tree loss. We analysed the fragile and fast-changing environment that is present on the lower ablation sector of the Miage Glacier (Mont Blanc Massif, Italian Alps) where some ice cliffs are present and backwasting and downwasting phenomena occur. Tree features and short-term evolution were analysed with respect to glacier variations (mainly surface displacements and ice ablation) and geometry changes of the two most representative ice cliffs. The supraglacial trees' life time resulted to be mainly controlled by glacier surface displacements and by the occurrence of backwasting and downwasting processes, whereas tree germination was associated with fine debris presence. These factors, controlling plants' life and growth on the glacier, are an actual limit when supraglacial trees are analysed to reconstruct past environmental changes occurred on the glacier tongue. Moreover, we found that a large number of trees die under conditions of dominating backwasting inducing the loss of debris substrate (condition met especially on the northern glacier lobe). Instead, in the case of prevalence of downwasting (condition mainly observed on the southern glacier lobe), trees more easily survive and flow downvalley transported by the glacier flux.