Past surface instability of M iage debris‐covered glacier tongue ( M ont B lanc M assif, I taly): a decadal‐scale tree‐ring‐based reconstruction

Debris‐covered glaciers may host several biological forms that colonize the debris cover, especially if the glacier tongue reaches sufficiently low altitudes (down to about 1700 m a.s.l. at M iage G lacier, W estern I talian A lps) thus allowing also tree growth. Supraglacial trees colonizing the debris‐covered tongue are strongly influenced in growth and distribution by substrate characteristics and instability. The tree age distribution at M iage G lacier presents a positive gradient towards the glacier terminus, which was found to be related to the decreasing glacier surface velocity. By analysing tree‐ring growth anomalies on the glacier and at a control site at the tree line over the 20‐year period 1987–2006, it was found that trees growing on the glacier presented the highest percentages of abrupt growth changes ( AGCs )>+70% with respect to the four previous years. Considering tree displacement on the glacier surface over the same 20‐year period and the recorded AGCs , it was found that the central‐lower portion of the southern lobe towards the margins was the most unstable. The temporal analysis of AGC >+40% confirmed a period of higher glacier surface instability, reaching a maximum in the years 1988 (on lobe S ) and 1989 (on lobe N ), probably related to the passage of a kinematic wave in the glacier tongue. Our analysis suggests that supraglacial trees hold useful information on the glacier tongue dynamics and that both AGC >+70% and AGC >+40% may be used as a proxy for substrate instability in spatio‐temporal reconstructions in the A lpine environment.