Debris-covered glaciers as habitat for plant and arthropod species : environmental framework and colonization patterns

Debris-covered glaciers are glaciers with the largest part of the ablation zone covered by a debris layer. Recent papers showed that debris-covered glaciers are able to support plant and arthropod life, advancing the hypothesis that such landforms could act as warm-stage refugia for cold-adapted species due to their microclimate features and thermal inertia. However, integrated research comparing debris-covered glaciers with surrounding landforms to outline their ecological peculiarities are currently lacking. We analyze some abiotic (glacier surface velocity, ice melting rate and supraglacial debris thickness; ground temperature and humidity; substrate physical and chemical parameters) and biotic features (vascular plant and arthropod communities) of an Alpine debris-covered glacier (Belvedere, Western Alps, Italy), and compare them with those of the surrounding iceless landforms as reference sites (stable slope and iceless moraine). Our data show remarkable differences between stable slopes and unstable landforms as a whole (iceless moraine and supraglacial debris). The iceless moraine and the supraglacial debris show similar substrate features, but different ground temperature (lower on supraglacial debris) and different occurrence of cold-adapted species (more frequent/abundant on supraglacial debris). Such differences could be attributed to the thermal effect of underlying ice. Our data support the hypothesis advanced by previous studies: the thermal contrast with the surrounding landforms and the ability to descend below the climatic treeline give debris-covered glaciers the ecological requirements to be considered potential warm-stage refugia for cold-adapted species. However, our data highlighted that biotic colonization of such landforms could be prevented by some glaciological features, like the mechanical disturbance due to the ordinary ice dynamics (e.g. high glacier surface velocity) and time since the last extraordinary ice dynamic (e.g. surge-type movements). The combined effect of such features is currently preventing colonization by low-dispersal taxa as some cold-adapted ground beetles.