Glacial and periglacial landforms as warm-stage refugia for cold-adapted plant and arthropod species:ecological and biogeographical significance of a habitat system

Abstract

Since 1880 air temperature at global scale rapidly increased of c. 0.85 \ub0C likely due to the increasing amount of anthropogenic greenhouse gas, and is forecast to increase over the 21st century as well. Cold-adapted species of the high latitude/altitude are the first to be threatened by such climate change, due to the progressive reduction of their suitable areas. Microclimatic heterogeneity at landscape level is supposed to play a key role in this scenario providing warm-stage refugia, sites able to preserve suitable climate conditions for cold-adapted species in spite of the ongoing climate warming. Active rock glaciers and debris-covered glaciers (periglacial and glacial landforms characterized by debris with underlying ice) were proposed as potential warm-stage refugia in mountain regions, due to the microclimate effect of underlying ice and the thermal inertia provided by the debris fraction. Besides microclimate heterogeneity at landscape level, macroclimate heterogeneity at regional scale (e.g. the contrast between continental and oceanic climates of inner and peripheral mountain ranges, respectively) can be important, since drive the distribution of periglacial and glacial landforms as potential warm-stage refugia and the effects of climate change themselves on high mountain landscapes (e.g. temporal pattern of glaciers retreat) and ecosystems (e.g. temporal pattern of primary successions on ice-free glacial landforms). Aim of this study was to analyze the ecological features of periglacial and glacial landforms (active rock glaciers and debris-covered glaciers on inner mountain ranges, glacier forelands and recent moraines on peripheral mountain ranges) to test whether they have the main key-requirements to act as potential warm-stage refugia for cold-adapted species. Concerning active rock glaciers and debris-covered glaciers, we considered as key-requirements a cold microclimate able to support cold-adapted species and the ability to cross the altitudinal zonation of mountain ecosystems, thus supporting cold-adapted species in adverse climate contexts. Concerning glacial landforms on peripheral mountain ranges, we considered as key-requirement a relatively slow colonization pattern able to long-lasting support pioneer cold-adapted species in spite of the primary succession development. The study was carried out in different areas of the Italian Alps (Adamello-Presanella, Ortles-Cevedale, Monte Rosa, Orobian Alps) in which different landforms were analyzed and compared. A multidisciplinary approach was followed, considering both the abiotic (ground surface temperature and humidity, soil physical and chemical parameters) and biotic variables (plant and arthropod communities). Our results show as active rock glaciers and debris-covered glaciers does not differ from the surrounding debris-featured iceless landforms for soil parameters, but for a cold microclimate likely due to the underlying ice. The microclimate can explain why these landforms differ from the surrounding ones for the occurrence of cold-adapted plant and arthropod species. On the Alps, active rock glaciers are unable to cross the altitudinal zonation of mountain ecosystems, thus can only enhance the occurrence of cold-adapted species within their normal altitudinal distribution. By contrast, debris-covered glaciers can cross the climatic treeline, allowing cold-adapted species to live even below their normal altitudinal distribution. However, the relative altitudinal distribution of active rock glaciers and debris-covered glaciers can be inverted on mountain systems drier than Alps, suggesting a similar ecological role of such landforms in opposite macroclimate conditions. Moreover, our results suggest that mountain systems characterized by oceanic macroclimate could prevent the extinction risk for pioneer cold-adapted species along a chronosequence of glacier retreat, due to the long-lasting persistence of glaciers at relatively low altitudes (as a consequence of high winter precipitations) and the slow colonization of ice-free glacial landforms (as a consequence of long-lasting snow cover and temperatures around zero). Peripheral mountain ranges in particular outlines as oceanic areas of crucial biogeographical importance during both the opposite climate extremes: in cold-climatic stages they can provide refugia for a lot of species, as a consequence of the relatively low ice-sheet expansion; in warm-climatic stages they may provide refugia for cold-adapted species in particular, as a consequence of their climatic, thus glaciological and ecological features