Glacier shrinkage and slope processes create habitat at high elevation and microrefugia across treeline for alpine plants during warm stages

The temperature rise at the global level and glacier shrinkage are deemed to induce new dynamics between biotic and abiotic factors, especially in mountain regions. In this work, we aimed to assess: (a) the area made available after glacier retreat since the Little Ice Age (LIA) for the colonization of alpine plants; (b) the ability of 14 alpine and nival species, typically growing on peaks and in deglaciated areas, to migrate and survive at lower elevations in microrefugia across the treeline ecotone. In the Adamello-Presanella Massif (Rhaetian Alps), we collected presence data on 14 alpine species within 3 × 3 or 5 × 5 m plots according to slope landforms and deposits. The analysis of the elevation range of the investigated species across the treeline was performed using Sentinel-2 satellite images to derive the actual position of the upper and lower treeline. The differences in the species elevation ranges across slope landforms, in relation to the treeline elevation and aspect, were detected with linear mixed models. Results showed that glaciers contracted sharply by more than 50% between the LIA and 2006, making large deglaciated areas available (more than 57 km2). All the studied species were found at low altitudes across the treeline ecotone, from about 500 to 1200 m below their typical habitat. Six species (Cardamine resedifolia, Cerastium uniflorum, Leucanthemopsis alpina, Luzula alpinopilosa, Oxyria digyna, Saxifraga oppositifolia) significantly exceeded the upper treeline in correspondence of alpine composite channels (p < 0.001 for all species). One species (Adenostyles leucophylla) exceeded the upper treeline in correspondence of composite slope deposits (p < 0.05). Two species (Ranunculus glacialis and S. oppositifolia) reached the lower treeline in composite channels and slope deposits (p = 0.800, and p = 0.519, respectively). The retreat of the alpine glaciers, coupled with the intensification of paraglacial processes, may favor new habitat opportunities at high elevation and microrefugia at low altitudes for nival and alpine species. The widespread presence of microrefugia for alpine plants downslope during warm periods suggests a low incidence of biodiversity loss in the alpine regions due to climate change.