How prevalent were nunataks as glacial refugia in the Alps? Insights from range hindcasting of the nival flora

Abstract

The European Alps were heavily glaciated during the Pleistocene, prompting debates about plants’ glacial refugia. While most alpine vegetation likely survived in lower mountain ranges, adjacent or disconnected from the Alps, or elsewhere in European lowlands, a century-old hypothesis suggests that some plants persisted on rocky peaks protruding from ice-covered areas. However, this so-called “nunatak” hypothesis has received relatively limited attention and support from phylogeographic studies. We modeled the potential distribution of 69 nival plant species during the Last Glacial Maximum (LGM, circa 20 to 25 ky ago) using species distribution models based on paleo-climatic conditions, paleo-ice extent, topographic metrics and bedrock lithologies. Two thirds of studied species and the majority of silicicolous species were predicted to have predominantly occurred on nunataks during the LGM in comparison to peripheral mountains or lowlands. Bedrock affinity, alongside topographic and climatic preferences, profoundly influenced predictions of species’ refugia locations and range contractions. Silicicolous species relied heavily on nunataks, where lithology and topographic ruggedness jointly favored their survival. Calcicolous species, in contrast, primarily found refugia in peripheral and extra-alpine massifs, the inner Alps being largely uninhabitable—either due to the extensive ice cover on calcareous massifs, often located at lower elevations, or because unglaciated ones were too unsuitable for their persistence. Generalist species, with broader ecological flexibility, persisted across more diverse refugia, favoring mainly the less harsh peripheral regions. Multivariate analyses identified seven cooccurring species groups associated with distinct potential refugia. Our findings challenge the view that nunatak refugia were undervalued during glacial periods, highlighting their significance for siliceous rock-specialist nival species. This work provides a well-delimited framework for further research on alpine paleobiogeography.