The late rise of sky-island vegetation in the European Alps.

Our understanding of the emergence of mountain floras rests on our ability to infer how orogeny, landscape dynamics and climate change altered their evolutionary trajectories. Here we reconstruct the assembly of the diverse sky-island flora of the European Alps and test the impact of key geo-climatic events. We use a dated 5,231-species phylogeny, including 96% of the sky-island flora. The assembly of this flora occurred through the colonization of over a thousand distinct lineages, of which 46% speciated from their lowland or non-Alpine ancestor and 6% underwent in situ cladogenesis. The young ages of extant sky-island lineages show that their accumulation was decoupled from ancient geo-climatic events but accelerated throughout the Plio-Pleistocene. The sky-island vegetation therefore assembled through recent lineage turnover, which was triggered, rather than impeded, by Pleistocene glacial intensification. This perspective challenges previous assumptions and highlights the complex interplay of geo-climatic factors in shaping the intricate tapestry of alpine floras.