The asynchronous rise of Northern Hemisphere alpine floras reveals general responses of biotic assembly to orogeny and climate change.

Understanding how biotic assembly processes responded to past geoclimatic changes is key to explaining the origins of mountain biodiversity and the causes of regional disparities in species richness. Here, we jointly reconstructed geographic ranges and biome-niche evolution for 34 diverse plant clades across five major Northern Hemisphere mountain systems and quantified how late Neogene cooling increased arctic-alpine habitat connections across regions. We reveal that, while alpine floras originated asynchronously and were assembled through distinct evolutionary processes over the past 30 million years, general biological responses to orogeny and environmental change are apparent. Across regions, in situ diversification was consistently elevated during heightened phases of tectonic activity. Over the past 5 million years, enhanced arctic-alpine connectivity facilitated biotic interchange and positioned the boreal-arctic region as a major biogeographic crossroads linking Eurasia and North America.