The dominant species of a biome can be regarded as its genuine indicator. Evergreen broadleaved forests (EBLFs) in subtropical East Asia harbor high levels of species biodiversity and endemism and are vital to regional carbon storage and cycling. However, the historical assembly of this unique biome is still controversial. Fagaceae is the most essential family in East Asian subtropical EBLFs and its dominant species are vital for the existence of this biome. Here, we used the dominant Fagaceae species to shed light on the dynamic process of East Asian subtropical EBLFs over time. Our results indicate high precipitation in summer and low temperature in winter are the most influential climatic factors for the distribution of East Asian subtropical EBLFs. Modern East Asian subtropical EBLFs did not begin to appear until 23 Ma, subsequently experienced a long‐lasting development in the Miocene and markedly deteriorated at about 4 Ma, driven jointly by orogenesis and paleoclimate. We also document that there is a lag time between when one clade invaded the region and when its members become dominant species within the region. This study may improve our ability to predict and mitigate the threats to biodiversity of East Asian subtropical EBLFs and points to a new path for future studies involving multidisciplinary methods to explore the assembly of regional biomes.

Diapensia L. is the second largest genus of Diapensiaceae. The taxonomic treatment within Diapensia and relationships within Diapensiaceae have been disputed. Chloroplast genome sequence data have proved to be useful for plant phylogenetic analyses and species delimitation. In this study, we de novo sequenced and assembled 22 chloroplast genomes of 15 species of Diapensiaceae, including all recognized species of Diapensia with multiple samples. A super‐matrix containing a total of 107 genes and 18 taxa was constructed for phylogenetic analyses to resolve phylogenetic relationships among genera of the family and within Diapensia. The resulting phylogenetic tree showed the following strongly supported relationships: (Galax, (Pyxidanthera, (Berneuxia, ((Schizocodon, Diapensia), and Shortia s.s.)))). The dated phylogeny and reconstructed lineage‐through‐time plot for the family indicated rapid diversification in the Neogene and an acceleration of diversification rate after c. 8 Ma. Biogeographic analysis suggested that Diapensia originated in the Northeast Asian mountains approximately 6.06 Ma, followed by northward dispersal to the Arctic and southwestward dispersal to the Himalaya–Hengduan Mountains. Phylogenetic relationships within Diapensia were well resolved. Based on the phylogenetic results, we proposed to reinstate the species status of Diapensia bulleyana Forrest ex Diels, and raised D. purpurea f. albida to the species rank (D. albida [W. E. Evans] J. F. Ye comb. & stat. nov.). The distribution ranges of all species delineated based on the phylogenetic results were revised accordingly based on specimen occurrences. Our study adds new examples for the power of plastid genome data for resolving phylogenetic relationships and clarifying taxonomic disputes among closely related species.