Differential Quaternary dynamics of evergreen broadleaved forests in subtropical China revealed by phylogeography of Lindera aggregata (Lauraceae)

Aim According to palaeo-biome reconstructions, the subtropical evergreen broadleaved forests (EBLF) in East Asia contracted to a narrow belt south of 24 degrees N during the Last Glacial Maximum (LGM). Yet numerous phylogeographical studies of EBLF plants have inferred glacial refugia north of this area. Here, we aim to further elucidate the Quaternary dynamics of EBLFs using the widespread shrub Lindera aggregata as model. Location Subtropical China. Taxon Lindera aggregata (Sims) Kosterm (angiosperms). Methods Four chloroplast DNA (cpDNA) fragments and 15 low-copy nuclear genes (LCGs) were sequenced in 19 populations. The time to the most recent common ancestor (TMRCA) was estimated using secondary calibrations. Bayesian algorithms were used to infer population clustering, phylogeny and divergence time. Historical gene flow was estimated using a maximum-likelihood algorithm. Potential habitats at present and during the LGM were predicted using ecological niche modelling. Results The TMRCA estimate was 1.78 Myr. We observed a shallow cpDNA network organized around one dominant haplotype. The LCGs revealed three distinct genetic clusters that reflected contrasting historical population dynamics. The most ancient cluster was located south of the Nanling and Wuyi Mountains and indicated long-term population persistence in multiple refugia. The area further north was occupied by two disjunct clusters that dated back to the LGM and showed signatures of a rapid northward expansion from two refugia. We detected noteworthy pollen-mediated secondary admixture between all three clusters including somewhat preferential southward gene flow. Main conclusions Lindera aggregata represents a rare example of extensive post-glacial range expansion across the EBLF biome (expansion-contraction model) that contrasts with its generalized long-term population stability in the southernmost range parts. Our findings also provide rare evidence for noteworthy post-glacial gene flow into long-term refugial populations. They correspond relatively well with palaeo-biome reconstructions and support the hypothesis that the EBLF has undergone extensive post-glacial community reshuffling.