4.1 Genetic divergence between eastern and western populations
The population divergence in the eastern portion of the geographic range of C. chuniana is estimated to be older (0.51‒0.34 Ma) and with smaller population sizes than within the Nanling Mts., where more recent and rapid population diversification occurred (0.37‒020 Ma) with larger population sizes (Figure 4). The phylogenetic analysis also revealed that populations of the Nanling Mts. formed a monophyly group and were separated distinctly from the eastern populations. This pattern agrees with the general pattern of genetic divergence observed between eastern and western China in other plant species with wide distributions (Chen, Deng, Zhou, & Sun, 2018; Gong et al. , 2008; Ha, Kim, Choi, & Kim, 2018; Hohmann et al., 2018; Lu et al. , 2018; Qiu, Guan, Fu, & Comes, 2009). One main factor contributing to the differences of population divergence time and level of diversification between the east and west is likely to be the different orientations of mountain ranges (Chen et al. , 2018). The southwest-northeast orientation of the Wuyi Mts. and East China Mts. are thought to present geographical barriers blocking southward migrations in times of cooler climate or northward postglacial population expansion, which is disadvantageous for increasing population size and diversification, and may have contributed to an older divergence as is seen in the eastern populations (WYS1, WYS2, LXS1 and LXS2). Conversely, the north-south orientation of the Nanling Mts., allowing various elevational shifts of plant species, can facilitate gradual retreat from north to south and short-distance migrations during glacial and interglacial periods, thus promoting more population diversification, larger population size and younger divergence as is seen in the populations of the Nanling Mts. (NLE1 and NLE2, and NLW1 through NLW5). Therefore, the orientation and physiography of the mountain ranges appear to have contributes to the geographic pattern of genetic variation between the eastern and western populations of C. chuniana .