4 | DISCUSSION
4.1 | Thepatterns of species diversity
Overall, there were difference in the species distribution between the Kunlun Mountains and the Hengduan Mountains: 55.89% of the species in the Kunlun Mountains occurred in both the Kunlun Mountains and the Hengduan Mountains, and the level of similarity was higher than 80% at the genus level. Similar patterns were observed in the endemic taxa, where 70% of the endemic species in the Kunlun Mountains were also distributed in the Hengduan Mountains; all the endemic genera in the Kunlun Mountains were found in the Hengduan Mountains. The TS declined across the 28 county-level geographical units with an increase in distance from the Hengduan Mountains (Table 1). Therefore, our study strongly indicated that mass species migrations occurred between the KMF and the HMF at both the genus and species levels.
Some studies have suggested that the diversity hotspots of Chinese endemic seed flora are in the Qinling Mountains and further south, or in the Hengduan Mountains and to the east of China (Huang et al., 2016). Similar rules exist regarding the hotspots of endemic woody seed plants in China (Huang et al., 2012). Around 20% of the total species are endemic to the QTP (Wu, 2008; Yan, Yang, & Tang, 2013; Yu, Zhang, Liu, Chen, & Qi, 2018), and 32.4% of the total species are endemic to the Hengduan Mountains (Zhang, Boufford, Ree, & Sun, 2009). The Kunlun Mountains were not a hotspot of Chinese endemic seed flora or a center of diversification for current plants. However, endemic species of seed plants accounted for 29.8% of the total species in the Kunlun Mountains. The proportion of endemic species of the Kunlun Mountains was higher than that of QTP, and lower than that of the Hengduan Mountains. Consequently, these endemic species in the Kunlun Mountains may have come from the current speciation centers. The results of SES-PDs also indicated that the KMF is relatively young flora (Table 2) and that the young flora may have come from the speciation centers.
The Kunlun Mountains reached their present altitude over the last 17 million years (Sun et al., 2015), during which they experienced dramatic climatic fluctuations (Deng, Wu, Wang, Su, & Zhou, 2019) and geological movements (Cui et al., 1998). The climatic conditions have changed from hot and humid to cold and dry (Zheng, 1999). In addition, since the Pleistocene, the region has experienced numerous glacial events (Su, 1998), for example, the Largest Glaciation (1.2–0.6 Ma) and the Last Glacial Maximum (Shi, Zheng, & Yao, 1997). In the Last Glacial Maximum, the Kunlun Mountains were mostly covered by a unified ice sheet (Shi, Zheng, & Yao, 1997; Su, 1998). These numerous glaciations have led to mass extinction events in the Kunlun Mountains. Many plant molecular phylogeography studies have reported that the QTP was recolonized by most current plants in the postglacial period, from the southern or eastern glacial refugia (Wiens & Donoghue, 2004; Qiu, Fu, & Comes, 2011; Yu & Zhang, 2013; Yan & Tang, 2019). Some hardy plants also persisted in micro-glacial refugia and dispersed into adjacent regions in the postglacial period. Consequently, after these glacial events, the current plants may have originated from refuges adjacent to the Kunlun Mountains.
The findings of the present study improve our understanding of the patterns of species diversity in the Kunlun Mountains. First, the KMF were tightly linked with the HMF at both the species and genera levels. Second, after numerous glacial events, it is probable that the current plants originated from adjacent refuges. The PD and SES-PD of the KMF also supported the conclusion that the current plants may have originated from a diversification center. The endemic species provided further evidence that they may have come from the current speciation centers. Third, previous studies revealed that the primary direction of species migration is from east to west or from south to north during the interglacial periods in the QTP and adjacent areas (Qiu, Fu, & Comes, 2011; Yu & Zhang, 2013; Yan & Tang, 2019). Fourth, The Hengduan Mountains have been confirmed as the largest refuge for species in the glacial periods (Liu, Luo, Li, & Gao, 2017), a center of species diversification for many current plants (Wen, Zhang, Nie, Zhong, & Sun, 2014; Chen, Deng, Zhou, & Sun, 2018), and a hotspot for Chinese endemic seed flora (Huang et al., 2016). Therefore, the above findings revealed that species recolonization underpinned the current patterns of species diversity in the Kunlun Mountains, and that the KMF originated primarily from the HMF.