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.