4.4 Population dynamics history within Notholirion
The Quaternary Ice Age has been a highly significant and widely studied
event in the field of biogeography. The climatic upheavals associated
with this ice age, along with interglacial cycles, have had a profound
impact on species ranges, often leading to population migration and even
extinction(G. M. Hewitt, 2004). Based on our previous research on
divergence timing, we have found that among the three species in theNotholirion , N. thomsonianum diverged as the early branch
during the late Miocene, whereas the other two species did not fully
diverge until the LGM period. Therefore, we speculate that the climatic
fluctuations during the Quaternary Ice Age, particularly the significant
temperature drop during the LGM, played a crucial role in shaping the
range of the ancestral taxa of the Notholirion genus. These
changes likely in turn contributed to the divergence of N.
bulbuliferum and N. macrophyllum , leading to the formation of
distinct species. According to the results predicted by our distribution
models for the three periods (LIG, LGM and present), the suitable
distribution area of Notholirion in the eastern part of the
Tibetan Plateau contracted significantly toward ITS eastern edge
during the Last Glacial Maximum (LGM), and then re-expanded toward the
eastern part of the Plateau to ITS present distribution pattern
after the LGM. Additionally, our mismatch distribution analysis based on
both ITS and cpDNA data exhibited non-smooth bimodal or
multi-peak curves (Figure). This, coupled with the non-star-like
phylogeographical structure of haplotypes and non-significant results of
the neutrality test, suggests that Notholirion had not recently
undergone rapid expansion events (Figure, Table). We propose that the
stability of Notholirion in ITS overall distribution
range, as well as the observed contraction and re-expansion in the
eastern Tibetan Plateau, can be attributed to ITS distinctive
characteristics as an autochthonous high-altitude taxon, along with the
combined influence of external environmental factors. These findings
also indicate that the response of plant taxa across the eastern Tibetan
Plateau and the Hengduan Mountains to Quaternary ice age climate change
is more intricate and nuanced than previously assumed.
Numerous studies have provided evidence that during the Quaternary
period, there was no uniform ice cap covering both the Tibetan Plateau
and the Hengduan Mountains(G. M. Hewitt, 2004; Taberlet, Fumagalli,
Wust-Saucy, & Cosson, 1998). Additionally, Notholirion , as a
high-altitude plant taxon primarily found at elevations around 3000 m,
exhibITS enhanced cold and frost tolerance and a greater capacity
to adapt to the environmental fluctuations of the ice age. Consequently,
it is plausible that certain widely distributed and cold-tolerant plant
taxa, such as Chamaesium , Allium sectionSikkimensia , and Cardiocrinum , includingNotholirion , survived the ice age by persisting in multiple
refugia and subsequently expanded their distribution following the ice
age(J. Li et al., 2022; C. Xie et al., 2019; H. Y. Zheng et al., 2021).
Furthermore, we observed notable levels of genetic diversity
(ITS : HD>0.7) and unique haplotypes specific in
populations from BDY, BLQ, and BZD. In the context of genealogical
geographic studies, regions exhibiting elevated genetic diversity or the
presence of ancient and endemic haplotypes are often regarded as
potential ice age refuges for plants(Tzedakis, Lawson, Frogley, Hewitt,
& Preece, 2002). Based on these findings, we propose that the Hengduan
Mountains and their neighboring areas could have served as refuges
during the ice age for Notholirion species inhabiting both the
eastern Tibetan Plateau and the Hengduan Mountains. Notably, this region
has been recognized as a significant refuge for numerous plant taxa on
the Tibetan Plateau, including Potentilla glabra , Rhodiola
alsia , Eriophyton wallichii , and Chamaesium (Q. B. Gao et
al., 2012; L. Y. Wang, Ikeda, Liu, Wang, & Liu, 2009; X. X. Wang, Yue,
Sun, & Li, 2011; H. Y. Zheng et al., 2021). These findings suggest that
the Quaternary ice age may have had less impact on the survival and
widespread distribution of these high-elevation cold-tolerant plants in
the subtropical region of the Tibetan Plateau than we thought. They
survived on multiple ice-age refuges and rapidly regained their
pre-glacial ranges after the ice age.