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.