Phylogeography and Population Demography of Parrotia subaequalis, a Hamamelidaceous Tertiary Relict ‘Living Fossil’ Tree Endemic to East Asia Refugia: Implications from Molecular Data and Ecological Niche Modeling

The diverse topography and mild monsoon climate in East Asia are considered to be important drivers for the long-term ecological success of the Tertiary relict 'living fossil' plants during the glacial/interglacial cycles. Here we investigated the phylogeographic pattern and demographic history of a hamamelidaceous Tertiary relict 'living fossil' tree (<i>Parrotia subaequalis</i>) endemic to the subtropical forests of eastern China, employing molecular data and ecological niche modeling. In the long evolutionary history, <i>P. subaequalis</i> has accumulated a high haplotype diversity. Weak gene flow by seeds, geographical isolation, and heterogeneous habitats have led to a relatively high level of genetic differentiation in this species. The divergence time of two cpDNA lineages of <i>P. subaequalis</i> was dated to the late Miocene of the Tertiary period, and the diversification of haplotypes occurred in the Quaternary period. Paleo-distribution modeling suggested that <i>P. subaequalis</i> followed the pattern of 'glacial expansion-interglacial compression'. The Dabie Mountain and Yellow Mountain in Anhui Province and the Tianmu Mountain and Simin Mountain in Zhejiang Province were inferred to be multiple glacial refugia of <i>P. subaequalis</i> in East Asia and have been proposed to be protected as 'Management Units'. Collectively, our study offers insights into the plant evolution and adaptation of <i>P. subaequalis</i> and other Tertiary relict 'living fossil' trees endemic to East Asia refugia.