image: Figure 1 Modern sclerophyllous evergreen broad-leaved forests and leaves of Quercus section Heterobalanus. (a), (b), (e), (f) Q. aquifolioides; (d) Q. monimotricha; (c), (g), (h) Q. guyavifolia; scale bars=1 cm; (a), (e), (f) all photographed in Yajiang, Sichuan (4,200 m a.s.l.); (b)–(d) photographed in Daocheng, Sichuan (4,000 m a.s.l.), Shangri-La, Yunnan (3,800 m a.s.l.), and Shangri-La, Yunnan (3,600 m a.s.l.), respectively. view more
Credit: ©Science China Press
Sclerophyllous evergreen broad-leaved forests, mainly made up of sclerophyllous oak, Quercus section Heterobalanus, Fagaceae, represent the most typical forest type in the Hengduan Mountains. The distribution pattern of Quercus sect. Heterobalanus is closely related to the growth and formation of the Tibetan Plateau. Molecular studies illustrated a rapid diversification of this group in the middle Miocene. Fossil records also reveal a range expansion and a niche shift of them, which evolved from being companion species in the subtropical evergreen broad-leaved forest in the Miocene to become a dominant and constructive species in the sclerophyllous evergreen broad-leaved forest in the Hengduan Mountains during the Pliocene.
The oldest fossil record of Quercus sect. Heterobalanus previous discovered is from the middle Miocene (15 Ma) of the Gazhacun Formation in Namling County, southern Tibet. However, reseachersrecently discovered leaf fossils of this group from the upper Eocene (~34 Ma) of Lawula Formation in Markam Basin, southeastern Tibet. This discovery illustrates the origin of Quercus sect. Heterobalanu is nearly 20 Myr older than previously assumed. Based on their morphology and the results of geometric morphometrics, reseachers assigned them as Quercus cf. presenescens Z.K. Zhou.
By integrating fossil records and biogeographic data, reseachers infer that the leaves of Quercus sect. Heterobalanus were already adapted to cool and dry conditions in some local regions no later than in the late Eocene. Then, with the growth of the and late Cenozoic global cooling, the expansion of cooler and drier habitats benefited the spread and development of this section and their leaves exhibited morphological stasis through stabilizing selection.
Reseachers argue that Quercus sect. Heterobalanus appeared in the subtropical evergreen and deciduous broad-leaved mixed forests of the southeastern margin of the Tibetan Plateau no later than in the late Eocene. Some taxa spread westwards along the Gangdese Mountains and later the Himalaya, and others spread eastwards and southeastwards, gradually becoming a dominant group of species in the Hengduan Mountains. This dispersal route is contrary to the previous “northwards hypothesis”, supporting the hypothesis of an East Asian origin for Quercus section Ilex.
See the article: Chen L, Deng W, Su T, Li S, Zhou Z. 2021. Late Eocene sclerophyllous oak from Markam Basin, Tibet, and its biogeographic implications. Science China Earth Sciences, 64(11): 1969-1981, https://doi.org/10.1007/s11430-020-9826-4
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Science China Earth Sciences