Cenozoic plant diversity of Yunnan: A review

Abstract

Yunnan in southwestern China is renowned for its high plant diversity. To understand how this modern botanical richness formed, it is critical to investigate the past biodiversity throughout the geological time. In this review, we present a summary on plant diversity, floristics and climates in the Cenozoic of Yunnan and document their changes, by compiling published palaeobotanical sources. Our review demonstrates that thus far a total of 386 fossil species of ferns, gymnosperms and angiosperms belonging to 170 genera within 66 families have been reported from the Cenozoic, particularly the Neogene, of Yunnan. Angiosperms display the highest richness represented by 353 species grouped into 155 genera within 60 families, with Fagaceae, Fabaceae, Lauraceae and Juglandaceae being the most diversified. Most of the families and genera recorded as fossils still occur in Yunnan, but seven genera have disappeared, including Berryophyllum, Cedrelospermum, Cedrus, Palaeocarya, Podocarpium, Sequoia and Wataria. The regional extinction of these genera is commonly referred to an aridification of the dry season associated with Asian monsoon development. Floristic analyses indicate that in the late Miocene, Yunnan had three floristic regions: a northern subtropical floristic region in the northeast, a subtropical floristic region in the east, and a tropical floristic region in the southwest. In the late Pliocene, Yunnan saw two kinds of floristic regions: a subalpine floristic region in the northwest, and two subtropical floristic regions separately in the southwest and the eastern center. These floristic concepts are verified by results from our areal type analyses which suggest that in the Miocene southwestern Yunnan supported the most Pantropic elements, while in the Pliocene southwestern Yunnan had abundant Tropical Asia (Indo-Malaysia) type and East Asia and North America disjunct type that were absent from northwestern Yunnan. From the late Miocene to late Pliocene through to the present, floristic composition and vegetation types changed markedly, presumably in response to altitude changes and coeval global cooling. An integration of palaeoclimate data suggests that during the Neogene Yunnan was warmer and wetter than today. Moreover, northern Yunnan witnessed a pronounced temperature decline, while southern Yunnan experienced only moderate temperature changes. Summer precipitation was consistently higher than winter precipitation, suggesting a rainfall seasonality. This summary on palaeoclimates helps us to understand under what conditions plant diversity occurred and evolved in Yunnan throughout the Cenozoic.

Keywords: Cenozoic; Floristic change; Fossil plants; Palaeobiodiversity; Palaeoclimate; Yunnan.

Fig. 1

Map showing the distribution of Cenozoic fossil floras in Yunnan, southwestern China.

Fig. 2

Selected fossil taxa comprising leaves, wood, fruits and seeds from the Cenozoic of Yunnan. Scale bars = 10 mm for images 1–3, 5 and 8–10; 0.5 mm for images 4, 7, 11 and 12.1. Drynaria callispora (Polypodiaceae) from the late Pliocene Longmen flora (Su et al., 2011). 2. Calocedrus shengxianensis (Cupressaceae) from the middle Miocene Wenshan flora (Zhang et al., 2015b). 3. Tsuga xianfengensis (Pinaceae) from the late Miocene Xianfeng flora (Xing et al., 2013). 4. Fossil wood of Pinus uniseriata (Pinaceae) from the Xianfeng flora (Wang et al., in press). 5. Mahonia mioasiatica (Berberidaceae) from the Wenshan flora (Huang et al., 2016a). 6. Quercus tenuipilosa (Fagaceae) from the late Pliocene Hunshuitang flora (Hu et al., 2014). 7. A fossil seed of Ranunculus (Ranunculaceae) from the Nanbanbang flora (unpublished data). 8. Rosa fortuita (Rosaceae) from the Wenshan flora (Su et al., 2016). 9. Celastrus caducidentatus (Celastraceae) from the middle Miocene Sanzhangtian flora (Liang et al., 2016). 10. Bauhinia wenshanensis (Fabaceae) from the Wenshan flora (Meng et al., 2014). 11. Eurya stigmosa (Theaceae) from the late Pliocene Nanbanbang flora (Zhu et al., 2016). 12. Aralia stratosa (Araliaceae) from the late Pliocene Fudong flora (Zhu et al., 2015).

Fig. 3

Species diversity of the fossil-richest genera from the Cenozoic of Yunnan.

Fig. 4

Selected fossil taxa now extinct in Yunnan. Scale bars = 10 mm for all images. 1. Seed scale of Cedrus angusta (Pinaceae) from the late Pliocene Longmen flora (Su et al., 2013b). 2, 3. Winged fruits of Cedrelospermum asiaticum (Ulmaceae) from the middle Miocene Maguan flora (Jia et al., 2015). 4. Leafy shoot of Sequoia maguanensis (Cupressaceae; Zhang et al., 2015a) from the middle Miocene Wenshan flora.

Fig. 5

Floristic regions in the late Miocene (A) and late Pliocene (B) of Yunnan (grey areas: no data). I. Tropical floristic region. II. Subtropical floristic region. III. Northern subtropical floristic region. IV. Subalpine floristic region.

Fig. 6

Composition of areal types for four late Miocene floras from Yunnan.

Fig. 7

Composition of areal types for four late Pliocene floras from Yunnan.