A biogeographical study on tropical flora of southern China

Abstract The tropical climate in China exists in southeastern Xizang (Tibet), southwestern to southeastern Yunnan, southwestern Guangxi, southern Guangdon, southern Taiwan, and Hainan, and these southern Chinese areas contain tropical floras. I checked and synonymized native seed plants from these tropical areas in China and recognized 12,844 species of seed plants included in 2,181 genera and 227 families. In the tropical flora of southern China, the families are mainly distributed in tropical areas and extend into temperate zones and contribute to the majority of the taxa present. The genera with tropical distributions also make up the most of the total flora. In terms of geographical elements, the genera with tropical Asian distribution constitute the highest proportion, which implies tropical Asian or Indo‐Malaysia affinity. Floristic composition and geographical elements are conspicuous from region to region due to different geological history and ecological environments, although floristic similarities from these regions are more than 90% and 64% at the family and generic levels, respectively, but lower than 50% at specific level. These differences in the regional floras could be influenced by historical events associated with the uplift of the Himalayas, such as the southeastward extrusion of the Indochina geoblock, clockwise rotation and southeastward movement of Lanping–Simao geoblock, and southeastward movement of Hainan Island. The similarity coefficients between the flora of southern China and those of Indochina countries are more than 96% and 80% at family and generic levels, indicating their close floristic affinity and inclusion in the same biogeographically floristic unit.


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HUA tropical flora of China are investigated, and the possible influences by geohistorical events associated with the uplift of the Himalayas have been also discussed.

| GEOGRAPHY
Areas with a tropical climate in China include southeastern Xizang (Tibet), southwestern to southeastern Yunnan, southwestern Guangxi, southern Guangdon, southern Taiwan, and Hainan. Both climatic and physical zonation indicate that the tropical zone is generally south of the Tropic of Cancer, with the exception of parts of southwest China (National Committee of Atlas Compilations, 1999), although the precise demarcation line for the tropical area has been debatable and varyingly applied. A line at c. 22°30′N was tentatively suggested as the northern boundary of the tropical zone in south and southeastern China based on the biogeographical patterns of Chinese seed plants, such as the dominance of tropical genera in this area (Zhu, 2013a;Zhu, Ma, Yan, & Hu, 2007).
This line corresponds well with the currently recognized northern boundary of the tropical monsoon and rain forests of China (Wu, 1980;Zhang, 2007;Zhu, 2017a). Despite a slightly lower annual cumulative temperature in the region of southern China surrounding c. 22°30′N, tropical genera account for more than 80% of the total genera in the lowland floras and exhibit a Southeast Asian floristic affinity (Zhu, 2008a(Zhu, ,b, 2013aZhu & Roos, 2004;Zhu et al., 2007). This line also coincides with the demarcation between two established floristic regions in China, the East Asiatic Kingdom (Wu & Wu, 1996) or Holarctic Kingdom, and the Paleotropical Kingdom (Takhtajan, 1978). The location of a boundary at c. 22°30′N is additionally supported by a similar periphery line drawn by Ashton (2014) to distinguish major zonal forest formations of lowland tropical Asia based on herbarium specimens and personal experience. I suggest that using the 22°30′N borderline is a suitable biogeographical boundary for the tropical areas in south and southeastern China, see Figure 1.

| MATERIALS AND METHODS
Based on the biogeographical boundary for the tropical areas in south and southeastern China, data on the respective tropical floras from these southern China provinces were collected: Motuo of Xizang (Yang & Zhou, 2015), southern Yunnan (Zhu & Yan, 2012), southeastern Yunnan (Zhu & Yan, 2009), southwestern Guangxi (Qin & Liu, 2010), and Hainan (Xing, Zhou, Wang, Zeng, & Liu, 2012). I checked and synonymized the native seed plants from these tropical areas in China, and was able to recognize 12,844 species of seed plants comprising of 2,181 genera in 227 families (not including Taiwan due to the lack of a tropical plant checklist) (see Appendix S1 and S2). The circumscriptions of families followed the APG III and APG IV technique (APG, 2009(APG, , 2016Chase & Reveal, 2009), and I followed w 3 TROPICOS (http:// mobot.mobot.org/W3T/Search/vast.html) for species nomenclature.
Patterns of seed plant distribution were quantified at the generic and the family levels following Wu (1991) and Wu, Zhou, Sun, Li, and Peng (2006) (Kress, DePilipps, Farr, & Kyi, 2003), Thailand (Smitinand, 2001), and Vietnam (Chan, 1999;Zhu, Yan, & Qin, 2003). As the usual, F I G U R E 1 Borderline suggested as the biogeographically northern boundary for the tropical area in south and southeastern China. (The figure was made by the Landscape Ecology Lab., Xishuangbanna Tropical Botanical Garden, CAS) I use the similarity coefficients at family and generic levels to clarify the affinity between these regional floras in this article.

| Floristic composition
I was able to recognize 12,844 species of seed plants comprising of 2,181 genera and 227 families from the tropical areas of southern China.
Among them were 14 families, which comprised of more than 200 species each, such as Ochidaceae (823 species/153 genera), Fabaceae   (61). Similar to the geographical patterns of families, I found that the majority of species-rich genera presented a pantropic distribution, but did extend into temperate areas.

| Biogeographical elements
The pantropic distribution at the family level makes up the highest ratio, with 86 families representing 37.89% of the total. Further, we found that cosmopolitan families make up 20.70%, and the north temperate families make up 13.66% (

| Variation in floristic composition and geographical elements of the tropical flora
The flora of southern China shows conspicuous variations in floristic composition from region to region. Despite this, I found that the floristic similarities at the family and generic levels were more than 90% and 64%, respectively, but at the specific level, there is less than 50% similarity among the compared regional floras from southwestern China to southeastern China (Table 5). More similar dominant families and genera exist between southeastern Xizang (Tibet) and southeastern Yunnan, especially the families Ericaceae and Aralicaceae (Zhu, 2017c).
In comparing floras, the tropical element is the most consistent regional geographical element. However, I found that the floras of  (Table 4).

| Comparison to the floras of Indochina countries
The flora of southern China has distinct similarities at both the family and generic levels with the floras of Indochina countries (Table 6).
They have similar coefficients at the family level and share more than 96%, and at generic level more than 80%. The family level coefficients between southern China and across Indochina countries are nearly identical; however, at the generic level, the highest similarity exists between southern China and Laos, up to 85.22%. The tropical flora of southern China undoubtedly belongs to the same biogeographical unit as those of Indochina countries.

| DISCUSSION
The differences seen between the characteristics and evolution of The Indian continent collided with Asia around 50 Ma (Rowley, 1996)   been suggested as one of the prominent fragments of the extruded Indochina block (Sato, Liu, Zhu, Yang, & Otofuji, 1999, 2001Sato et al., 2007). As a whole, the Simao Terrane was displaced southward by 800 km and rotated clockwise 30° (Figure 3). The rotation processes are believed to have remained active until at least the Miocene (Chen et al., 1995;Schärer et al., 1990). Such geological events may have directly affected the evolution of the flora of southwestern China (Zhu, 2012(Zhu, , 2013b(Zhu, , 2015.
Paleomagnetic studies indicate that during the Mesozoic, Hainan Island was in effect connected to North Vietnam and Guangxi (Mo & Shi, 1987). Blocks reconstruction of Asia reveals that Hainan Island was in a position adjacent to North Vietnam and Guangxi at 40 Ma (Replumaz & Tapponnier, 2003). From the late Mesozoic to early Cenozoic, the Beibu Gulf lithosphere was drawn away, and Hainan moved southeast along the Red River fault and revolved clockwise to its present location (Ma et al., 2014). The Red River fault system also gave rise to the Tonkin Gulf in a widespread extension across a 100-km-wide zone prior to 30 Ma (Rangin, Klein, Roques, Le Pichon, & Van Trong, 1995). This evidence suggests the geological evolution of the Tonkin-Beibu Gulf may have caused Hainan Island's southeast moment. Rock magnetism and paleomagnetism show that Hainan was 5-6° north of its present geographic position in the late Cretaceous (Fu et al., 2010;Liu & Morinaga, 1999).
However, it has also been suggested that the rotation of Hainan Island may have occurred during the mid-Tertiary, when large-scale left-lateral motion occurred along the Red River fault as a result of the collision of the Indian Plate into Eurasia, causing extrusion of the Indochina block and the opening of the South China Sea (Li, Metcalfe, & Wang, 1995).
Recent biogeographical evidence suggests that Hainan Island may have been in contact with northern Vietnam and Guangxi during the Eocene and drifted to its present location by moving southeast due to the extrusion of the Indochina block (Zhu, 2016a).
In this study, dominant families and genera shared higher similarities between southeastern Tibet (Xizang) and southeastern Yunnan could be explained by the geological history in southwestern China. In the late Eocene India collided with northern Myanmar and Tibet and since the late Cretaceous, northern Myanmar and Tibet have moved northward relative to the Asian plate to the east (Mitchell, 1993). With the southeastward extrusion of the Indochina geoblock, the Lanping-Simao geoblock experienced clockwise rotation and southeastward movement (Chen et al., 1995;Funahara et al., 1993;Harrison et al., 1992;Leloup et al., 1995;Sato et al., 1999Sato et al., , 2001Sato et al., , 2007 causing southeastern Yunnan to move further southward related to the northwestern Yunnan. These events may have shaped the distribution pattern along the so-called Tanaka line (Tanaka, 1954; and eastern Yunnan was suggested as the "Hua line" (Zhu, 2011(Zhu, , 2013b. Evidence for this biogeographical line is presented by Zhang et al. (2012) using a cluster analysis of species' presence/absence in Yunnan and their phylogenetic relatedness, taxonomic composition, and regional phylogenetic structure (Li, Kraft, Yang, & Wang, 2015).
After the southeast movement resulting from the uplift of the Himalayas and the extrusion of the Indochina block from its location near Vietnam and Guangxi during the Eocene, the present Hainan Island has a typical tropical climate at 18°10′04″-20°9′40″N in the southernmost of China, which results in the highest number of tropical elements in its flora (Zhu, 2016a). The flora of Taiwan appears to mainly derive from the East Asian flora due to the uplifting of the island after the Late Tertiary, although tropical flora is found in the southernmost part of Taiwan with its tropical climate (Zhu, 2016b).

CONFLICT OF INTEREST
None declared.

AUTHOR CONTRIBUTIONS
HZ contributed to the conceptualization, data curation, formal analysis, funding acquisition, investigation, methodology, project administration, resources, validation visualization, writing-original draft, and writing-review and editing.