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Biogeographical Divergence of the Flora of Yunnan, Southwestern China Initiated by the Uplift of Himalaya and Extrusion of Indochina Block

The floral composition of Yunnan is conspicuously linked to the biogeographical history of this extremely species-rich province in southwestern China. The floristic compositions of three representative regions in Yunnan were compared to reveal their variation with geography. From southern Yunnan, 4150 native species (including subspecies and varieties) from 1240 genera and 183 families of seed plants were recognized. From central Yunnan 3389 native species from 1095 genera and 167 families of seed plants were recognized. From northwestern Yunnan 6807 native species from 1296 genera and 166 families of seed plants were recognized. Although these three floras across Yunnan are similar in familial composition, similarities between the floras of southern and northwestern Yunnan are low at the generic and specific levels. The flora of northwestern Yunnan is dominated by families and genera with cosmopolitan and north temperate distributions, while the flora of southern Yunnan is dominated by tropical families and genera. Northwestern Yunnan is composed largely of temperate genera, of which the highest proportion has a north temperate distribution. In contrast, southern Yunnan has mainly tropical genera, of which most have a tropical Asian distribution. The flora of central Yunnan is a combination of southern and northwestern Yunnan. These three floras might be derived from a common Tertiary tropical or subtropical East Asian flora, but the geological history of each region has influenced its flora, and they have remained divergent since the late Tertiary. The flora of northwestern Yunnan has evolved with the uplift of the Himalayas and by gradual proliferation of mainly cosmopolitan and north temperate floristic elements, while the flora of southern Yunnan has evolved with extrusion of the Indochina block and the influence of mainly tropical Asian elements.


Introduction
The ''Eastern Asiatic floristic region'' was delineated by Takhtajan [1] in his floristic regionalization of the world. The region is considered to be one of the major centres of development of higher plants, as it is especially rich in gymnosperms and primitive angiosperms [2]. Yunnan province of south-western China is the core area of the west wing of the ''Eastern Asiatic floristic region'' with extremely rich biodiversity, and is among the hotspots for conservation priorities in the world [3]. The region is also important for understanding historical biogeography due to its location in a transitional zone between tropical south-east Asia and temperate east Asia, and its position in geological history at a sutural zone between Gondwana and Laurasia [4,5].
Yunnan is a mountainous region with an extremely diverse topography (from 76.4 m at the lowest valley in the southeast to 6740 m at the highest mountain summit in the northwest). Due to its diverse topography and climate, as well as its unique geological history, Yunnan is extremely rich in species and vegetation types, and the landscape varies vastly from tropical rain forests in southern Yunnan, to Taiga-like cold temperate coniferous forest in northwestern Yunnan.
The plant geography of Yunnan was primarily studied by Li and Walker [6]. There are many floristic works on local areas and nature reserves in Yunnan, with fewer large area and regional scale studies, but exceptions are the large Hengduan Mountains [7,8,9], southern Yunnan [10,11], southwestern Yunnan [12,13], and central Yunnan [14]. Southern Yunnan, with a tropical monsoon climate and lower mountain-basin topography, has a tropical flora of Malaysian affinity. Central Yunnan, with a subtropical climate and middle mountain-valley topography, has close affinities to the subtropical flora of East Asia. Northwestern Yunnan, with a temperate climate and alpine-deep valley topography, has a temperate Himalayan flora. These patterns in the flora and vegetation of Yunnan are of extreme interest to botanists. In the present study, we select three representative regions with different altitudes across this extremely biodiverse region of China, southern, central and northwestern Yunnan, to study their floristic compositions and variation with geography, and their evolution with geological history, as well as biogeographical affinities.

General Geography
Yunnan is in southwestern China between 21u099 and 29u159 N, 97u329 and 106u129 E ( Figure 1) and occupies an area of 394,100 km 2 . It has a mountainous topography with the mountain ridges generally running in a north-south direction, decreasing in elevation southward. Yunnan is extremely diverse in habitat and topography. The general climatic pattern consists of tropical wet climates in the southern lowlands (annual mean temperature 19-22uC), tropical dry climates in deep valleys below 1000 m alt. (20-24uC) due to the foehn effect, subtropical climates on the central plateau (14-18uC), and temperate to cold temperate climates in the northern high mountains (5-14uC). The climate changes conspicuously with altitude. Yunnan is therefore a region with tropical areas as the horizontal base [15].
Yunnan supports an extremely rich biodiversity and various vegetation types. A total of 13,245 species in 2,137 genera and 222 families of native seed plants are recognized from Yunnan [16], contributing to 50% of the total seed plant species in China. Twelve vegetation types including 169 formations were recognized in the vegetation classification of Yunnan [17], including tropical rain forest, subtropical evergreen broad-leaved forest, warmtemperate deciduous broad-leaved forest, temperate coniferous and broad-leaved mixed forest, cold temperate coniferous forest, alpine shrubs and meadows, as well as thorny shrubs and savanna in dry-hot valleys [18]. Southern Yunnan is generally covered by tropical rain forest at its lowlands [19]. Most of central Yunnan is between 1300-2200 m, and is covered by subtropical evergreen broad-leaved forest and secondary Pinus yunnanensis forest. Northern Yunnan, with an elevation above 1900 m, is covered mainly by temperate sclerophyllous oak forest and cold-temperate coniferous forest [20]. The distribution of vegetation in Yunnan corresponds more with elevation than latitude [10].

Materials and Methods
This study focuses on three representative regional floras from southern, central and northwestern Yunnan. Southern Yunnan (Xishuangbanna administrative region) lies between 21u099 and 22u369 N, 99u589 and 101u509E and has an area of 19690 km 2 with altitude varying from 480 m at the bottom of the lowest valley (Mekong River) to 2430 m at the highest mountain top. Central Yunnan, here including 7 counties, lies between 23u539 and 25u119 N, 100u329 and 101u58 9E. It includes the core area of Wuliang and Ailao Mountains, and has an area of 25,424 km 2 . Its altitude varies from 422 m at the lowest valley in the south (Red River) to 3156.9 m at the highest mountain top. Northwestern Yunnan, here including three counties, lies between 27u109 and 28u279 N, 98u539 and 99u429 E. It has an area of 23870 km 2 with altitude varying from 1900 m at the lowest valley to 6740 m at the highest mountain summit ( Figure 1).
Complete lists of the total native seed plants of southern, central and northwestern Yunnan were based on the recently completed Flora of Yunnan [16], the data base of seed plants from KUN (herbarium of Kunming Institute of Botany, Chinese Academy of Sciences), and floristic inventories [21,22,23]. Circumscription of families and species followed the nomenclature of w 3 TROPICOS (http://mobot.mobot.org/W3T/Search/vast.html).
Based on these plant lists, floristic and geographical attributes of the floras of the three areas were analyzed. Patterns of seed plant distributions of these floras were quantified at the generic level based on Wu's documentation [24] and at the family level following Wu et al. [25]. Comparisons of both floristic composition and geographical elements were made to assess floristic similarities and variation, as well as to determine biogeographical affinities between the three areas. The dominant families from these three floras are summarised in Table 1.

Floristic Composition
Genera with the highest species richness in southern Yunnan include  Table 2).

Geographical Elements
Geographical elements at family level. Twelve distribution types of family are recognized from northwestern, central, and southern Yunnan (Table 3).
In the flora of southern Yunnan, there are 106 (57.92%) families with tropical distributions. Those with pantropic distributions make up 73.9% of the tropically distributed families, and 42.08% of the total number of families, such as Acanthaceae, Anacardiaceae, Annonaceae, Apocynaceae, Araceae, Arecaceae, Burseraceae, Clusiaceae, Myristicaceae, Sapotaceae and Icacinaceae. The remainder have old world tropic distributions (including Pittosporaceae, Pandanaceae and Musaceae), tropical Asian and tropical American disjunct distributions (including Araliaceae, Elaeocarpaceae, Gesneriaceae, Staphyleaceae and Styracaceae), and tropical Asian distributions (such as Crypteroniaceae and Sabiaceae). Cosmopolitan families make up 25.68% of the total number of families in southern Yunnan, such as Asteraceae, Poaceae, Rosaceae, Fabaceae etc. Families with mainly temperate distributions contribute 16.39% to the total flora, including those with north temperate distributions (such as Caprifoliaceae, Betulaceae, Buxaceae and Salicaceae), east Asia and north America disjunct distributions (Magnoliaceae, Nyssaceae and Saururaceae), and east Asian distributions (Actinidiaceae, Cephalotaxaceae and Stachyuraceae).
In  Table 4.

Biogeographical Divergence of the Flora of Yunnan
The floristic similarity between these regional floras of Yunnan is more than 88% at the family level and more than 55.89% at the generic level, and lowest at the species level (26.72%-48.45%) ( Table 5). Central Yunnan shares a nearly equal proportion of flora with both southern and northwestern Yunnan. The family Theaceae is particularly species rich in central Yunnan, but otherwise the dominant families of central Yunnan are also common to southern or northwestern Yunnan.
The dominant families in the floras of southern and northwestern Yunnan are different, with the exception of the most dominant families (see Table 1). The families Zingiberaceae, Cucurbitaceae, Apocynaceae, Annonaceae, Convolvulaceae, Are- A majority of the thirty most dominant genera in central Yunnan (Table 2) are also common to either southern or northwestern Yunnan. However, southern and northwestern Yunnan do not share any dominant genera.
Comparisons of geographical elements (distribution types) at family level from these regional floras revealed that, excluding cosmopolitan families, families with tropical distributions in all (Table 3, type 2-7) contribute to a majority of the total number of families in all three floras of Yunnan, and families with pantropic distributions make up the highest proportion among geographical elements.
Tropical genera in all (Type 2-7) make up 77.02% of the total number of genera in the flora of southern Yunnan, and genera with temperate distributions (Type 8-14) contribute only to Of the genera that are shared by two of the three floras, 621 genera are found only in central and southern Yunnan, and 664 genera are present only in the floras of central and northwestern Yunnan. Among the genera found only in central and southern Yunnan, 551 genera have tropical distributions, including 253 genera from tropical Asia, 102 genera with pantropic distributions, and 78 genera with tropical Asia to tropical Australia distributions. Among the genera only to the floras of central and northwestern Yunnan, 495 genera have temperate distributions, including 146 north temperate genera, 137 genera of east Asia, and 75 genera with old world temperate distributions. It is evident that the flora of central Yunnan has floristic attributes of both southern and northwestern Yunnan, and is related to the flora of southern Yunnan mainly by tropical elements, especially tropical Asia and pantropic elements, while related to the flora of northwestern Yunnan mainly through temperate elements, especially north temperate and east Asian elements.

Discussion and Conclusions
The flora of Yunnan is noticeably divergent along altitude, latitude and topography. Southern Yunnan has a tropical flora of Malaysian affinity, while northwestern Yunnan has a temperate Himalayan flora. Whether these floral patterns are a consequence of mainly ecological divergence or historical-biogeography is of important significance in revealing the formation and evolution of the flora of Yunnan.
Although the floras of southern and northwestern Yunnan have a similar composition at the family level (88% similarity), they differ at the generic (55.89% similarity) and especially at specific (26.72% similarity) levels.
The flora of northwestern Yunnan is dominated by families and genera with cosmopolitan and north temperate distributions, while Table 3. Geographical elements of seed plants at the family level of these three compared floras.  the flora of southern Yunnan, although sharing some large cosmopolitan families, is additionally dominated by tropical families and genera. The flora of northwestern Yunnan is comprised more of tropical families (45.18) than temperate families (24.7%), but temperate genera are more dominant (47.19%). The flora of southern Yunnan is comprised mainly of tropical families (57.92%) and genera (77.02%). Furthermore, the flora of northwestern Yunnan is characterized by a relatively high proportion of genera with northern temperate, east Asian and old world temperate distributions, while the flora of southern Yunnan is characterized by a relatively high proportion of genera with tropical Asian (making up the highest proportion), pantropic and tropical Asia to tropical Australia distributions. Among genera that are known from southern Yunnan but not central or northwestern Yunnan, the majority have tropical distributions. Among genera in northwestern Yunnan only, the majority have temperate distributions. These factors reveal that the flora of northwestern Yunnan is different from the flora of southern Yunnan in floristic attributes. The flora of northwestern Yunnan is temperate in nature, while Table 5. Comparison of floristic similarities at the family, generic and specific levels between these floras of southern, centre and north-western Yunnan.  Table 6. Ggeographical elements of seed plants at the generic level in these floras respectively and shared by two of them. the flora of southern Yunnan is tropical in nature and has strong tropical Asian affinities. Furthermore, if we look at global species numbers from each family and consider the proportion of species that are found in each of these regional floras (excluding cosmopolitan families), the families characteristic of southern Yunnan are quite different from those of northwestern Yunnan. In the flora of southern Yunnan, tropical families, such as Tetramelaceae, Cardiopteridaceae, Sphenocleaceae, Sladeniaceae, Musaceae, Hernandiaceae, Gnetaceae and Opiliaceae are most characteristic; while in northwestern Yunnan, temperate families, especially those with east Asian distributions, such as Helwingiaceae, Tetracentraceae, Stachyuraceae, Cephalotaxaceae, Eupteleaceae, and those with north temperate distributions, such as Diapensiaceae, Caprifoliaceae, Hydrangeaceae, Betulaceae, Pinaceae, Juncaceae and Papaveraceae are characteristic.
The conspicuous divergence between the floras of southern and northwestern Yunnan might be attributed to differences in geological history as well as differences in ecological habitat. The northwestern Yunnan had a temperate and subtropical flora during the Tertiary [26], and underwent a quick uplift with the Himalayas after the Pleistocene [27]. Uplift of Himalaya mountains began about 50 Myr ago, and further significant increases in altitude of the Tibetan plateau are thought to have occurred about 1068 Myr ago [28,29,30]. With the uplift of Himalaya, east Asian monsoons were appeared about 9-8 Myr ago, and intensified about 3.6-2.6 Myr ago [31]. Studies on fossil mammalian, pollen, as semblages and sedimentclay mineralogy as well as carbon isotope data from fossil tooth enamels and paleosol carbonates revealed that the central Himalaya had a much warmer environment in the late Neogene, and the ''paleothermometer'' were 2166uC at c. 7 Myr ago, which is 1966uC higher than the present-day, and it was suggested that these area have been raised by c.2-2.5 km since 7 Myr to its current elevation of 4100-4500 m above sea level [32]. The northwestern Yunnan, which is part of eastern Himalaya, should have the similar environmental changes and tectonic uplift as the central Himalaya since the late Neogene. On the other hand, collision between India and Asia also caused lateral extrusion of southeast Asia between 32 Ma and 10 Ma [28,33,34,35,36,37].
It could be inferred that these floras of Yunnan could derived from a common Tertiary tropical or subtropical Asian flora. Thereafter, the flora of northwestern Yunnan has evolved with the uplift of the Himalayas by gradual proliferation of mainly cosmopolitan and north temperate floristic elements, while the flora of southern Yunnan has evolved with extrusion of the Indochina block to southeast Asia by the influence of mainly tropical Asian elements.