Biogeography of the savanna-like vegetation in hot dry valleys in southwestern China with reference to their floristic origin and evolution

Savanna-like vegetation and dry thickets occur in hot dry valleys across southwestern China. Here, the flora and biogeography of these vegetations are studied. Native seed plants of 3,217 species from 1,038 genera in 163 families are recorded from the hot dry valleys in SW China. The biogeographical elements with a tropical distribution contribute 57.18%, but the ones with a temperate distribution contribute 36.45% of the total genera of the flora. This shows that the flora has proliferated by temperate elements via their evolution, although the flora occur in tropical habitats in the hot dry valleys. Floristic divergence across these hot dry valleys is obvious. The floras in the Yuanjiang (the upper reaches of the Red River) and the Nujiang (the upper reaches of the Salween River) valleys are dominated by tropical elements (77.26% and 74.49 of the total genera, respectively), but the flora of the Jinshajiang (the upper reaches of the Yangtze River) valley is composed of half tropical (47.27%) and half temperate (44.96%) genera. Regarding floristic similarities, the Jinshajiang shows the highest similarity to the Yuanjiang although these river valleys are located a great distance from each other. Our results could be well explained from the geological events since the Cenozoic, such as the uplift of Himalayas, the extrusion of Indochina, the river capture of the Jinshajiang separating from the Yuanjiang, and the northward movement of the Burma Plate. Further floristic comparison between the flora in hot dry valleys of SW China and southern Africa supports the consideration that the flora of savanna-like vegetations of SW China could have floristic affinity to African savannas over the course of its evolutionary history by the Indian Plate from southern Africa colliding with Eurasia in the Cenozoic.