The Simao Terrane, which forms the present west and south parts of Yunnan, has been suggested as one of the prominent fragments of the extruded Indochina block It was displaced southward by 800 km, together with a clockwise rotation of 30°. The rotation processes were believed active until at least the Miocene. This unique geological history may thus influence the abundant patterns of dominant geographical elements across Yunnan.
Prof. ZHU Hua of Xishuangbanna Tropical Botanical Garden (XTBG) used a database of 38 regional floristic studies (mostly in well-protected nature reserves) covering Yunnan to illustrate the distribution patterns of their floristic elements. He compared the distribution patterns of Yunnan seed plants and geological events. He assigned the seed plants to 15 distribution patterns according to their worldwide geographical distributions.
According to the study, the abundances of geographical elements across Yunnan show complicated patterns. The abundance pattern of the pantropic genera showed high proportion at dry-hot valleys despite latitude and altitude. The abundance pattern of the typical temperate genera showed some relations to latitude and altitude. The abundance pattern of the east Asia genera showed also some relations to latitude and altitude, but a higher proportion in southeast Yunnan.
The researcher considered that the relatively high proportion of tropical Asian genera in western and southwestern Yunnan matched the clockwise rotation of the Simao Terrane and the southeastward extrusion of the Simao-Indochina geoblock that facilitated migrations of SE Asian plants. These oblique patterns in Yunnan cannot be explained by climate alone, but can be explained at least partly by the geological history.
