In the southern mountain ranges of Yunnan province, China, deep valleys of several large rivers create rain shadows with hot dry summers, and are locally designated tropical; towards the north, notably in the Lancang (Upper Mekong) valley, these regions may experience frost during winter. The woody forest canopy of these valleys is predominantly deciduous, with evergreen elements in the north, where the canopy is open and the forest savanna-like. However, we here present tall forest with a closed deciduous canopy and semi-evergreen subcanopy observed in hot dry valleys of these rivers and their tributaries in the tropical south. The structure and physiognomy of these forests resemble the tall (moist) deciduous forest formation widespread in South Asia and Indo-Burma. Furthermore, these forests are largely composed of tropical elements at both the generic (80%) and the species level (>70%), indicating that these forests are indeed tropical. We originally hypothesized that these isolated forests represent refugia of a pre-Holocene extension of tall (moist) deciduous forest formation of South Asia and Indo-Burma. The sample plot we established to test this hypothesis confirmed that these forests share the structure and physiognomy of the tall (moist) deciduous forest formation; however, the plots also showed that these forests lack the characteristic and dominant species of the formation's Indo-Burmese range. The tree flora, in particular, indicates that both deciduous and evergreen elements are instead mostly derived from the adjacent tropical semi-evergreen forests of tropical southern China; yet they also include an important endemic element, which implies that these forests have survived as refuges possibly since the Pliocene. The exceptional representation of evergreen elements in these forests indicates that they have rarely been subject to hot fires or domestic cattle browsing, adding to the unique nature of the forests and further justifying their strict conservation.
