Molecular phylogenetidsts often find that a diversification of western Chinese plant taxa took place in the Miocene and link this to the Neogene uplift of the Tibetan Plateau. This link is made despite abundant geological evidence showing that a high but topographically complex Tibet already existed in the Paleogene. To evaluate and constrain molecular phylogenetic trees and better understand Asian plant diversification requires accurate systematic assignment of well-dated plant megafossils. Here, as part of an ongoing programme of absolute dating and taxonomic evaluation of fossil floras across southwestern China and the Himalaya, we present a new and expanded systematic treatment of the late Miocene leaf flora from the Gazhacun Formation near the village of Wang b'dui in Namling County, central southern Tibet. This flora, whose age (15 Ma) and paleoelevation (similar to 5 km) are well constrained, comprises 9 families, 13 genera and 25 species (including 22 new species). The paleoflora represents a typical boreal temperate mostly deciduous broad-leaved forest attesting to a cool humid climate. At the paleolatitude of the site, <28 degrees N, this is compatible with the high elevation (4700-5200 m) for this part of Tibet quantified by both leaf physiognomy and isotopic analyses. Plant fossil evidence also witnesses a much wetter Tibetan upland environment before a rising Himalaya obstructed northward-moving moist air from the Indian Ocean. (C) 2019 Published by Elsevier B.V.