Origins and evolution of plant diversity in the Hengduan Mountains, China

Abstract

The Hengduan Mountains region (HDM) in southwest China, one of the earth's 34 biodiversity hotspots, is characterized by its unique geology, dramatic topography, a climate where snow and below freezing temperatures can occur on any day of the year, by its location at elevations averaging between (1400–) 2000 and 4500 (–5300) meters above sea level (m a.s.l.), and by one of the richest floras in the temperate Northern Hemisphere. The formation and evolution of the plant diversity in the area and uplift of the Qinghai–Tibet Plateau (QTP) were synchronous (Sun, 2002, Sun and Li, 2003). During its geological history, as part of the QTP, the area was still below the Tethys Sea or on the coast of the Tethys in the late Cretaceous (Zhang, 2012). In the early Tertiary, the collision between the Indian plate and Eurasia led to the retreat of the Tethys Sea and the QTP entered an era of land evolution (Zheng, 2013). From the Eocene to the Oligocene, the Tethys gradually narrowed as the Indian plate continued its northward movement. By the end of the Oligocene, the sea had since dropped out and the main body of the QTP was transformed into a land environment (Zhang, 2012). The Hengduan Mountains (the eastern edge of the QTP) had formed as north–south asymmetric wavy creases and large intervening fault zones (gorges). The geological evolution of the HDM coincided with the uplift process and was synchronous with the evolution of the QTP (Zhang, 2012). Therefore, the flora of the HDM began its development and evolution with the retreat of the Tethys Sea and appearance of land in the early Paleogene. Along with the uplift of the QTP and the evolution of the geological environment, the flora underwent a change from a thermophilic flora in the early Paleogene to a xerophytic and temperate to alpine flora by the mid-to late Neogene (Sun, 2002). Additionally, many plant groups migrated into the region from various sources resulting in a very rich and complex flora, which the HDM has preserved in many ways to the present. After the Neogene, global temperatures decreased, resulting in the Quaternary glacial periods. Plant diversity and the flora of the Arctic-Tertiary in many parts of the Northern Hemisphere, such as in Europe and N America, suffered devastating destruction and a large number of species became extinct, resulting in the extant flora in these regions becoming poor and reduced to fragments of their former richness (Kubitzki and Krutzsch, 1996, Tiffney and Manchester, 2001). It is therefore difficult for us to study the origin, differentiation and formation mechanisms of plant diversity of the earth in those areas. The complex and diverse habitats in the HDM, however, were relatively little affected by climatic and geological processes. They therefore provided optimal conditions for evolution and diversification to take place while at the same time maintaining refugia where plants could ride out the glacial cycles. The synchronous evolution of plant diversity and geological events provide a relatively integrated framework for interpreting the evolutionary history of the flora. Not only are there some ancient remnants or relics, but also, more prominently, a large number of clades that experienced rapid radiations, forming relatively complete lineages and many young species and infraspecific taxa. The HDM is therefore not only a natural historical ‘museum’ that has preserved plant diversity since the Cenozoic era, but also a ‘cradle’ where many new species were born and flourished. The preservation of the old while giving rise to the new has resulted in the incredible plant diversity that has made the HDM the hotspot that it is today and the key area and natural laboratory for the study of the origins, evolution and dispersal of that diversity.