Evolutionary Triggers of Plant Speciation in the Himalaya-Tibet-Hengduan Biodiversity Epicentre

Background: Past studies linking geologic and climatic controls on speciation in the Himalaya, Tibet and Hengduan (HTH) regions do not fully capture the mechanisms underlying patterns of plant diversity and endemism in hyperdiverse mountain ecosystems. Aims: This study sheds new light on the orogenic and climatic drivers of plant endemism and diversity in the HTH region and unravels the mechanisms behind plant speciation and assembly. Materials & Methods: We re-examine the past findings on geo-climatic controls on speciation in the HTH. We also analyse new datasets on the elevational patterns of endemic species richness in the HTH regions and correlate these with the hypsometric profiles based on digital elevation models of these regions. Results: We discover that the phased evolution of HTH mountain-building and climate oscillations, and species radiations were not exactly synchronous, but were interspersed with periods of inactivity. We find an inverse yet overlapping relationship between elevation-area troughs (EATs) of hypsometric profiles and peaks of endemic plant richness in the HTH, indicating that most endemic species occupy the higher elevational belts with the least geographic areas. These elevational belts comprise geographically and biologically isolated habitats. Discussion and Conclusions: We infer that orogeny, monsoon and glaciations have shaped the highly dissected HTH landscapes, characterised by high-relief topography, leading to population isolation, speciation and endemism, albeit with temporal lags. Glacial–interglacial cycles have further promoted isolation, co-habitation, hybridisation and polyploidy among plant populations, generating and accelerating endemic radiations. Our results provide crucial evidence linking mountain-building, climatic events and landform development with plant species richness and endemism in the HTH regions and potentially in other mountain ecosystems.