1 Department of Botany, National Museum of Natural History, Smithsonian Institution, Washington, DC, United States.
2 Department of Biology, University of Mississippi, Oxford, MS, United States.
3 School of Life Science, Shanxi University, Taiyuan, Shanxi, China.
4 Key Laboratory of Biodiversity Formation Mechanism and Comprehensive Utilization of the Qinghai-Tibet Plateau in Qinghai Province, School of Life Sciences, Qinghai Normal University, Xining, China.
5 Wolfson College, Oxford University, Oxford, United Kingdom.