A comparison of the taxonomic richness of vascular plants in China and theUnited States

Numbers of taxa at the level of order, family, genus, and species were tabu lated for 12 subclass-level taxonomic groups of vascular plants in the flor as of China and the United States. Analysis of these data showed that the f lora of China is significantly more diverse than that of the United States. Furthermore, the difference in diversity arises at and below the level of genera. Finally. the euasterids II and Caryophyllidae are exceptions to the general trend in being more diverse in the United States. As a result, the floras of China and the United States are different nonrandom samples of t he floras of the North Temperate Zone and the world. Phylogenetically older groups have a larger proportion of genera shared between China and the Uni ted States and also lend to have larger proportions of taxa belonging to ge nera with tropical affinities. The two subclass-level groups that are more diverse in the United States have primarily temperate affinities and are re latively young phylogenetically. We conclude that the patterns of diversity of Chinese and U.S. vascular plants have been influenced by the longer and more open access of temperate eastern Asia to tropical regions, the presen ce in southern China of a larger area of subtropical climate with complex t opography, and the reduced impact of late Tertiary climate cooling in easte rn Asia compared to North America. The greatest differences in diversity oc cur among older groups having stronger tropical affinities and, perhaps, or iginating in eastern Asia. In the North American flora, these groups appear to be disproportionately small relicts of a formerly widespread "boreotrop ical flora" that was relatively homogeneous at the genus level across the B ering Land Bridge between eastern Asia and North America. Groups that arose and diversified later, and that have fewer genera in common between Asian and North American floras, particularly the euasterids II, were better adap ted to the cooler and drier climates of the Neogene temperate latitudes and evidently were relatively unaffected by late Neogene glaciations. These gr oups are thus as diverse, or more diverse, in North America as in eastern A sia. Understanding the relative-diversity of two regions requires an apprec iation of the historical development of the floras in the context of large- scale processes and events.