A Comparison of the Taxonomic Richness of Vascular Plants in China and the United States.

Numbers of taxa at the level of order, family, genus, and species were tabulated for 12 subclass-level taxonomic groups of vascular plants in the floras of China and the United States. Analysis of these data showed that the flora 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 the floras of the North Temperate Zone and the world. Phylogenetically older groups have a larger proportion of genera shared between China and the United States and also tend to have larger proportions of taxa belonging to genera with tropical affinities. The two subclass-level groups that are more diverse in the United States have primarily temperate affinities and are relatively 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 presence in southern China of a larger area of subtropical climate with complex topography, and the reduced impact of late Tertiary climate cooling in eastern Asia compared to North America. The greatest differences in diversity occur among older groups having stronger tropical affinities and, perhaps, originating in eastern Asia. In the North American flora, these groups appear to be disproportionately small relicts of a formerly widespread "boreotropical flora" that was relatively homogeneous at the genus level across the Bering 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 adapted to the cooler and drier climates of the Neogene temperate latitudes and evidently were relatively unaffected by late Neogene glaciations. These groups are thus as diverse, or more diverse, in North America as in eastern Asia. Understanding the relative diversity of two regions requires an appreciation of the historical development of the floras in the context of large-scale processes and events.