American amphitropical disjuncts: Perspectives from vascular plant analyses and prospects for future research

PREMISE OF THE STUDY: Historical patterns and processes of plants with an American amphitropical disjunct (AAD) distribution have long interested botanists and biogeographers. Here we update examples of AAD vascular plants, their biogeographic history, and aspects of their biology elucidated by recent studies to make inferences about common patterns of AAD plants and formulate future research questions. METHODS: All known examples of AAD vascular plants were tabulated, along with data on plant duration and habit, chromosome number, dispersal direction, and divergence time. The data were then compared with regard to taxonomic categories, AAD bioregions, and character evolution. KEY RESULTS: We clarify the definition of amphitropical and summarize features of AAD plants. We identify 237 AAD plant divergence events. Timing of these events generally corresponds with taxonomic category. Plant duration and habit are associated with AAD bioregions. Increases in chromosome number mostly occurred in members of the recipient area. The AAD plants of bipolar or temperate bioregions entirely or largely dispersed from North to South America, whereas almost half of desert plants dispersed from South to North America. CONCLUSIONS: Tabulating AAD plants by taxonomic group and bioregion yields insight into character evolution and processes of divergence. Phylogenetic studies provide information on the timing and direction of dispersal. However, more research on AAD plants is needed to draw inferences regarding general patterns and processes, especially those at the clade level. Our AAD Working Group website provides current information on AAD vascular plants to aid workers doing research in this field.