Floristic relationships between Eastern Asia and North America: test of Gray's hypothesis.

Similarities in the temperate floras of eastern Asia and North America have been appreciated for more than 200 yr, but the generality of the floristic relationships among eastern Asia (EAS), eastern North America (ENA), and western North America (WNA), postulated by Asa Gray about 150 yr ago, has not been tested until now. In this article, floristic relationships based on genera shared among EAS, ENA, and WNA were examined at different spatial scales for different phylogenetic groups using complete floras. Floristic similarity between EAS and ENA is higher than that between EAS and WNA, and the floras of ENA and WNA are more closely related to each other than are the floras of EAS and ENA. Compared with ENA and WNA, the number of genera common to EAS and ENA is significantly higher in basal angiosperms and significantly lower in asterids. Floristic similarities tend to decrease from more basal to more modern lineages between EAS and ENA and between EAS and WNA but not between ENA and WNA. Similarly, from more basal to more modern divisions, the fraction of shared genera decreases between EAS and ENA and between EAS and WNA, whereas the floristic similarity between ENA and WNA tends to increase. Furthermore, floristic similarity between EAS and ENA increases with latitude. The causes of the observed patterns of floristic similarity between EAS, ENA, and WNA are discussed.