The phylogeographic patterns of Northern Hemisphere angiosperms reflect the dynamic interplay between plate tectonics, climate and terrestrial biota during the last 100 Ma. Barriers including oceans, mountains, deserts, day length, available precipitation, and seasonal temperature influenced the dispersal, genetic isolation, and evolutionary diversification of terrestrial plants and their dependent animals. The major physiographic barriers to early Tertiary land organisms were oceanic, restricting exchange to higher latitudes between Eastern Asia and North America (The Bering Land Bridge) and North America and Europe (The North Atlantic Land Bridge). Europe and Asia were separated by the Turgai Strait. While boreal climate was relatively warm, winter day length may have posed an added barrier at the more northerly latitudes. Global cooling near the end of the Eocene introduced the further barrier of continental climates hosting herb-dominated biomes in midcontinental North America and Eurasia, aided locally by the growth of the Rockies (North America), the Alpine Orogeny (Europe), and the retreat of the Turgai Strait (Eurasia). By the Late Tertiary, continued tectonism, including the rising Himalayan Mountains, increased the intensity of seasonal changes. The Pleistocene ice ages further restricted intercontinental exchange to cool-temperate and Arctic forms, while decimating warm-temperate and subtropical clades in Europe and North America. The basic geological and paleoclimatological evidence for these environmental changes is now relatively well known. However, several finer-scale questions remain unanswered. Does short winter day length really pose a barrier to evergreen plants in warm northerly latitudes? Did ephemeral migration routes allow brief pulses of biotic exchange during climatic transitions, e.g., allowing warm-temperate elements to move between Europe and Asia in the middle Tertiary? What is the role of herbs in early Tertiary floras, where they are generally poorly represented?

Key words: Angiosperm, Phytogeography, Tertiary