Patterns in the assembly of temperate forests around the Northern Hemisphere

Recent studies of Northern Hemisphere biogeography have highlighted potentially significant differences between disjunction patterns in plants versus animals. To assess such differences, we compiled a larger sample of relevant plant phylogenies from which disjunction patterns, ancestral areas and directions of movement could be inferred. We considered 66 plant clades with species variously endemic today to eastern Asia (EA), Europe (including southwestern Asia), eastern North America (ENA), and/or western North America (WNA). Within these clades we focused on 100 disjunctions among these major areas, for 33 of which absolute divergence times have also been inferred. Our analyses uphold the view that disjunctions between EA and ENA are exceptionally common in plants, apparently more so than in animals. Compared with animals, we find few disjunctions between EA and WNA, consistent with increased extinction in WNA or failure of some groups to colonize that region. Taken at face value, our data also support the view that many temperate forest plant groups originated and diversified within EA, followed by movement out of Asia at different times, but mostly during the last 30 Myr. This favours Beringia over a North Atlantic land bridge as the primary path between the Old World and the New World. Additional studies are needed, especially to evaluate the impacts of differential extinction on these patterns, to more confidently establish divergence times, and to assess the statistical significance of these findings. Fortunately, many more plant groups show relevant disjunction patterns and could soon be added to such analyses.