Gregory W Stull 

Biogeographic disjunction patterns, where multiple taxonomic groups are shared between isolated geographic areas, represent excellent systems for investigating the historical assembly of modern biotas as well as fundamental biological processes such as speciation, diversification, niche evolution, and evolutionary responses to climate change. Studies on plant genera disjunct across the Northern Hemisphere (NH), particularly between eastern North America (ENA) and eastern Asia (EAS), have yielded tremendous insight on the geologic history and evolutionary assembly of rich temperate floras. However, one of the most prevalent disjunction patterns involving ENA forests has been largely overlooked: that of taxa disjunct between the eastern US (EUS) and cloud forests of Mesoamerica (MAM), with examples including sugar maple (Acer saccharum), dogwood (Cornus florida), sweetgum (Liquidambar styraciflua), redbud (Cercis Canadensis), hop hornbeam (Ostrya virginiana), and beechdrops (Epifagus virginiana). Despite the remarkable nature of this disjunction pattern, which has been recognized for over 70 years, there have been surprisingly few recent efforts to empirically examine its evolutionary and ecological origins. Here I synthesize previous systematic, paleobotanical, and phylogenetic and phylogeographic studies to establish what is known, and unknown, about this disjunction pattern to provide a roadmap for future research. I argue that this disjunction pattern, and the evolutionary history and fossil record of the Mexican flora more broadly, represents a key missing piece in the broader puzzle of Northern Hemisphere biogeography. I also suggest that the EUS-MAM disjunction pattern represents an excellent system for examining fundamental questions about how plant traits and life history strategies mediate plant evolutionary responses to climate change, and for predicting how broadleaf temperate forests will respond to the ongoing climatic pressures of the Anthropocene.