Cold origins limit the establishment of northern temperate plants in the Southern hemisphere

Plants with amphitropical distributions have closely related populations in both Northern and Southern Hemispheres, but are absent from the intervening tropics. They provide a unique opportunity to study the constraints shaping the distribution of temperate lineages through time. Using grasses from the ecologically diverse supertribe Melicodae, an emerging study system with species distributed throughout the temperate regions, we test the hypothesis that geography and/or environmental niche constrain which lineages successfully cross the tropics to establish in the opposite hemisphere. Biogeographic and evolutionary modelling was conducted on well resolved plastid and nuclear phylogenies constructed from whole-genome sequencing of 178 accessions of 103 Melicodae species. Results show that species from cold regions are much less likely to successfully cross the tropics, with successful lineages all sharing warmer niches that evolved prior to their establishment in the opposite hemisphere. Evidence suggests that this result is explained both by the greater distances that high-latitude, cold-origin lineages must disperse to cross the tropics, and inherent limitations associated with colder thermal niches. In particular, our results suggest that traits allowing species to cope with cold winters, rather than an inability to cope with warm summers, limit their ability to establish in the opposite hemisphere, hinting at important trade-offs between cold-tolerance and biogeographic potential. These results provide insight into the drivers of the distribution and diversity of plants, and the challenges facing cold-origin lineages in a rapidly warming world. If cold-origin species occupy a smaller proportion of their potential range, and are unlikely to establish in new areas with suitable climates, their ability to track preferred habitat as climates warm may be worse than currently expected.