A concept on my mind for a few years now is the intriguing idea of refugia—places less affected by glaciation (or other difficult situations) during ice ages to where plants and animals could migrate.
Florida is certainly a possible refugium, as it was never glaciated. While it’s not unusual to think of animals migrating to more favorable locales, it is startling to think of plants migrating, but they do. If they don’t, or don’t do so quickly enough, they face may extinction if conditions change for the worse.
The refugium concept has been used to help explain the distribution of some current-day plant and animal communities. One that has stuck in my mind for a time now is that of the paw paw tree, Asimina triloba, and other species in the same genus.
The paw paw is a real oddity. It’s a member of the Annonaceae plant family, a family containing many tropical and sub-tropical plants, with a few stragglers in temperate regions, like paw paw. I believe Asimina is the only genus in the family to venture north. As an edible fruit, it’s got an interesting history with attempts to commercialize it and people swearing it’s the most insanely delicious fruit they’ve eaten.
While pond apple (Annona glabra) for example, another Annonaceae, does well in the hottest parts of southern Florida, paw paw does not sink below four counties in the northernmost Florida panhandle, according to the USDA distribution maps. And its range apparently extends well into southern Canada. Quite un-tropical!
How is/was Florida a refugium? While the edible paw paw, A. triloba, doesn’t love the sub-tropics, there are eight Asimina species and a naturally occurring hybrid. All of them, excepting A. Triloba, grow in Florida:
Asimina angustifolia (slimleaf pawpaw) grows only in Florida, Georgia and small parts of Alabama.
Asimina incana (woolly pawpaw) grows only in Florida and small parts of Georgia.
Asimina × nashii [angustifolia × incana] (Nash’s pawpaw) grows only in Florida and very small parts of Georgia.
Asimina obovata (bigflower pawpaw) is found only in Florida.
Asimina parviflora (smallflower pawpaw) is exceptional in its range covering Florida, Georgia, Alabama, west to parts of Texas and north to parts of southern Virginia.
Asimina pygmea (dwarf pawpaw) is found only in Florida and two bordering Georgia counties.
Asimina reticulata (netted pawpaw) grows throughout Florida (south to Miami-Dade, unlike other the other species) and into Georgia.
Asimina tetramera (fourpetal pawpaw) is restricted to only Palm Beach and Martin Counties in Southeast Florida!
You get the picture. There’s lots of paw paw species in Florida. What I can imagine is glaciation in North America causing southern migration of paw paw species to the south, where they could wait out the ice. Thereafter, when temps increased and ice disappeared, they could slowly migrate to areas north, with common paw paw (A. triloba) being the most successful.
Whereas animals may migrate back to more favorable locations fairly quickly and leave little evidence behind of their travails, plants are a lot slower in doing so, thereby giving us the chance to see the action in their re-inhabiting formerly icy areas, or at least some trail of it.
Or, it may be Florida was a refugium to one or just a few Asimina species, and given enough time they were able to differentiate and speciate into the paw paw species variety we find in Florida today. And again, some migrated north with A. triloba being the best at re-establishing itself up north.
Then again, maybe humans are responsible for dispersing the most edible of the species up north.
I have not yet found any research done on this topic, but I dearly want to know if there’s any evidence for this idea one way or the other. Maybe DNA research can answer some questions by determining the age of the paw paw species.
Read my piece on paw paw and Dr. David Fairchild here.
There’s also a fantastic book on the fruit: Pawpaw: In Search of America’s Forgotten Fruit.