Sabal minor subsp. louisiana origin theories?

[L.A.M.]

I have to wonder whether S. louisiana may be a different species than S. minor, although given the lack of genetic evidence we have with S. brazoriensis, I'm inclined to consider it a subspecies for now. It's clearly too distinct to just be a regional variety though, just as the Picea glauca subsp. densata is distinct enough from standard P. glauca to be even more fiercely heat-tolerant and also be far more drought-tolerant. In any case, I digress. What do you think caused S. minor subsp. louisiana to speciate well beyond a typical regional variety and enough to possibly even be an unproven separate species? Here's my theory.

Similarly to a distinct subspecies or even distinct species, S. louisiana reaches the size of a small tree and grows much faster than standard S. minor, and it's substantially less reliably cold-hardy (only to about 5 degrees Fahrenheit above zero instead of 0, and even 5 can damage it if it stays below 10 for 24 hours or longer). I suspect both S. louisiana and S. brazoriensis in neighboring Texas evolved from S. minor on parallel evolutionary tracks within the last 120,000 years or so. Whereas S. brazoriensis has probably evolved to consistently stay in Brazoria County (it's reliably cold-hardy to 0 despite being bigger than S. louisiana) ever since Eemian sea levels fell from their higher-than-modern peak, S. louisiana clearly would've had to jump from dry land to the exposed continental shelf to survive the harshest periods of the Last Glacial Period around 70,000 and 26,000 years ago then jump back to higher ground before Holocene sea levels reached near-modern levels. This could explain why they're smaller and less cold-hardy than S. brazoriensis; S. louisiana clearly evolved to migrate as sea levels and climate changed, not to necessarily reach their full potential in terms of size and cold-hardiness like S. brazoriensis did.

I made a similar speculative thread about S. brazoriensis a few months ago. The mysterious extra cold-hardy tree-sized palmettos of the northwestern Gulf Coast have fascinated me for some time! Someone published an animation of prehistoric North American climate data based on 120,000 years of Greenland Ice Core data, and although their winter isotherm distinguishing between temperate and continental climates was -3 degrees Celsius (26.6 degrees Fahrenheit), not 0 Celsius (32 Fahrenheit), it still gives us a pretty good idea of where palms could and couldn't have survived then; areas within a few miles of the transition zone would've been too hostile in light of the ground still freezing.

P.S.: As for why we don't see any attempts at speciation in Mississippi, Alabama and the Florida panhandle, nor in the Old South? They clearly must've failed further east than Louisiana along the Gulf Coast due to the bad winters during the two most severe climate periods even on the continental shelf. Don't forget, as I mentioned, the video creator chose to use the -3 degrees Celsius winter isotherm, not the 0 degrees Celsius isotherm that's more reflective of the ability of subtropical or oceanic climate vegetation to avoid freezing to death in a decades- or centuries-long life. I noticed periods as early as about 77,500 years ago and as late as about 15,200 years ago where the coasts at the time of Mississippi, Alabama and the Florida panhandle probably would've had at least one month averaging somewhat below freezing. On the contrary, the southwestern corner of modern land in Brazoria County and some land at the time off the coast of modern Louisiana probably retained suitable habitat for palms (albeit barely about 26,000 years ago in the Brazoria County case) as long as they at least adapted to be more cold-hardy than S. palmetto, with S. brazoriensis being further tested to redevelop the full extent of S. minor's cold-hardiness to survive winters comparable to modern Tennessee and Kentucky (something I theorized in my thread about that) and S. louisiana constantly seeding new areas to avoid being wiped out by fluctuations between freezing winters higher up or rising sea levels lower down.

Nascent Sabal palm varieties evolving towards subspecies in the Old South, I suspect, probably simply got outcompeted by S. palmetto along the Atlantic coast and Florida Peninsula even in favorable climate periods - perhaps even also outcompeted by the shrubby Serenoa and Rhapidophyllum which still have aboveground trunks, unlike standard Sabal minor - or hybridized into oblivion. Furthermore, the Atlantic coast has a third, major issue during glacial periods; although Greenland Ice Core data doesn't show it, the immediate Atlantic coastal plain sometimes got covered in a marine layer cold enough for permafrost (guaranteeing a D- or E-type climate) during Heinrich events as far south as Jacksonville. Although it didn't form Carolina Bays west of the Florida Peninsula's dividing ridge nor on the Piedmont (the similar-looking Grady ponds of the Florida panhandle and southern Alabama are formed by carbonate soil/bedrock, not permafrost-related thermokarst), that too probably would've been a death blow to any nascent ill-adapted palm species along the Atlantic coast in Heinrich events, with Serenoa and Sabal palmetto probably restricted to the Florida Peninsula (and Bahamas in the latter case) at times and Rhapidophyllum and Sabal minor possibly also present in favorable microclimates along the Fall Line and entire immediate Gulf Coast except in the absolute coldest periods. There are iceberg grooves as far south as the Florida Keys dated to Heinrich event 3 and many more along Cape Hatteras dated to other Heinrich events, indicating a meltwater-supercharged Labrador Current during those times! In a comment, the author of that climate video conceded that including those climate events along the Atlantic during his video would've pushed the model to its limits, although it still seems sound to rely on the ice core data for areas west of the Atlantic Fall Line and the Florida Peninsula's continental divide. Qualified scientists have also used this same Greenland ice core data in PDFs of published studies about the Last Glacial Period and shown Trewartha climate classifications (although the maps I saw in the one study I saw doing so erroneously merged the Trewartha Dca and Do classifications).