Don,
I'ts a native tree, and was growing in the wild, just as you saw it, long before Europeans - and probably before the 'native' Americans came. Birds eat the seeds, and deposit them hither and yon.
I've been gone from my native AL for close to 20 years now, and had largely forgotten how common 'wild' M.grandiflora seedlings were in the forests until we took a trip to Gulf shores last spring. They're everywhere.
Although M.grandiflora, and M.virginiana are common in landscape plantings here in western KY, I can't say that I've ever seen an 'volunteer' seedling up here, though I can grow 'em from seed very easily, and the bigleaf magnolia clan(cucumbertree, bigleaf, umbrella) is pretty prominent in the wild when you get over into the eastern part of the state.

Thanks for the note, Lucky. Yes, I knew Mag.g. is native in La. & Ga. - I probably didn't express my question clearly. Sometimes a plant will naturalize in an ideal environment to which it isn't native (kudzu, lonicera, etc). I didn't expect Mag. g. to be growing wild as far "north" as S. Carolina. So, I suppose the question more correctly would be: is it occurring in natural distribution, or by inroduction. We have some of the deciduous ones (cucumber) native as far north as western Maryland, and the Mag. vir. is native in southern Maryland. All are wonderful trees !

How hard is it to grow grandiflora from seeds?

Bull Bay Magnolia is probably not native to the Piedmont and Mountain areas of S. Carolina, but certainly is in the Lowcountry.

I have a couple of 2 yr. old M. grandifloras in pots that I grew from seed. The seeds shouldn't be allowed to dry out. I kept mine vernalized for 3 months before sowing, but I would think winter sowing would be easier!