exactly. A nigra won't last a couple of years outdoors in ontario(CAN), but a rubra may live to 75.
but have yet to hear of a 500 year old rubra anywhere, whereas nigras in milder-winter areas as the mediterranean region could attain that age, maybe even more.
thus said, i was pertaining to the most hospitable growing conditions for each species.
intriguingly, nigras could live for hundreds of years in england, which isn't exactly mediterranean.
yes, but it is still a hit or miss endeavor, and involves thick calipers, which not many in usa could afford to do. Here in vegas, nigras grow excruciatingly slow, despite our relatively long growing seasons, and have never come across anyone being successful with cuttings(since presumably only using thin caliper stems). So when we plant grafted nigras deep with the scion wood partly underground, the intent is for the scion wood to assume truncheon status someday should the alba rootstoc start declining.