I do know they can be grafted onto any prunus tree, like peach, cherry, nectarine, etc.. Peach is common. But I don't know of any thornes on any of these. Look carefully. Maybe somethng else sprouted at the dead tree's base.

American (native) plums have thorns from my experience. I'm not telling you what you have is an American plum, but just that wild plums have thorns.

Will grafting a plum onto peach tree stock make a semi-dwarf plum tree? How tall?

Prunus americana and prunus spinosa have thorns, probably some other plums as well. Americana is used as a rootstock for some northern plums so it is probably americana.

Grafting plum onto peach will not dwarf it; peach is vigorous as a rootstock.

Scott

Thanks for the help everyone.
I think I will plant a companion plum for pollination and see what develops