tupelo vs black gum

tupelo and black gum both are nouns.

Word	Noun	Adjective	Verb	Adverb
tupelo	Yes	No	No	No
black gum	Yes	No	No	No

As nouns, black gum is a hyponym of tupelo; that is, black gum is a word with a more specific, narrower meaning than tupelo:

tupelo: any of several gum trees of swampy areas of North America
black gum: columnar tree of eastern North America having horizontal limbs and small leaves that emerge late in spring and have brilliant color in early fall

Other hyponyms of tupelo include Nyssa aquatica, water gum, Nyssa sylvatica, pepperidge, sour gum.

tupelo (noun)	black gum (noun)
any of several gum trees of swampy areas of North America	columnar tree of eastern North America having horizontal limbs and small leaves that emerge late in spring and have brilliant color in early fall
pale soft wood of a tupelo tree especially the water gum	a small mallee with rough dark-colored bark toward the butt; yields a red eucalyptus kino gum