black gum vs mallee

black gum and mallee both are nouns.

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

As nouns, mallee is a hypernym of black gum; that is, mallee is a word with a broader meaning than black gum:

black gum: a small mallee with rough dark-colored bark toward the butt; yields a red eucalyptus kino gum
mallee: any of several low-growing Australian eucalypts

black gum (noun)	mallee (noun)
columnar tree of eastern North America having horizontal limbs and small leaves that emerge late in spring and have brilliant color in early fall	any of several low-growing Australian eucalypts
a small mallee with rough dark-colored bark toward the butt; yields a red eucalyptus kino gum