common European ash vs ash tree

common European ash and ash tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
common European ash	Yes	No	No	No
ash tree	Yes	No	No	No

As nouns, ash tree is a hypernym of common European ash; that is, ash tree is a word with a broader meaning than common European ash:

common European ash: tall ash of Europe to the Caucasus having leaves shiny dark-green above and pale downy beneath
ash tree: any of various deciduous pinnate-leaved ornamental or timber trees of the genus Fraxinus

Other hypernyms of common European ash include ash.

common European ash (noun)	ash tree (noun)
tall ash of Europe to the Caucasus having leaves shiny dark-green above and pale downy beneath	any of various deciduous pinnate-leaved ornamental or timber trees of the genus Fraxinus