Ash tree and White ash

Ash tree noun - Any of various deciduous pinnate-leaved ornamental or timber trees of the genus Fraxinus.

White ash and ash tree are semantically related. In some cases you can replace phrase "White ash" with "Ash tree", this phrases are similar.

White ash noun - Spreading American ash with leaves pale green or silvery beneath and having hard brownish wood.

Ash tree and white ash are semantically related. You can use "Ash tree" instead a noun phrase "White ash".