Ash tree and Blue ash

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

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

Blue ash noun - Ash of central and southern United States with bluish-green foliage and hard brown wood.

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