Ash tree and Brown ash

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

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

Brown ash noun - Vigorous spreading North American tree having dark brown heavy wood; leaves turn gold in autumn.

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