Birch tree and Downy birch

Birch tree noun - Any betulaceous tree or shrub of the genus Betula having a thin peeling bark.

Downy birch and birch tree are semantically related. In some cases you can replace phrase "Downy birch" with "Birch tree", this phrases are similar.

Downy birch noun - European birch with dull white to pale brown bark and somewhat drooping hairy branches.

Birch tree and downy birch are semantically related. You can use "Birch tree" instead a noun phrase "Downy birch".