Poplar and White aspen

Poplar noun - Any of numerous trees of north temperate regions having light soft wood and flowers borne in catkins.

White aspen and poplar are semantically related. In some cases you can replace phrase "White aspen" with "Poplar", this terms are similar.

White aspen noun - A poplar that is widely cultivated in the United States; has white bark and leaves with whitish undersurfaces.

Poplar and white aspen are semantically related. You can use "Poplar" instead a noun phrase "White aspen".