You write a poem about a tree. Should no one ever again be able to write a poem about a tree without giving you a royalty?
You seem not to understand the basic concept of literature. The copyright isn’t based on subject matter - a tree, a rose, the sunrise, etc.
There are many poems, songs, etc. on many subjects. It is the unique work of the author that has the copyright. The topic of trees is not the point. The point is the individual work - Housman’s poem, “Loveliest of trees! The cherry now/is hung with bloom along the bough...”