RE:
Do names of trees and flowers need to be capitalized when used in a sentence?

When you analyse it, it’s a particular species of tree, so the River Birch is a proper name, and it should be capitalised. Having said that, it seems to me you should trust your instincts on this and if it feels much more natural without capitals, I’d go with that. I doubt very much if readers will object either way. Your judgment should really be, what will the readers find most natural.

Ex. Tammy loves nature. She enjoys sitting among the natural beauty of the oak and maple trees, and seeing tulips and lilacs swaying in the spring breeze.

I love daffodils.