Hello,
it depends. If your mesh resolution is 10 cm and, roughly spoken, the diameter of your trees is equal or bigger than your mesh resolution, than you can include the trees directly in the mesh. However, I would recommend to discretize the tree with several mesh nodes in order to compute a reasonable drag force.
The option with vertical structures is convenient if the structure is (much) smaller than the local mesh size. Than you have to estimate the drag force coefficient from literature etc.
Hope this helps,
Clemens